777f mg sistemas

SERV18XXDecember 2006

TECHNICAL PRESENTATION

777F (JRP)OFF-HIGHWAY TRUCK

POWER TRAIN, STEERING, HOIST, AND BRAKINGSYSTEMS

Service Training Meeting Guide(STMG)

GLOBAL SERVICE LEARNING

TABLE OF CONTENTS

POWER TRAIN ...........................................................................................................................3Torque Converter Hydraulic System ......................................................................................6Transmission Hydraulic System ...........................................................................................17Rear Axle ..............................................................................................................................29Transmission/Chassis Electronic Control System ................................................................31

STEERING SYSTEM ................................................................................................................42

HOIST SYSTEM........................................................................................................................57

BRAKE SYSTEM ......................................................................................................................77Brake Electronic Control System .......................................................................................104Automatic Retarder Control System...................................................................................109Traction Control System .....................................................................................................111

CONCLUSION.........................................................................................................................118

SERV18xx - 2 - Text Reference12/06

1

POWER TRAIN

The 777F Off-highway Truck power train is electronically controlled. TheTransmission/Chassis ECM controls the ECPC transmission shifting and the torque converterlockup clutch operation. The transmission has seven forward speeds and one reverse speed.

Power flows from the engine to the rear wheels through the power train. The main power traincomponents are:

- Torque converter (1)

- Drive shaft (2)

- Transfer gears (3)

- Transmission (4)

- Differential (5)

- Final drives (6)

Other power train components visible in this illustration are the transmission charge filters (7),torque converter charging filter (8), and two-section hydraulic tank (9).


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These illustrations show the location of the main electronic components in the power train. TheTransmission/Chassis ECM (1) is located behind the cab seat and is accessed by removing apanel at the rear of the cab. The transmission modulating valves (2) are located on top of thetransmission planetary gears and are accessed by removing a cover plate. The torque converterlockup clutch solenoid valve (3) is located on the rear of the torque converter.

NOTE: The Transmission/Chassis ECM receives input signals from severalcomponents located on the machine to control transmission shifting and the torqueconverter lockup clutch operation. The electronic components will be covered later inthe presentation.

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Shown is the transmission and torque converter hydraulic system for the 777F. A five sectionpump is located at the rear of the torque converter housing. The first section (attached to pumpdrive at rear of torque converter) scavenges oil from the bottom of the torque converter caseand returns the oil to the hoist, torque converter, and brake hydraulic tank. The second sectionpumps charge oil through the torque converter filter to the torque converter. The third sectionsends oil through the lockup clutch filter and provides pilot oil to the following circuits:

- Lockup clutch valve

- Variable speed fan clutch control

- Hoist pilot signal resolver

- Traction control valve

The fourth section scavenges oil from the transmission sump and sends oil to the transmissionoil cooler and the transmission hydraulic tank.

The fifth section sends charge oil through the transmission oil filters to the transmission controlvalves.


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Torque Converter Hydraulic System

This schematic shows the oil flow from the torque converter pump through the torque converterhydraulic system.

The scavenge pump section pulls oil through a screen from the torque converter housing andsends the oil to the hoist, torque converter, and brake hydraulic tank.

The torque converter charging pump section sends oil through the torque converter chargingfilter to the torque converter inlet relief valve. Oil flows from the inlet relief valve through thetorque converter to the outlet relief valve. Oil flows from the outlet relief valve to the brake oilcooling circuit.

The lockup clutch valve pump section sends oil through the lockup clutch valve filter to thetorque converter lockup clutch valve. When oil pressure in the lockup clutch valve circuit istoo high, the lockup clutch relief valve allows oil to flow to the brake cooling circuit.

Oil from the lockup clutch valve pump section also flows to the TCS valve, variable speedclutch control and hoist pilot signal resolver.


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The five sections of the power train pump (from the front to the rear) are:

- Torque converter scavenge (1)

- Torque converter charging (2)

- Lockup clutch valve, hoist pilot circuit, TCS valve, and variable speed fan clutch (3)

- Transmission scavenge (4)

- Transmission charging (5)

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In DIRECT DRIVE, the lockup clutch is engaged by hydraulic pressure and locks the turbine tothe impeller. The housing, impeller, turbine, and output shaft then rotate as a unit at enginerpm. The stator, which is mounted on a one-way clutch, is driven by the force of the oil in thehousing. The one-way clutch permits the stator to turn freely in DIRECT DRIVE when torquemultiplication is not required.

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This sectional view shows a torque converter in CONVERTER DRIVE. The lockup clutch(yellow piston and blue discs) is not engaged. During operation, the rotating housing andimpeller (red) can rotate faster than the turbine (blue). The stator (green) remains stationaryand multiplies the torque transfer between the impeller and the turbine. The output shaft rotatesslower than the engine crankshaft, but with increased torque.

The five section power train pump (1) is located at the bottom rear of the torque converter.

The inlet relief valve (2) limits the maximum pressure of the supply oil to the torque converter.The torque converter inlet relief pressure can be checked by removing a plug and installing apressure tap. Normally, the inlet relief pressure will be slightly higher than the outlet reliefvalve pressure.

Oil flows through the inlet relief valve and enters the torque converter. Some of the oil willleak through the torque converter to the bottom of the housing to be scavenged. Most of the oilin the torque converter is used to provide a fluid coupling and flows through the torqueconverter outlet relief valve (3).

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The outlet relief valve maintains the minimum pressure inside the torque converter. The mainfunction of the outlet relief valve is to keep the torque converter full of oil to preventcavitation. The outlet relief pressure can be measured at the tap (4) on the outlet relief valve.

The torque converter lockup clutch valve (5) directs oil to engage the torque converter lockupclutch. The torque converter lockup clutch pressure can be checked at the tap (6) on top of thelockup clutch valve.

Excess oil that accumulates in the bottom of the torque converter is scavenged by the firstsection of the pump through a screen behind the access cover (7) and returned to the hoist,torque converter, and brake hydraulic tank.


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The torque converter lockup clutch modulating valve contains a proportional solenoid thatreceives a signal from the Transmission/Chassis ECM to engage and release the torqueconverter lockup clutch.

In this illustration, the lockup clutch modulating valve is shown with no current signal appliedto the solenoid (TORQUE CONVERTER DRIVE or NEUTRAL). The Transmission/ChassisECM controls the rate of oil flow through the lockup clutch modulating valve to the lockupclutch by changing the signal current strength to the solenoid. With no current signal applied tothe solenoid, the transmission modulating valve is DE-ENERGIZED and oil flow to the clutchis blocked.

Pump oil flows into the valve body around the valve spool and into a drilled passage in thecenter of the valve spool. The oil flows through the drilled passage and orifice to the left sideof the valve spool to a drain orifice. Since there is no force acting on the pin assembly to holdthe ball against the drain orifice, the oil flows through the spool and the drain orifice past theball to the tank.

The spring located on the right side of the spool in this view holds the valve spool to the left.The valve spool opens the passage between the clutch passage and the tank passage and blocksthe passage between the clutch passage and the pump supply port. Oil flow to the clutch isblocked. Oil from the clutch drains to the tank preventing clutch engagement.


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In this illustration, the modulating valve is shown with a maximum current signal commandedto the solenoid. When the modulation cycle stops, the Transmission/Chassis ECM sends themaximum specified current signal to fully engage the lockup clutch (DIRECT DRIVE).

The constant current signal pushes the pin firmly against the ball in the solenoid valve. The pinforce against the ball blocks more oil from flowing through the drain orifice. This restrictioncauses an increase in pressure on the left side of the valve spool. The valve spool moves to theright to allow pump flow to fully engage the clutch.

In a short period of time, maximum pressure is felt at both ends of the proportional solenoidvalve spool. This pressure along with the spring force on the right end of the spool cause thevalve spool to move to the left until the forces on the right end and the left end of the valvespool are balanced.

The valve spool movement to the left (balanced) position reduces the flow of oil to the engagedclutch. The Transmission/Chassis ECM sends a constant maximum specified current signal tothe solenoid to maintain the desired clutch pressure.


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A torque converter outlet temperature sensor (arrow) provides an input signal to theTransmission/Chassis ECM, which sends a signal to the monitoring system to inform theoperator of the torque converter outlet temperature.

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The filter has a bypass switch (3) which provides an input signal to the monitoring system, viathe Transmission/Chassis ECM, to inform the operator if the filter is restricted. The filterhousing has an S•O•S tap (4) and a lockup clutch circuit pressure tap (5).

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Oil from the lockup clutch valve pump section flows to the lockup valve oil filter (1) and thento the lockup clutch modulating valve (2). The filter is located inside of the left frame rail.

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The lockup clutch relief valve (1) is located inside the left frame rail in front of the lockupclutch filter (2). This view is looking up from the bottom of the truck. When oil pressure inthe lockup clutch valve circuit is too high, the lockup clutch relief valve allows oil to flow tothe brake cooling circuit.

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The torque converter filter includes an S•O•S port (2) located on the bottom of the filter.

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The torque converter charging filter (1) is located on the right frame rail, behind the right fronttire. Oil from the torque converter charging pump section flowS through the torque converterfilter to the torque converter inlet relief valve.

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Transmission Hydraulic System

The transmission scavenge pump section pulls oil from the bottom of the transmission casethrough a magnetic screen and sends the oil through the transmission oil cooler to thetransmission tank. The magnetic screen should always be checked for debris if a problem withthe transmission is suspected.

The transmission charging pump section pulls oil from the transmission hydraulic tank.Charging oil flows from the pump through two transmission charging filters to the transmissionmain relief valve and seven modulating valves.

The main relief valve regulates the supply pressure inside the transmission hydraulic system.Oil unseats the check ball and forces the spool to the right if the transmission system pressurebecomes greater than the spring force on the right of the spool. Excess oil will flow to thelubrication circuit and the lube relief valve. The lubrication circuit oil and oil from the luberelief valve flows to the transmission sump. The relief valve is adjustable by turning theadjusting screw on the right end of the valve.


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The clutch modulating valves control the engagement of the transmission clutches. Thesolenoids are controlled by a pulse width modulated (PWM) signal from theTransmission/Chassis ECM. Supply oil flows into the clutch modulating valves and through apassage in the center of the spool. Oil then flows to the tank if the solenoid is not energized.Oil flow is blocked by a ball and seat if the solenoid is energized. The spool will shift downand the clutch will begin to fill. The signal from the Transmission/Chassis ECM determineshow long it takes to fill each clutch.

The transmission lubrication relief valve limits the transmission lubrication oil pressure.


The transmission scavenge pump section (1) pulls oil from the bottom of the transmission casethrough a magnetic screen and sends the oil through the transmission oil cooler (2) to thetransmission tank. The oil cooler is located on the right side of the engine.

The transmission charging pump section (3) pulls oil from the bottom of the transmissionhydraulic tank through a magnetic screen and sends the oil through the transmission filters tothe transmission hydraulic controls.

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Oil from the transmission charging pump section is sent to the transmission charge oil filters (1) located on the cross member on the right side of the machine.

The rear filter housing has an S•O•S tap (2) and a charge pressure tap (3). The rear filterhousing also has a bypass switch (4) which provides an input signal to the monitoring system,via the Transmission/Chassis ECM, to inform the operator if the filter is restricted.

The ECPC transmission hydraulic controls can be accessed by removing a cover plate (5) ontop of the transmission. The transmission input speed sensor (6) is located on top of thetransfer gear housing. The transmission input speed sensor sends an input to theTransmission/Chassis ECM which checks the speed of the drive shaft to the speed of theengine.

The transmission has pressure taps located on the outside of the transmission which aids inpreventing contamination from entering the transmission as well as saving time when checkingthe pressures on the 777F transmission.

Shown in the lower right illustration are the transmission control valve pressure taps. The lubeoil pressure tap (7) and the transmission hydraulic system pressure tap (8) are located towardthe rear of the transmission. Oil pressure for the seven clutches can be checked at theremaining seven taps (9) on the transmission.

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The transmission modulating valves control the oil to corresponding transmission clutches. Thesolenoid valves are:

- Clutch No. 1 Solenoid valve (1)- Clutch No. 2 Solenoid valve (2)- Clutch No. 3 Solenoid valve (3)- Clutch No. 4 Solenoid valve (4)- Clutch No. 5 Solenoid valve (5)- Clutch No. 6 Solenoid valve (6)- Clutch No. 7 Solenoid valve (7)

The main relief valve (8) controls the transmission hydraulic pressure, and the lubrication reliefvalve (not visible) controls the lubrication pressure. The lubrication relief valve is locatedbelow the main relief valve.

Also located on the transmission hydraulic control valve is the transmission hydraulic oiltemperature sensor (9). The temperature sensor sends a signal to the Transmission/ChassisECM indicating transmission oil temperature.


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The table in this illustration lists the solenoids that are energized and clutches that are engagedfor each transmission speed. This table can be useful for transmission diagnosis.


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In this illustration, the transmission modulating valve is shown with no current signal applied tothe solenoid. The Transmission/Chassis ECM controls the rate of oil flow through thetransmission modulating valves to the clutches by changing the signal current strength to thesolenoid. With no current signal applied to the solenoid, the transmission modulating valve isDE-ENERGIZED and oil flow to the clutch is blocked.

Pump oil flows into the valve body around the valve spool and into a drilled passage in thecenter of the valve spool. The oil flows through the drilled passage and orifice to the left sideof the valve spool to a drain orifice. Since there is no force acting on the pin assembly to holdthe ball against the drain orifice, the oil flows through the spool and the drain orifice past theball to the tank.

The spring located on the right side of the spool in this view holds the valve spool to the left.The valve spool opens the passage between the clutch passage and the tank passage and blocksthe passage between the clutch passage and the pump supply port. Oil flow to the clutch isblocked. Oil from the clutch drains to the tank preventing clutch engagement.


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In this illustration, the modulating valve is shown with a signal to the solenoid that is below themaximum current. Clutch engagement begins when the Transmission/Chassis ECM sends aninitial current signal to ENERGIZE the solenoid. The amount of commanded current signal isproportional to the desired pressure that is applied to the clutch during each stage of theengagement and disengagement cycle.

The start of clutch engagement begins when the current signal to the solenoid creates amagnetic field around the pin. The magnetic force moves the pin against the ball in proportionto the strength of the current signal from the Transmission/Chassis ECM.

The position of the ball against the orifice begins to block the drain passage of the oil flow fromthe left side of the valve spool to the tank. This partial restriction causes the pressure at the leftend of the valve spool to increase. The oil pressure moves the valve spool to the right againstthe spring. As the pressure on the right side of the valve spool overrides the force of the spring,the valve spool shifts to the right.

The valve spool movement starts to open a passage on the right end of the valve spool for pumpsupply oil to fill the clutch. Oil also begins to fill the spring chamber on on the right end of thespool.


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In the initial clutch filling stage, the Transmission/Chassis ECM commands a high current pulseto quickly move the valve spool to start filling the clutch. During this short period of time, theclutch piston moves to remove the clearances between the clutch discs and plates to minimizethe amount of time required to fill the clutch. The ECM then reduces the current signal whichreduces the pressure setting of the proportional solenoid valve. The change in current signalreduces the flow of oil to the clutch. The point where the clutch plates and discs start to touchis called TOUCH-UP.

Once TOUCH-UP is obtained, the Transmission/Chassis ECM begins a controlled increase ofthe current signal to start the MODULATION cycle. The increase in the current signal causesthe ball and pin to further restrict oil through the drain orifice to tank causing a controlledmovement of the spool to the right. The spool movement allows the pressure in the clutch toincrease.

During the MODULATION cycle, the valve spool working with the variable commandedcurrent signal from the Transmission/Chassis ECM acts as a variable pressure reducing valve.

The sequence of partial engagement is called desired slippage. The desired slippage iscontrolled by the application program stored in the Transmission/Chassis ECM.


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In this illustration, the modulating valve is shown with a maximum current signal commandedto the solenoid. When the modulation cycle stops, the Transmission/Chassis ECM sends themaximum specified current signal to fully engage the clutch.

The constant current signal pushes the pin firmly against the ball in the solenoid valve. The pinforce against the ball blocks more oil from flowing through the drain orifice. This restrictioncauses an increase in pressure on the left side of the valve spool. The valve spool moves to theright to allow pump flow to fully engage the clutch.

In a short period of time, maximum pressure is felt at both ends of the proportional solenoidvalve spool. This pressure along with the spring force on the right end of the spool cause thevalve spool to move to the left until the forces on the right end and the left end of the valvespool are balanced.

The valve spool movement to the left (balanced) position reduces the flow of oil to the engagedclutch. The Transmission/Chassis ECM sends a constant maximum specified current signal tothe solenoid to maintain the desired clutch pressure.


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The different maximum specified pressures for each clutch is caused by different maximumcurrent signals being sent by the Transmission/Chassis ECM to each individual modulatingvalve. The different maximum signal causes a difference in the force pushing the pin againstthe ball to block leakage through the drain orifice in each solenoid valve. The different rate ofleakage through the spool drain orifice provides different balance positions for the proportionalsolenoid valve spool. Changing the valve spool position changes the flow of oil to the clutchand the resulting maximum clutch pressure.

The operation of the proportional solenoid to control the engaging and releasing of clutches isnot a simple on and off cycle. The Transmission/Chassis ECM varies the strength of thecurrent signal through a programmed cycle to control movement of the valve spool.

The clutch pressures can be changed using Caterpillar Electronic Technician (ET) during thecalibration procedure.


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The transmission hydraulic control relief valve is used to regulate the pressure to the maincomponents in the transmission.

Oil enters the relief valve at the supply port. The pressure of the oil unseats the ball and movesthe spool toward the right. Oil flows past the spool and to the tank to regulate transmission oilpressure.

The adjustment screw alters the preload on the spring to adjust the relief pressure.


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Rear Axle

Check the differential oil level by removing the magnetic inspection plug (1). The oil shouldbe level with the bottom of the fill plug opening. The magnetic inspection plug should beremoved at regular intervals and checked for metal particles. The plug (2) at the bottom of thedifferential housing is used to drain the oil.

The optional remote grease fittings (3) are located on top of the differential.

Inspect the condition of the rear axle breather (4) at regular intervals. The breather preventspressure from building up in the axle housing. Excessive pressure in the axle housing cancause brake cooling oil to leak through the Duo-Cone seals in the wheel brake assemblies. Thebrake cooling oil pressure can be checked at the pressure taps (5) on top of the axle.

A differential carrier thrust pin is located behind the small cover (6). The thrust pin preventsmovement of the differential carrier during high thrust load conditions.

The backup alarm (7) is located on top of the rear frame. When the machine is in reverse, theTransmission/Chassis ECM sends a signal to sound the backup alarm.

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Shown is the differential removed from the rear axle housing. The differential is located in therear axle housing behind the transmission. Power flows from the transmission to thedifferential. The differential divides the power to the right and left axle shafts. Torque istransmitted equally from the differential through the two axle shafts to the final drives. Thedifferential adjusts the speed of the axle shafts for vehicle cornering, therefore, the powerdelivered to the axle shafts is unequal during cornering.

The differential thrust pin contacts the differential carrier at the location shown (arrow). Whenhigh thrust loads are transmitted from the differential pinion to the differential ring gear, thecarrier tries to move away from the pinion. The thrust pin prevents movement of thedifferential carrier during high thrust load conditions.

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Transmission/Chassis Electronic Control System

Shown in this illustration are the transmission/chassis electronic control system inputs andoutputs for the 777F trucks.

The main purpose of the Transmission/Chassis ECM is to determine the desired transmissiongear and energize the appropriate solenoids to shift the transmission up or down as requiredbased on information from both the operator and machine. The Transmission/Chassis ECMalso controls all the hoist functions, the steering disable function, and other functions asdescribed in this presentation.

The Transmission/Chassis ECM receives information from various input components such asthe shift lever switch and the transmission output speed sensors.

Based on the input information, the Transmission/Chassis ECM determines whether thetransmission should upshift, downshift, engage the lockup clutch, or limit the transmission gear.These actions are accomplished by sending signals to various output components.


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Power train output components include the transmission modulating valve solenoids and lockupclutch solenoid. Several other Transmission/Chassis ECM output components are coveredthroughout the presentation.

The Engine ECM, the monitoring system, the Transmission/Chassis ECM, and the Brake ECMall communicate with each other through the CAT Data Link. Communication between theelectronic control modules allows the sensors of each system to be shared. Many additionalbenefits are provided, such as Controlled Throttle Shifting (CTS). CTS occurs when theTransmission/Chassis ECM tells the Engine ECM to reduce or increase engine fuel during ashift to lower stress to the power train.

The Electronic Technician (ET) Service Tool can be used to perform several diagnostic andprogramming functions.

NOTE: Some of the Transmission/Chassis ECM input and output components areshown during the discussion of other systems.


The Transmission/Chassis ECM (arrow) is located in the compartment at the rear of the cab.The Transmission/Chassis ECM performs the transmission control functions, plus some othermachine functions (hoist and secondary steering control). Because of the functionality of thecontrol, it is referred to as the Transmission/Chassis ECM.

The Transmission/Chassis ECM is an A4M1 module with two 70-pin connectors. TheTransmission/Chassis ECM communicates with the Engine ECM, Brake ECM, and monitoringsystem over the CAT Data Link and can communicate with some attachments over the CANDatalink.

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At the base of the shift lever (1) is a position sensor (2) which provides input signals to theTransmission/Chassis ECM when the operator moves the lever. The shift lever position sensoris a Hall-Effect position sensor. The shift lever is connected to a device which contains twomagnets. One magnet (3) is visible in the bottom left view.

As the lever is moved, the magnets pass over the Hall Cell (4) and the change in the magneticfield produces a signal. The internal electronics (5) of the sensor process the signal and send aPWM signal to the ECM.

The lever position sensor receives 24 VDC from the machine electrical system. The sensorcontains a fourth pin that is used for calibration on some machine applications.

The following measurements would be typical for the position sensor with the sensor connectedto the Transmission/Chassis ECM and the key switch turned ON:

• Pin 1 to Pin 2 -- Supply Voltage

• Pin 3 to Pin 2 -- .7 - 6.9 DCV on DC volts scale

• Pin 3 to Pin 2 -- 4.5 - 5.5 KHz on the KHz scale

• Pin 3 to Pin 2 -- 5% - 95% duty cycle on the % scale

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Also shown in the top right illustration is the drive gear UP switch (6) and the drive gearDOWN switch (7). The drive gear switches are toggle switches that send a signal to theTransmission/Chassis ECM. When the drive gear UP switch is pressed, the high gear limit canbe increased up to seventh gear. When the drive gear DOWN switch is pressed, the high gearlimit can be decreased down to third gear.

The transmission shift lever lock button (8) unlocks the transmission shift lever when pressed.


The transmission output speed sensors are located on the transfer gear housing on the input endof the transmission behind a cover (arrow). Although the sensors are physically located nearthe input end of the transmission, the sensors are measuring the speed of the transmissionoutput shaft. The sensors are two wire passive sensors. The passive speed sensor uses thepassing teeth of the output shaft to provide a frequency signal. The signal from the sensor isused for automatic shifting of the transmission. The signal is also used to drive thespeedometer and as an input to other electronic controls.

The Transmission/Chassis ECM also performs a check between the two measured transmissionoutput speeds and the transmission input speed to ensure that the ECM calculates an accuratetransmission speed. This check also uses the speeds to determine the direction of motion of themachine.

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The engine speed sensor (arrow) is located at the rear of the engine on the left side of the gearhousing. The engine speed sensor sends a frequency signal to the Transmission/Chassis ECMindicating engine speed. The Transmission/Chassis ECM uses the engine speed signal input todetermine actual engine speed. The actual engine speed is one of the parameters used todetermine the proper transmission shift points.

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The transmission oil level switch (arrow), located near the bottom of the transmission tank,sends a signal to the Transmission/Chassis ECM indicating the hydraulic oil level in thetransmission tank.

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The body up switch (1) is located on the frame near the body pivot pin. This magnetic switchis normally open. When the body is raised, a magnet (2) mounted on the body passes theswitch and causes the switch to close. The resulting ground signal is sent to theTransmission/Chassis ECM. This signal is used to limit the top gear into which thetransmission will shift when the body is up.

The body up top gear value is programmable from FIRST to THIRD utilizing the Cat ETService Tool. The ECM comes from the factory with this value set to FIRST gear. Whendriving away from a dump site, the transmission will not shift past FIRST gear until the body isdown. If the transmission is already above the set limit gear when the body is raised, nolimiting action will take place.

The body up switch signal is also used to control the SNUB position of the hoist control valve.As the body is lowered and the magnet passes the body up switch, the Transmission/ChassisECM signals the hoist lower solenoid to move the hoist valve spool to the SNUB position. Inthe SNUB position, the body float speed is reduced to prevent the body from making hardcontact with the frame.

The body up switch input provides the following functions:

- Body up gear limiting

- Body up sound reduction

- Hoist snubbing

- Lights the body up dash lamp

- Signals a new load count (after 10 seconds in the RAISE position)

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A diagnostic code occurs if the Transmission/Chassis ECM does not receive a closed (ground)signal from the switch within four hours of operation time or an open signal from the switchwithin one hour of operation time. The body up switch must be adjusted properly for all of thefunctions to operate correctly.

Two LEDs are located on the body up switch. The green LED indicates that battery power ispresent. The amber LED indicates that the switch is closed (grounded).

The body position switch can be raised or lowered slightly in the bracket notches to start theSNUB feature sooner or later.

NOTE: The snub feature can also be adjusted in the Cat ET hoist configuration screenby selecting the "Hoist lower valve adjustment status".


37

Besides controlling the Transmission Shifting and Torque Converter Lockup, theTransmission/Chassis ECM also controls other functions as shown above, such as ControlThrottle Shifting (CTS), Directional Shift Management, and Top Gear Limit.

There are several programmable parameters available with the Transmission/Chassis ECM.

NOTE: Refer to the Transmission/Chassis Electronic Control System Operation,Troubleshooting, Testing, and Adjusting manual (RENR8342) for more information onthe additional Transmission/Chassis ECM functions and programmable parameters.


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38

STEERING SYSTEM

The steering system on the 777F is similar to the 777D except a steering disable solenoid valvehas been added and some of the component locations have changed.

When energized, the steering disable solenoid valve stops the oil flow coming from the steeringpump. This prevents the front wheels from turning to allow servicing to be conducted safely inthe front wheel area.

The steering system uses a load sensing, pressure compensated pump. Minimal horsepower isused by the steering system when the truck is traveling in a straight path. Steering hydraulichorsepower requirements depend on the amount of steering pressure and flow required by thesteering cylinders.

This illustration shows the following main steering components:

- Steering pump (1)

- Steering disable valve andsteering valve (2)


- HMU (3)

- Steering tank (4)

- Secondary steering pump (5)

15

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The steering system tank is located on the right platform

Check the steering system oil level at the sight gauge (1).

The steering system oil filter (2) is located on the side of the steering tank.

The steering system uses a pressure compensated piston type pump. Case drain oil from thesteering pump returns to the hydraulic tank through a case drain filter (3) on the side of thesteering tank.

Before removing the cap to add oil to the steering system, depress the pressure release button (4) on the breather to release any remaining pressure from the tank.

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The steering system filter base and the case drain filter base have bypass valves that allow thesteering oil to bypass the filters if they are plugged.


The 777F Trucks are equipped with a load sensing, pressure compensated, piston-type pump.The steering pump operates only when the engine is running and provides the necessary flow ofoil for steering system operation. The steering pump contains a load sensing controller withtwo valves. The high pressure cutoff valve (1) functions as the primary steering system reliefvalve.

The flow compensator valve (2) is used to adjust the low pressure standby setting. When thetruck is traveling in a straight path, virtually no flow or pressure is sent to the steeringcylinders, and the pump destrokes to low pressure standby.

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When the truck is traveling in a straight path, the steering cylinders require virtually no flow orpressure. The HMU provides a very low pressure load sensing signal to the flow compensatorin the load sensing controller.

Pump oil (at low pressure standby) flows to the swashplate piston and past the lower end of thedisplaced flow compensator spool to the actuator piston. The actuator piston has a largersurface area than the swashplate piston. The oil pressure at the actuator piston overcomes thespring force and the oil pressure in the swashplate piston and moves the swashplate to destrokethe pump. The pump is then at minimum flow, low pressure standby.

Pump output pressure is equal to the setting of the flow compensator plus the pressure requiredto compensate for system leakage.


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During a turn, when steering pressure and flow are required, pressure increases in the HMUload sensing signal line. The pressure in the signal line is equal to the pressure in the steeringcylinders. The pump load sensing controller is spring biased to vent the actuator pistonpressure to drain. Venting pressure from the load sensing controller and the actuator pistonpositions the spring biased swashplate to maximum displacement (maximum flow).

As pressure increases in the HMU load sensing signal line, pump supply pressure is sensed onboth ends of the flow compensator. When pressure is present on both ends of the flowcompensator, the swashplate is kept at maximum angle by the force of the spring in the pumphousing and pump discharge pressure on the swashplate piston. The pistons reciprocate in andout of the barrel and maximum flow is provided through the outlet port. Since the pump isdriven by the engine, engine rpm also affects pump output.


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The steering disable valve (1) is located behind the shock on the right frame rail.

When the steering disable solenoid valve (2) is energized, oil flow from the steering pump tothe steering valve is blocked by the steering disable valve, which allows servicing behind thefront wheels with the machine running.

When the machine lockout switch, located under a panel on the left stairway, is toggled, asignal is sent to the Transmission/Chassis ECM. The Transmission/Chassis ECM energizes thesteering disable solenoid allowing service to be performed behind the front wheels safely.

Also located on the steering disable valve is a pressure tap (3) for checking the load sensingsignal to the pump, and an S•O•S tap (4).

44

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Steering oil flows from the pump through the steering disable valve to the steering valve (1)located on the frame behind the right front suspension cylinder. The primary steering pressureswitch (2) monitors the output of the steering pump. The steering pressure switch providesinput signals to the Transmission/Chassis ECM which sends a signal to the monitoring systemto inform the operator of the steering system condition. A steering system warning is displayedif the pressure is too low.

The steering pressure switch cannot tolerate high steering system pressures. A pressurereducing valve (not visible) reduces the steering system pressure to the steering pressureswitch.

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Two relief valves are located on the left side of the steering valve. The top relief valve (3) is aback-up relief valve for the secondary steering system. The secondary steering back-up reliefvalve protects the secondary steering system if the relief valve on the secondary steering pumpmalfunctions.

The lower relief valve (4) is a back-up relief valve for the primary steering system. Theprimary steering back-up relief valve protects the primary steering system if the high pressurecutoff valve on the steering pump malfunctions. Primary steering pressure is first controlled bythe high pressure cutoff valve located on the steering pump.

Check valves are used to separate the primary and secondary steering systems. The secondarycheck valve (5) is behind the left plug, and the primary check valve (6) is behind the right plug.

Steering system pressures can be measured at the steering system pressure tap (7).


This illustration shows the location of the HMU (arrow) for the 777F. Serviceability hasimproved for the HMU on the 777F due to the redesigned walkways. The HMU is connectedto the steering wheel and controlled by the operator.

The HMU meters the amount of oil sent to the steering cylinders by the speed at which thesteering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steeringcylinders, and the faster the wheels will change direction.

The steering system is referred to as "Q-amp" which means flow amplification. During asudden steering change (steering wheel speed greater than 10 rpm), additional steering pump oilflow will bypass the gerotor pump in the HMU and flow directly to the steering cylinders.Steering oil flow to the cylinders is equal to the gerotor pump oil flow plus the bypass oil flowfrom the steering pump. The steering oil flow is amplified up to 1.6 to 1. The purpose of theflow amplification is to provide quick steering response when sudden steering changes areneeded.

Two crossover relief valves are installed in the top of the HMU. The crossover relief valves areinstalled in series with the left and right turn ports. If an outside force is applied to the frontwheels while the steering wheel is stationary, the crossover relief valves provide circuitprotection for the steering lines between the steering cylinders and the HMU. The crossoverrelief valves allow oil to transfer from one end of the steering cylinders to the opposite end ofthe cylinders.

48


To test the right crossover relief valve, install two tees with pressure taps in the right turnsteering hose at the steering cylinders. Steer the truck completely to the right against the stops,and shut off the engine. An external pump supply must be connected to one of the pressuretaps on the right turn hose. Connect a pressure gauge to the other pressure tap on the right turnhose. Pressurize the steering system, and the reading on the gauge will be the setting of theright crossover relief valve.

To test the left crossover relief valve, install two tees with pressure taps in the left turn steeringhose at the steering cylinders. Steer the truck completely to the left against the stops, and shutoff the engine. An external pump supply must be connected to one of the pressure taps on theleft turn hose. Connect a pressure gauge to the other pressure tap on the left turn hose.Pressurize the steering system, and the reading on the gauge will be the setting of the leftcrossover relief valve.


The electric secondary steering pump (1) and motor (2) on the 777F are the same as the 777D,however the location has changed. The pump and motor are now located on the front of thefront crossmember. The pump and motor assembly also includes the brake release pumpsection (3) and the prelubrication (QuickEvac) pump section (4).

The secondary pressure switch (5) is also mounted next to the secondary steering pump. Thepressure switch detects if the wheels are being turned via the steering wheel when secondarysteering is activated. When the wheel is turned in a secondary steering condition, the pressureswitch will signal the Transmission/Chassis ECM and the QuickEvac function will be disabled.

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If the primary steering pressure switch signals the Transmission/Chassis ECM that the steeringsystem pressure is low, the ECM will energize the secondary steering relay located behind thecab. The secondary steering relay will then energize a second larger relay located on the frameabove the steering valve, which will then energize the secondary steering motor.

The primary relief valve for the secondary steering is accessible through the small allen headplug (6). To check the setting of the secondary steering primary relief valve, do not start thetruck. Turn ON the key start switch and depress the secondary steering switch in the cab. Turnthe steering wheel hard to the left or right while the secondary steering pump is running.Secondary steering system pressures can be measured at the steering system pressure tap.


51

Shown is a schematic of the steering hydraulic system in the HOLD position. The primarysteering pump pulls oil from the steering tank. All piston-type pumps produce a small amountof leakage to the case drain circuit for lubrication and cooling. The case drain oil flows to thesteering tank through a case drain filter.

Steering oil flows from the pump to the steering disable valve. When the steering disable valveis energized, oil is allowed to flow to the steering valve.

In the steering valve, a steering pressure switch monitors the output of the steering pump. Thesteering pressure switch cannot tolerate high steering system pressures. A pressure reducingvalve lowers the steering system pressure to the steering pressure switch.

If the steering pressure switch signals the Transmission/Chassis ECM that the steering systempressure is low, the ECM will then energize the secondary steering motor. Secondary steeringsupply oil will flow to the steering valve.


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Two relief valves are installed in the steering valve. The secondary steering back-up reliefvalve protects the secondary steering system if the relief valve on the secondary steering pumpmalfunctions. The primary steering back-up relief valve protects the primary steering system ifthe high pressure cutoff valve on the steering pump malfunctions.

Two check valves are located on the steering valve. The check valves are used to separate theprimary and secondary steering systems.

Steering supply oil flows to the HMU from the steering valve. Return oil from the HMU flowsthrough the steering valve and the steering filter to the steering tank.

The HMU meters the amount of oil sent to the steering cylinders by the speed at which thesteering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steeringcylinders, and the faster the wheels will change direction.

Two crossover relief valves are installed in the top of the HMU. The crossover relief valves areinstalled in series with the left and right turn ports. If an outside force is applied to the frontwheels while the steering wheel is stationary, the crossover relief valves provide circuitprotection for the steering lines between the steering cylinders and the HMU. The crossoverrelief valves allow oil to transfer from one end of the steering cylinders to the opposite end ofthe cylinders.

When the Transmission/Chassis ECM energizes the secondary steering motor, load sensingsignal oil will flow from the secondary steering load sensing valve through the load sensingresolver to the HMU. The load sensing valve uses the load sensing signal pressure to controlthe amount of flow from the secondary steering pump to the steering valve.

The 777F Trucks use a dynamic load sensing steering system the same as the late model "D Series" Trucks. In a dynamic system, there is load sensing pressure and flow between theHMU and the steering pumps.

A load sensing pilot signal resolver valve is located in the steering disable valve. The resolvervalve allows load sensing signal oil to flow between the HMU and the primary steering pumpor the secondary steering pump. In the NO STEER position, oil flows to the HMU. In a LEFTor RIGHT STEER position, oil also flows from the HMU to the resolver valve.

Normally, the secondary steering pump is OFF and the resolver is closed from the HMU to thesecondary steering pump. The flow from the primary steering pump holds the resolver openand load sensing pilot signal pressure is present between the HMU and the piston pump flowcompensator.

The load sensing signal flow from the primary steering pump is also used for "thermal bleed"through the HMU. The "thermal bleed" is used to keep the HMU temperature the same as therest of the steering system. Keeping the HMU the same temperature prevents sticking.


52

HOIST SYSTEM

The hoist system on the 777F Update trucks is electronically controlled by theTransmission/Chassis ECM. The hoist control system operates similar to the 777D trucks.

The main components in the hoist system are:

- Hoist control lever and position sensor (in cab)

- Hoist pump (1)

- Hoist control valve (2)

- Hoist cylinders (3)

- Hydraulic oil tank (4)


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The operator controls the hoist lever (arrow). The four positions of the hoist lever are RAISE,HOLD, FLOAT and LOWER. The hoist valve has a fifth position referred to as the SNUBposition. The operator is unaware of the SNUB position because a corresponding lever positionis not provided. When the body is being lowered, just before the body contacts the frame, theTransmission/Chassis ECM signals the hoist lower solenoid to move the hoist valve spool to theSNUB position. In the SNUB position, the body float speed is reduced to prevent the bodyfrom making hard contact with the frame.

The hoist system can be enabled or disabled using ET. All trucks shipped from the factorywithout bodies installed are set at the Hoist Enable Status 2. The Hoist Enable Status 2 is a testmode only and will prevent the hoist cylinders from accidentally being activated. After thebody is installed, change the Hoist Enable Status to 1 for the hoist system to function properly.

The truck should normally be operated with the hoist lever in the FLOAT position. Travelingwith the hoist in the FLOAT position will make sure the weight of the body is on the frame andbody pads and not on the hoist cylinders. The hoist control valve will actually be in the SNUBposition.

If the transmission is in REVERSE when the body is being raised, the hoist lever sensor is usedto shift the transmission to NEUTRAL. The transmission will remain in NEUTRAL until thehoist lever is moved into the HOLD or FLOAT position and the shift lever has been cycled intoand out of NEUTRAL.

NOTE: If the truck is started with the body raised and the hoist lever in FLOAT, thelever must be moved into HOLD and then FLOAT before the body will lower.

53


The hoist lever (1) controls a position sensor (2). The PWM sensor sends duty cycle inputsignals to the Transmission/Chassis ECM. The hoist lever position sensor is a Hall-Effectposition sensor and operates the same as the transmission shift lever sensor (3) previouslydescribed. Depending on the position of the sensor and the corresponding duty cycle, one ofthe two solenoids located on the hoist valve is energized.

The four positions of the hoist lever are RAISE, HOLD, FLOAT, and LOWER, but since thesensor provides a duty cycle signal that changes for all positions of the hoist lever, the operatorcan modulate the speed of the hoist cylinders.

The hoist lever sensor performs three functions:

- Raises and lowers the body

- Neutralizes the transmission in REVERSE

- Starts a new TPMS cycle

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Shown is the hoist, converter, and brake hydraulic tank. The oil level is checked by openingthe small door (1) and looking at the sight gauge. The oil level should first be checked withcold oil and the engine stopped. The level should again be checked with warm oil and theengine running.

The lower sight gauge (2) can be used to fill the tank when the hoist cylinders are in theRAISED position. When the hoist cylinders are lowered, the hydraulic oil level will increase.After the hoist cylinders are lowered, check the hydraulic tank oil level with the upper sightgauge as explained above.

Check the hoist, converter, and brake hydraulic tank breather (3) for restriction. Clean the filterif it is restricted.

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Shown is the rear of the hoist, converter, and brake hydraulic tank. The hoist pump pulls oilfrom the tank through the suction screen (1) located in the rear of the tank. Oil returns from thehoist valve through the port (2).

Brake cooling oil returns to the hydraulic tank through the three upper ports (3).

Other ports located on the hydraulic tank are:

- Transmission charging pump suction (4)

- Transmission return (5)

- Torque converter pump suction (6)

- Brake cooling pump suction (7)

- Torque converter inlet relief valve return (8)

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The hoist pump (1) is a gear type pump that is attached to the drive gear at the rear of theengine. Mounted to the hoist pump is the brake cooling pump and the brake charging pump.Oil flows from the hoist pump to the hoist control valve.

The hoist system relief pressures are different in the RAISE and LOWER positions.

The body up switch must be in the RAISE position before the LOWER relief valve setting canbe tested. Move a magnet past the body up switch until the body up alert indicator on the dashturns ON. If the body up switch is in the LOWER position, the Transmission/Chassis ECMwill hold the hoist valve in the SNUB position and the LOWER relief valve will not open.

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In the HOLD, FLOAT and SNUB positions, the gauge will show the brake cooling systempressure, which is a result of the restriction in the coolers, brakes and hoses (normally muchlower than the actual oil cooler relief valve setting). The maximum pressure is limited by theoil cooler relief valve.

Hoist pump pressure can be checked at the pressure tap (2) on the pump.


59

The hoist control valve (1) is located behind the engine on the right side of the frame. Thehoist valve is the same as the hoist control valve on the 777D.

The hoist valve uses torque converter lockup clutch pump oil as the pilot oil to shift thedirectional spool inside the hoist valve. Lockup clutch pump oil enters the hydraulic actuators (2) on both ends of the hoist valve.


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Pilot oil pressure is always present at both ends of the directional spool. Two solenoid valvesare used to drain the pilot oil from the ends of the directional spool, which then allows thespool to move. The solenoid on the right is the RAISE solenoid valve (1), and the solenoid onthe left is the LOWER solenoid valve (2).

The left pressure tap (3) is used to check the pilot pressure of the hoist lower solenoid. Theright pressure tap (4) is used to check the pilot pressure of the raise solenoid.

When the Transmission/Chassis ECM receives an input signal from the hoist lever sensor, theECM sends an output signal current between 0 and 1.9 amps to one of the solenoids. Theamount of current sent to the solenoid determines how much pilot oil is drained from the end ofthe directional spool and, therefore, how far the directional spool travels toward the solenoid.

An oil cooler relief valve is located in the hoist control valve behind the large plug (5). Therelief valve limits the brake oil cooling pressure when the hoist valve is in the HOLD, FLOATor SNUB position.

The hoist system relief pressures are controlled by the two relief valves located on top of thehoist valve. The RAISE relief valve (6) limits the pressure in the hoist system during RAISE.The LOWER relief valve (7) limits the pressure in the hoist system during LOWER.

NOTE: The hoist valve LOWER position (snub adjustment) is an adjustable parameterin the Transmission/Chassis ECM using Cat ET. The slight adjustment provides ameans to compensate for valve differences. This is the snub adjustment.

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The hoist cylinder lower circuit pressure tap (1) and raise circuit pressure tap (2) are located onthe cross-tube between the lower hoist cylinder mounts.

The relief valve pressure setting is tested with the engine at HIGH IDLE and the hoist valve inthe RAISE or LOWER position.

The body up switch at the frame near the body pivot pin must be in the RAISE position beforethe LOWER relief valve setting can be tested. Move a magnet past the body up switch until thebody up alert indicator on the dash turns ON. If the body up switch is in the LOWER position,the Transmission/Chassis ECM will hold the hoist valve in the SNUB position and the LOWERrelief valve will not open.

An orifice plate is installed between the upper hose and the rod end port on both hoistcylinders. The orifice plate restricts the flow of oil from the rod end of the hoist cylinders.

The orifice plate also prevents cavitation of the cylinders when the body raises faster than thepump can supply oil to the cylinders (caused by a sudden shift of the load).

NOTE: If the snub feature is not adjusted correctly, residual pressure will exist in thehead side of the cylinders and the body will not rest on the frame. The raise circuitpressure tap should be used to ensure there is no residual pressure in the head side ofthe cylinders.

Otherwise, when checking the raise (high) circuit pressure, the pressure tap on the hoist pumpis easier to access.

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This illustration shows a sectional view of the hoist control valve in the HOLD position. Pilotoil pressure is present at both ends of the directional spool. The spool is held in the centeredposition by the centering springs and the pilot oil. Passages in the directional spool vent thedual stage relief valve signal stem to the tank. All the hoist pump oil flows through the brakeoil coolers to the rear brakes.

The position of the directional spool blocks the oil in the head end and rod end of the hoistcylinders.

A gauge connected to a pressure tap at the pump while the hoist valve is in the HOLD positionwill show the brake cooling system pressure, which is a result of the restriction in the coolers,brakes and hoses. The maximum pressure in the circuit should correspond to the setting of thebrake oil cooler relief valve.


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In the RAISE position, the raise solenoid is ENERGIZED and drains pilot oil from the upperend of the directional spool. The directional spool moves up. Pump oil flows past the loadcheck valve and the directional spool to the head end of the hoist cylinders.

When the directional spool is initially shifted, the load check valve remains closed until thesupply pressure is higher than the pressure in the hoist cylinders. The load check valveprevents the body from dropping before the RAISE pressure increases.

The directional spool also sends hoist cylinder raise pressure to the dual stage relief valvesignal stem. The dual stage relief valve signal stem moves down and blocks the supplypressure from opening the low pressure relief valve.

Oil flowing from the rod end of the hoist cylinders flows freely through the brake oil cooler tothe brakes.

If the pressure in the head end of the hoist cylinders exceeds the relief valve settings, the highpressure relief valve will open. When the high pressure relief valve opens, the dump valvemoves to the left and pump oil flows to the tank.


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The high pressure hoist relief valve setting is checked at the hoist pump pressure tap or thehead end pressure tap. Check the relief pressure with the hoist lever in the RAISE position andthe engine at HIGH IDLE.


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In the LOWER (power down) position, the LOWER solenoid is energized and drains pilot oilfrom the lower end of the directional spool. The directional spool moves down.

Supply oil from the pump flows past the load check valve and the directional spool to the rodend of the hoist cylinders. Oil in the head end of the hoist cylinders flows to the tank throughholes in the directional spool. The supply oil in the rod end of the cylinders and the weight ofthe body move the cylinders to their retracted positions.

Just before the body contacts the frame, the body up switch sends a signal to theTransmission/Chassis ECM to move the directional spool to the SNUB position. In the SNUBposition, the directional spool moves slightly to restrict the flow of head end oil through onlysome of the holes in the spool which allows the body to lower gradually.

The directional spool also vents the passage to the dual stage relief valve signal stem. The dualstage relief valve signal stem allows supply pressure to be limited by the low pressure reliefvalve.

If the pressure in the rod end of the hoist cylinders is too high, the low pressure relief valve willopen. When the low pressure relief valve opens, the dump valve moves to the left and pump oilflows to the tank.


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The low pressure hoist relief valve setting is checked at the rod end pressure tap. Check therelief pressures with the hoist lever in the LOWER position and the engine at HIGH IDLE.

The body up switch must be in the RAISE position before the LOWER relief valve setting canbe tested. Move a magnet past the body up switch until the body up alert indicator on the dashturns ON. If the body up switch is in the LOWER position, the Transmission/Chassis ECMwill hold the hoist valve in the SNUB position and the LOWER relief valve will not open.


65

In the FLOAT position, the LOWER solenoid is partially energized and drains some of the pilotoil at the lower end of the directional spool to the tank. The directional spool moves down.Because the pilot oil is only partially drained, the directional spool does not move down as faras during LOWER (power down).

Pump supply oil flows past the load check valve and the directional spool to the rod end of thehoist cylinders. Oil in the head end of the hoist cylinders flows to the tank. The position of thedirectional spool permits the pressure of the oil flowing to the brake oil cooler to be felt at therod end of the hoist cylinders.

The truck should normally be operated with the hoist lever in the FLOAT position. Travelingwith the hoist in the FLOAT position will make sure the weight of the body is on the frame andbody pads and not on the hoist cylinders. The hoist valve will actually be in the SNUBposition.


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In the SNUB position as the body is lowered, just before the body contacts the frame, the bodyup switch sends a signal to the Transmission/Chassis ECM to move the directional spool to theSNUB position. In the SNUB position, the directional spool moves slightly to a positionbetween HOLD and FLOAT. The SNUB position restricts the flow of oil and lowers the bodygradually.

The operator does not control the SNUB position. When the hoist lever is in the LOWER orFLOAT position and the body up switch is in the DOWN position, the hoist control valve is inthe SNUB position.

A gauge connected to the rod end pressure tap while the hoist control valve is in the SNUBposition will show the brake cooling system pressure, which is a result of the restriction in thecoolers, brakes and hoses. The maximum pressure in the circuit should correspond to thesetting of the brake oil cooler relief valve.


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Two-stage hoist cylinders (1) are used to raise the body. Oil flows from the hoist control valveto the two hoist cylinders when the directional spool in the hoist control valve is not in HOLD.

Check the condition of the body pads (2) for wear or damage.

Hoist pilot pressure is required to lower the body with a dead engine. The towing pump can beused to provide the hoist pilot oil.


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68

This illustration shows the hoist hydraulic system in the HOLD position. The hoist pump pullsoil from the hydraulic tank through the suction screen located in the rear of the tank. Oil flowsfrom the hoist pump to the hoist control valve.

When the hoist control valve is in the HOLD, FLOAT or SNUB position, all the hoist pump oilflows through the brake oil coolers located on the right side of the engine. Oil flows from theoil coolers, through the brakes, and returns to the hydraulic tank.

NOTE: If the truck is equipped with the optional caliper type front brake system, thebrake cooling pump is not installed and oil from the hoist pump will flow to only therear brakes.

A brake cooling relief valve is located in the hoist control valve. The relief valve limits thebrake oil cooling pressure when the hoist control valve is in the HOLD, FLOAT or SNUBposition.

The hoist valve uses torque converter lockup clutch pump oil as the pilot oil to shift thedirectional spool inside the hoist control valve. Oil flows from the lockup clutch pump to bothends of the hoist control valve.


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Pilot pressure is always present at both ends of the directional spool. Two solenoid valves areused to drain the pilot oil from the ends of the directional spool, which then allows thecentering springs and the pressure on the opposite end of the spool to move the spool. Whenthe RAISE solenoid is energized, the directional spool will move toward the RAISE solenoid.

The RAISE and LOWER solenoid valves constantly receive approximately 300 millivolts at afrequency of 80 Hz from the Transmission/Chassis ECM when they are in any position exceptHOLD. The excitation, referred to as "dither," is used to keep the solenoids in a ready state forquick response.

When the Transmission/Chassis ECM receives an input signal from the hoist lever sensor, theECM sends an output signal current between 0 and 1.9 amps to one of the solenoids. Theamount of current sent to the solenoid determines how much pilot oil is drained from the end ofthe directional spool and, therefore, the distance that the directional spool travels.

The truck should normally be operated with the hoist lever in the FLOAT position. Travelingwith the hoist in the FLOAT position will make sure the weight of the body is on the frame andbody pads and not on the hoist cylinders. The hoist valve will actually be in the SNUBposition.

When the hoist control valve is in the RAISE position, pump supply oil flows to the head endof the hoist cylinders. Pump supply oil also flows to the dual stage signal spool and moves thespool to the left. When the dual stage signal spool moves to the left, pump supply oil isblocked from the LOWER relief valve, and the RAISE relief valve will limit the hoist systempressure.

When the hoist control valve is in the LOWER (power down), FLOAT or SNUB position,pump supply oil flows to the rod end of the hoist cylinders. Pump supply oil is blocked fromthe dual stage signal spool and the spring holds the spool in the right position. When the dualstage signal spool is in the right position, pump supply oil can flow to the LOWER relief valve,and hoist system pressure is controlled by the LOWER relief valve.

An orifice plate is installed between the upper hose and the rod end port on both hoistcylinders. The orifice plate prevents cavitation of the cylinders when the body raises fasterthan the pump can supply oil to the cylinders (caused by a sudden shift of the load).


69

BRAKE SYSTEM

Two separate brake systems are used on the 777F. The two brake systems are theservice/retarder brake system and the parking/secondary brake system. The parking/secondarybrakes are spring engaged and hydraulically released. The service/retarder brakes arehydraulically engaged and spring released.

The braking system is also equipped with a Brake ECM that controls the braking systemfunctions, including the Automatic Retarder Control (ARC) and the Traction Control System (TCS).

The air system on the previous model trucks has been completely removed.

The main components in the braking system are:

- Brake charging pump (1)

- Brake cooling pump (standard oilcooled front brakes) (2)

- Accumulator charging valve (3)

- Brake accumulators (4)


- Cab brake manifold (5)

- Service brake valve (6)

- Brake oil filter (7)

- Front slack adjuster (8)

- Brake accumulator check valve (9)

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The rear brakes on the 777F Trucks are oil cooled. Shown is a cutaway illustration of an oilcooled brake assembly. The brakes are environmentally sealed and adjustment free. Oilcontinually flows through the brake discs for cooling. Duo-Cone seals prevent the cooling oilfrom leaking to the ground or transferring into the axle housing. The wheel bearing adjustmentmust be maintained to keep the Duo-Cone seals from leaking.

The smaller piston (yellow) is used to engage the secondary and parking brakes. The parkingbrakes are spring engaged and hydraulically released.

The larger piston (purple) is used to engage the service and retarder brakes. The service andretarder brakes are engaged hydraulically and released by spring force.

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The standard oil cooled front brakes are also environmentally sealed and adjustment free. Thepiston (yellow) is used to ENGAGE the service/retarder brakes. The front brakes do not have asecond piston for the parking/secondary brakes.

When the wheel is removed for service, the small plug at the lower left must be removed (thebrake assembly is equipped with two similar plugs). Two 3/8 inch bolts must be installed at theplug locations to hold the brake discs and plates in position during wheel removal. The boltsensure proper alignment of the teeth on the discs and plates during installation.

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With the optional disc and caliper design brakes, the brake caliper assemblies are fastened tothe spindle and do not rotate. The brake disc is fastened to the wheel and rotates with thewheel. Air can be bled from the front brakes through the bleed valves.

During brake application, hydraulic oil from the brake cylinders forces the brake pistons againstthe brake carrier linings (brake pads). The brake linings are forced against the disc to stop therotation of the wheel.

72


The brake charging pump (1), the brake oil cooling pump (2), and the hoist pump (3) aremounted to the pump drive gear on the left rear side of the engine. The 777F brake systemaccumulators are charged by the brake charging pump, which supplies oil to the accumulatorcharging valve. The oil cooling pump sends oil to the oil coolers before the oil flows to thefront and rear brakes for brake cooling.

NOTE: The brake oil cooling pump is not installed on trucks with the optional calipertype front brakes.

73

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The brake system filter (1) is located on the left outer rear frame next to the left rear strutmount. The brake filter includes a filter bypass switch (2), which sends a signal to the BrakeECM if the filter is restricted. The Brake ECM sends a signal to the monitoring system, whichilluminates the brake system-check indicator lamp.

75


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The accumulator charging valve (1) is located on the left side of the frame near the brakeaccumulators. The accumulator charging valve directs oil to the brake accumulators, brake oilcoolers, and the tank. Once the accumulators are charged, the excess oil flow is sent to cool thebrakes before returning to the tank.

The Brake ECM monitors the pressure in the service brake accumulators with the brakeaccumulator pressure switch (2). If the pressure in the service brake accumulators is low, theBrake ECM will signal the monitoring system to turn on the brake system-check indicatorlamp. A relief (3) valve limits the pressure in the brake charging circuit.

A pressure tap (4) on the line between the brake charging pump and the accumulator chargingvalve is used to check the charge oil pressure from the pump. The pressure tap (5) on thecharging valve is used to check the oil pressure in the service brake accumulators.

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The accumulator charging valve maintains the pressure in the accumulators at a constant ratewhile the engine is running. If the machine has lost power or the hydraulic pump has failed, thepressure in the accumulators will permit several applications of the service brakes.

This illustration shows the accumulator charging valve in the CUT-IN position. When theaccumulator oil pressure decreases below a certain point, the accumulator charging valvereaches the cut-in pressure setting. The pressure decrease allows spring force to move the cut-in/cut-out spool to the left and oil flows to the right end of the unloading valve. The orificein the unloading valve restricts the pump flow to the brake cooling system. Oil flow to thebrake accumulators increases and the accumulators are charged.

The accumulator oil pressure switch sends a signal to the Brake ECM to alert the operator whenthe brake oil pressure drops below the minimum operating pressure.


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This illustration shows the accumulator charging valve in the CUT-OUT position. When theaccumulator oil pressure increases to the cut-out pressure setting, the increased pressure causesthe cut-in/cut-out spool to move right against spring force. Oil at the right end of the unloadingvalve flows to the tank. Oil pressure on the left end of the unloading spool overcomes thedecreased oil pressure on the right end of the spool and spring force. Most of the brakecharging pump oil now flows to the brake cooling system.

The check valve prevents high accumulator oil pressure from flowing to the brake coolingsystem.

The accumulator charging valve remains in the CUT-OUT position until the pressure in theaccumulators decreases to the cut-in pressure setting.

The pressure relief valve regulates the oil pressure in the brake circuit. Any excess oil that isnot required by the brake cooling system or the brake circuit is diverted back to the hydraulicoil tank.


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There are the three brake accumulators for the 777F located on the left side of the frame. Theservice brake accumulators (1) and parking brake accumulator (2) are charged by the brakecharging pump and supply the required oil flow to engage the front and rear service brakes andrelease the rear parking brakes.

A check valve in the circuit between the parking brake accumulator and the service brakeaccumulators allows only the parking brake accumulator to be charged when using the electricbrake release pump.

80

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The cab brake manifold (1) is mounted below the cab on the left upper frame. The cab brakemanifold contains the ARC control solenoid (2) and the front service brake solenoid (3).

The ARC control solenoid is part of the ARC system. The ARC system uses the rear servicebrakes and the front oil cooled brakes to automatically control the speed of the truck.

The service brake pressure switch (4) is located near the cab brake manifold toward the front ofthe machine. The service brake pressure switch sends a signal to the Brake ECM when theservice brakes are engaged. The Brake ECM will use the signal from the pressure switch toenergize the stop lamp relay (located in cab) and turn on the brake lights. In a low pressuresituation, the Brake ECM will signal the monitoring system to activate the brake system-checkindicator.

82

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The service brake valve (1) is mounted below the floor of the operator’s cab. When the servicebrake pedal (2) is depressed, an internal spool directs oil flow from the service brakeaccumulators to the rear service brakes.

The amount of oil flow to the front service brakes is determined by the Brake ECM based on asignal from the service brake pedal position sensor (3). The Brake ECM allows some oil fromthe brake accumulators to flow to the front brakes by controlling the position of the front brakesolenoid located in the cab brake manifold.

NOTE: If the front brake switch (optional front caliper type brakes only) is activated,the Brake ECM will command all oil to flow to the rear brakes.

84

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When the manual retarder lever (1) is activated, a PWM signal is sent to the Brake ECM. TheBrake ECM sends a signal to the ARC solenoid and the front brake solenoid. The solenoidscontrol the amount of oil flow to the service brakes based on the position of the retarder lever.

If the ARC switch (2) is activated, the Brake ECM sends a signal to the ARC solenoid and thefront brake solenoid. The solenoids control the amount of oil flow to the service brakes basedupon the input signals that the Brake ECM receives from the engine speed sensor.

NOTE: If the truck is equipped with the optional front caliper type brakes, the BrakeECM will command all oil to flow to the rear brakes when the retarder lever is movedor the ARC switch is activated.

The optional engine brake switch (3) is also an input to the Brake ECM. The Brake ECMcommunicates the status of the brake switch to the Engine ECM via the Cat Data Link. TheEngine ECM controls the compression brake application (if equipped).

The front brake switch (4) is installed on machines with caliper type front brakes. Whenactivated, the front brake switch sends a signal to the Brake ECM which allows the front brakesto be engaged when the brake pedal is depressed. When the front brake switch is in the OFFposition, only the rear brakes will be engaged when the brake pedal is depressed.

86


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The 777F has two slack adjusters. The top illustration shows the rear slack adjuster (1). Therear slack adjuster is located above the rear differential. The bottom illustration shows the frontslack adjuster (2). The front slack adjuster is located on the left strut frame support.

The slack adjusters compensate for brake disc wear by allowing a small volume of oil to flowthrough the slack adjuster and remain between the slack adjuster and the brake piston underlow pressure. The slack adjusters maintain a slight pressure on the brake piston at all times.

Brake cooling oil pressure maintains a small clearance between the brake discs.

The service brake oil pressure can be tested at the taps (3) located on top of the slack adjusters.

87

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This illustration shows sectional views of the slack adjuster when the brakes are RELEASEDand ENGAGED.

When the brakes are ENGAGED, oil from the brake cylinder enters the slack adjuster and thetwo large pistons move outward. Each large piston supplies oil to one wheel brake. The largepistons pressurize the oil to the service brake pistons and ENGAGE the brakes.

Normally, the service brakes are FULLY ENGAGED before the large pistons in the slackadjuster reach the end of their stroke. As the brake discs wear, the service brake piston willtravel farther to FULLY ENGAGE the brakes. When the service brake piston travels farther,the large piston in the slack adjuster moves farther out and contacts the end cover. The pressurein the slack adjuster increases until the small piston moves and allows makeup oil from thebrake cylinder to flow to the service brake piston.

When the brakes are RELEASED, the springs in the service brakes push the service brakepistons away from the brake discs. The oil from the service brake pistons pushes the largepistons in the slack adjuster to the center of the slack adjuster. Makeup oil that was used toENGAGE the brakes is replenished at the brake cylinder from the makeup tank.


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The spring behind the large piston causes some oil pressure to be felt on the service brakepiston when the brakes are RELEASED. Keeping some pressure on the brake piston providesrapid brake engagement with a minimum amount of brake cylinder piston travel.

The slack adjusters can be checked for correct operation by opening the service brake bleedscrew with the brakes RELEASED. A small amount of oil should flow from the bleed screwwhen the screw is opened. The small flow of oil verifies that the spring behind the large pistonin the slack adjuster is maintaining some pressure on the service brake piston.

A more accurate test for the slack adjuster is discussed on the next page.


The service brake bleed screw (1) is identified by an "S" on the brake anchor plate casting nextto the screw. The parking brake bleed screw (2) is identified by a "P" on the casting.

Another check to verify correct slack adjuster operation is to connect a gauge to the pressuretap on top of the slack adjuster and another gauge at the service brake bleed screw location onthe brake anchor plate casting.

With the service brake pedal depressed, the pressure reading on both gauges should beapproximately the same. When the brakes are RELEASED, the pressure at the slack adjustershould return to zero. The pressure at the service brake bleed screw location should return tothe residual pressure held on the brakes by the slack adjuster piston.

If the slack adjuster residual pressure is too low, it could indicate a failed slack adjuster. Highresidual pressure may indicate a failed slack adjuster or warped brake discs. To check forwarped brake discs, rotate the wheel to see if the pressure fluctuates. If the pressure fluctuateswhile rotating the wheel, the brake discs are probably warped and should be replaced.

To check for brake cooling oil leakage, block the brake cooling ports and pressurize each brakeassembly to a maximum of 138 kPa (20 psi). Close off the air supply source and observe thepressure trapped in the brake assembly for five minutes. The trapped pressure should notdecrease.

90


1

2

The parking brake valve (1) is located on the inside left frame rail behind the center crossmember. The parking valve receives oil flow from the parking brake accumulator. Containedwithin the valve is a parking brake solenoid valve (2) and a purge solenoid valve (3).

When the parking brake solenoid is energized by the Brake ECM, the parking brake valvedirects oil flow through the TCS valve to release the rear parking brakes. There are no parkingbrakes on the front wheels. When the transmission shift lever is moved to PARK a signal issent to the Brake ECM to engage the parking brakes. There is not a separate parking brakecontrol switch. The secondary brake pressure switch (4) sends a signal to inform theTransmission/Chassis ECM that the secondary/parking brake is engaged.

When the machine is shut down, the purge solenoid is energized by the Transmission/ChassisECM and the purge valve drains the brake accumulators to tank.

91

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The parking brake pressure can be checked at the left parking brake tap (1) and at the rightparking brake tap (2).

93


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The secondary brake pedal position sensor (arrow) is located on the back of the secondarybrake pedal. The position sensor sends a signal to the Brake ECM indicating the position of thesecondary brake pedal. The Brake ECM sends a signal to the parking brake solenoid whichcontrols the secondary brake application at the rear brakes.

94


The secondary steering/brake release/QuickEvac pump and motor are located on the front of thefront crossmember as previously shown. The brake retract pump section (arrow) provides oil torelease the parking brakes and hoist pilot oil for lowering the body on trucks with a deadengine.

95


The diverter (towing) valve (arrow) is located on the left hoist cylinder frame support. Thediverter valve is used to unlock the brakes for towing and must be manually shifted beforetowing.

Once the valve is shifted, oil flow from the electric secondary steering/brake retract pump isdirected to the parking brake valve to release the parking brake.

To release the parking brakes for service work or towing, the electric motor on the pump isenergized by the brake release switch located in the cab.

When the key start switch is turned ON, the secondary steering system is energized for threeseconds to check the system. Since the towing pump is driven by the same electric motor asthe secondary steering pump, the diverter valve allows the towing pump oil to flow directly tothe hydraulic tank during the secondary steering test.

To shift the diverter valve, loosen the two diverter valve clamp bolts and slide the plate and thespool to the left. After the spool is shifted, tighten the diverter valve clamp bolts. When theelectric motor is energized, supply oil can flow from the towing pump, through the divertervalve, to the parking brake valve.

The brake release pump is also used to provide pilot oil to lower the body when the engine isoff.

96


97

This schematic shows the oil flow through the brake cooling system on the 777F Trucks withstandard oil cooled front brakes. The brake cooling pump supplies oil to the brake coolers andthe front and rear brakes. The brake cooling system also receives oil from the followingcomponents:

- Hoist valve (in the HOLD, FLOAT, and SNUB positions)

- Accumulator charging valve

- Torque converter lockup clutch relief valve

- Torque converter outlet relief valve

The pressure in the brake cooling system is limited by a relief valve located in the hoist valve.The relief valve is usually needed only when the brake cooling oil is cold. When brake coolingoil is at operating temperature, the brake cooling oil pressure is usually much lower than thesetting of the oil cooling relief valve.

NOTE: On trucks equipped with the optional caliper type front brakes, the brakecooling system oil flows only to the rear brakes.


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The brake oil coolers (arrows) are located on the right side of the engine. Engine coolant fromthe water pump flows around the brake oil coolers and back to the cylinder block. The enginecoolant transfers the heat from the brake oil system to the engine coolant.

Oil from the brake cooling pump flows through screens (not shown) before flowing through thebrake oil coolers.

98

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The brake cooling pump (1) is a gear type pump that is attached to the drive gear at the rear ofthe engine. The brake cooling pump is located between the hoist pump (2) and the brakecharging pump (3). Oil flows from the brake cooling pump to the brake oil coolers.

NOTE: The brake oil cooling pump is not installed on trucks with the optional calipertype front brakes.

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Shown is the left rear brake housing. Brake cooling oil pressure can be tested at the two taps (arrows) located in the brake cooling oil tubes. One tap is located on the brake coolinginlet tube and another tap is located on the brake cooling outlet tube. The pressure measured atthe brake inlet tube (from the oil coolers) will always be higher than the pressure measured atthe brake outlet tube.

A brake oil temperature sensor is located in a brake oil cooling tube on the truck. The brake oiltemperature sensor sends a signal to the Brake ECM indicating brake oil temperature. TheBrake ECM will send a signal over the Cat Data Link, which informs the monitoring system todisplay the temperature on the brake temperature gauge.

The most common cause of high brake cooling oil temperature is operating the truck in a gearrange which is too high for the grade and not maintaining a high enough engine speed. Theengine speed should be maintained at approximately 1900 rpm during long downhill hauls.

Make sure the oil cooling relief valve is not stuck open. Also, make sure the pistons in theslack adjuster are not stuck and holding too much pressure on the brakes.

100


101

This schematic shows the major components of the brake system with the standard oil cooledfront brakes. The front slack adjuster is not included on the optional caliper type front brakesystem.

Oil is drawn from the hydraulic tank by the brake charging pump. Oil flows through the brakefilter to the accumulator charging valve. The accumulator charging valve directs supply oil tothe brake accumulators. The accumulator charging valve also controls the cut-in and cut-outpressure. When the accumulators are charged, the charging valve will direct excess pump flowto the brake cooling system.

The service brake accumulators provide oil flow through the cab manifold to the service brakecontrol valve. Oil flowing into the cab manifold also flows to the ARC control solenoid andfront brake solenoid. When the operator depresses the service brake pedal, the service brakecontrol valve directs pump flow to the rear service brakes to stop the truck.

The front brakes are only engaged when the Brake ECM energizes the front brake solenoid.With the standard oil cooled front brakes, the Brake ECM determines when to energize thefront brake solenoid when the service brake pedal is depressed. With the optional caliper typefront brakes, the Brake ECM will energize the front brake solenoid when the front brakelockout switch in the cab is activated.


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The Brake ECM controls the modulation of the ARC solenoid and front brake solenoid, whichcontrols truck braking when the ARC system is ON.

Oil from the parking brake accumulator flows to the parking brake valve and the towingdiverter valve. When the parking brake is activated, the supply oil for releasing the parkingbrakes is directed to the tank and the parking brakes are engaged by spring force. When theparking brake solenoid is energized (parking brake de-activated), the parking brake valvedirects oil to the TCS valve. The pressure reducing valves in the TCS valve direct oil to releasethe parking brakes.

The diverter valve, under normal operation, is closed and blocks the oil flow from the electricbrake retract pump. If the truck is to be towed with a dead engine, the diverter valve must beshifted manually. When manually shifted, the diverter valve directs oil flow from the electricbrake retract pump to the parking brake valve to release the rear brakes.


102

Brake Electronic Control System

The 777F Trucks are equipped with a Brake ECM for controlling the parking brake and frontservice brake applications, the ARC system, and the TCS. Two possible arrangements can beinstalled on a truck:

- ARC only

- ARC and TCS

The Brake ECM receives information from various input components such as the engine speedsensor, the service brake pedal position sensor, the ARC switch, and the wheel speed sensors.

Based on the input information, the Brake ECM controls the front service brake applicationwhen the service brake pedal is depressed or the front and rear service brake application whenthe ARC system is activated. The Brake ECM also controls when the parking brakes shouldengage for the TCS and parking brake application when the parking brake is manuallyactivated.


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Output components include the ARC solenoid, the front service brake solenoid, the TCSselector and proportional solenoids, and the parking brake solenoid.

The compression brake switch is also an input to the Brake ECM. When the compressionbrake switch is activated, the Brake ECM sends a signal over the Cat Data Link to the EngineECM. The Engine ECM controls the engine compression brake, which was covered earlier inthe presentation.


The Brake ECM (arrow) is located in the compartment at the rear of the cab. The Brake ECMperforms the brake control functions, and controls the ARC system and TCS.

The Brake ECM is an A4M1 module with two 70-pin connectors. The Brake ECMcommunicates with the Engine ECM, Transmission/Chassis ECM, and monitoring system overthe CAT Data Link and can communicate with some attachments over the CAN Datalink.

103


104

When the service brake pedal is depressed, the service brake valve directs oil from the servicebrake accumulators to the rear brakes and sends a PWM signal to the Brake ECM via theservice brake pedal position sensor. The Brake ECM then determines what signal to send to thefront service brake solenoid based on the following conditions:

1. If the truck is equipped with the standard oil cooled front brakes, the Brake ECMsignals the front service brake solenoid to direct oil from the service brake accumulatorsto the front and rear brakes.

2. If the truck is equipped with the optional caliper type front brakes, the Brake ECMreceives a signal from the front brake lockout switch in the cab. If the lockout switch isOFF, the Brake ECM signals the front service brake solenoid to direct oil from theservice brake accumulators to the front and rear brakes the same as the oil cooled frontbrakes.

NOTE: Oil flow to the front and rear brakes may not be proportional. When the pedalis initially depressed, more oil is directed to the rear brakes. As the pedal is depressedfarther more oil is sent to the front brakes in proportion to the rear until full front brakepressure is present at full pedal travel.


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3. If the truck is equipped with the optional caliper type front brakes, and the lockoutswitch is ON, the Brake ECM de-energizes the front service brake solenoid. Oil flow tothe front brakes is blocked and only the rear brakes are used to stop the truck.

The Brake ECM also de-energizes the ARC solenoid when the ARC switch in the cab is OFFand the manual retarder lever is in the NEUTRAL position. The manual retarder lever alsocontrols the service brake application using the front brake solenoid and the ARC solenoid.

When the retarder lever is moved, a PWM signal is sent to the Brake ECM. The Brake ECMthen determines what signal to send to the ARC solenoid and front service brake solenoid basedon the following conditions:

1. If the truck is equipped with the standard oil cooled front brakes, the Brake ECMsignals the ARC solenoid and the front service brake solenoid to divide the oil flowfrom the service brake accumulators between the front and rear brakes.

2. If the truck is equipped with the optional caliper type front brakes, the Brake ECM de-energizes the front service brake solenoid. Oil flow to the front brakes is blockedand only the rear brakes are used to stop the truck with the retarder lever.


105

Automatic Retarder Control (ARC)

The ARC system receives signals from several switches and sensors. The main inputs to theBrake ECM for the ARC system are the ARC switch and engine speed sensor. The Brake ECManalyzes the various input signals and sends output signals to the ARC solenoid and frontservice brake solenoid.

NOTE: If the truck is equipped with the optional front caliper type brakes, the BrakeECM will de-energize the front service brake solenoid when the ARC system isactivated.

The ARC system function is to modulate truck braking (retarding) when descending a longgrade to maintain a constant engine speed. The ARC system engages the rear service brakesand the front oil cooled service brakes. If the ARC switch is moved to the ON position, theARC system will be activated if the throttle pedal is not depressed and the parking/secondarybrakes are RELEASED. The ARC system is disabled when the throttle is depressed or whenthe parking/secondary brakes are ENGAGED.


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The ARC is set at the factory to maintain a constant engine speed of 1900 ± 50 rpm (enginespeed setting is programmable). When the ARC initially takes control of retarding, the enginespeed may oscillate out of the ± 50 rpm target, but the engine speed should stabilize within afew seconds.

For proper operation of the ARC system, the operator needs only to activate the control withthe ARC switch and select the correct gear for the grade, load, and ground conditions. TheARC system is designed to allow the transmission to upshift to the gear selected by the shiftlever. After the transmission shifts to the gear selected by the operator and the engine speedexceeds 1900 rpm, the ARC system will apply the retarder as needed to maintain a constantengine speed.

The ARC system also provides engine overspeed protection. If an unsafe engine speed isreached, the ARC will engage the brakes, even if the ARC switch is in the OFF position and thethrottle is depressed.

Trucks approaching an overspeed condition will sound a horn and activate a light at 2100 rpm.If the operator ignores the light and horn, the ARC will engage the retarder at 2180 rpm. If theengine speed continues to increase, the Transmission/Chassis ECM will either upshift (one gearonly above shift lever position) or unlock the torque converter (if the shift lever is in the topgear position) at 2300 rpm.

The ARC also provides service personnel with enhanced diagnostic capabilities through the useof onboard memory, which stores possible faults, solenoid cycle counts and other serviceinformation for retrieval at the time of service.


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Traction Control System (TCS)

The Traction Control System (TCS) uses the rear parking/secondary brakes (spring engaged andhydraulically released) to decrease the revolutions of a spinning wheel. The TCS allows the tirewith better underfoot conditions to receive an increased amount of torque. The system iscontrolled by the Brake ECM and operates the same as the 777D TCS.

The Brake ECM monitors the drive wheels through three input signals: one at each drive axle,and one at the transmission output shaft. When a spinning drive wheel is detected, the BrakeECM sends a signal to the selector and proportional valves which ENGAGE the brake of theaffected wheel. When the condition has improved and the ratio between the right and left axlesreturns to 1:1, the Brake ECM sends a signal to RELEASE the brake.


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The service brake pressure switch provides an input signal to the Brake ECM from theTransmission/Chassis ECM through the CAT Data Link and performs two functions:

1. When the service brakes or retarder are ENGAGED, the TCS function is stopped.

2. The service brake pressure switch provides the input signal needed to perform adiagnostic test. When the TCS test switch and the retarder lever are ENGAGEDsimultaneously, the TCS will engage each rear brake independently. Install twopressure gauges on the TCS valve, and observe the pressure readings during the testcycle. The left brake pressure will decrease and increase. After a short pause, the rightbrake pressure will decrease and increase. The test will repeat as long as the TCS testswitch and the retarder lever are ENGAGED.

The TCS valve has left and right brake release pressure taps. Cat ET can also be used to viewthe left and right parking brake pressures during the test discussed above in function No. 2. When the proportional solenoid is ENERGIZED, Cat ET will show 44% when thebrake is FULLY ENGAGED.

NOTE: During the diagnostic test, the parking/secondary brakes must be released.


Shown is the right rear wheel speed sensor (arrow) looking toward the rear of the truck. TheTCS monitors the drive wheels through four input speed signals: one at each drive axle, andtwo at the transmission output shaft.

The transmission output speed sensors monitor the ground speed of the machine and provideinput signals to the Brake ECM through the CAT Data Link. The TCS uses the transmissionoutput speed sensors to disable the TCS when ground speed is above 19.3 km/h (12 mph).

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The TCS valve is mounted inside the left frame rail toward the rear of the machine. Twosolenoids are mounted on the valve.

Electrical signals from the Brake ECM cause the selector solenoid valve (1) to shift and selecteither the left or right parking brake. If the selector valve shifts to the left parking brakehydraulic circuit, the control oil is drained. The left reducing spool of the control valve can thenshift and engage the parking brake.

The proportional solenoid valve (2) controls the volume of oil being drained from the selectedparking brake control circuit. The rate of flow is controlled by a signal from the Brake ECM.

The pressure taps (3) can be used to test the left and right brake release pressures whenperforming diagnostic tests on the TCS. At HIGH IDLE, the pressure at the taps in the TCSvalve will be approximately 138 kPa (20 psi) less than the brake release pressure tested at thewheels.

The pressure taps are also used to provide parking brake dragging information to the servicetechnician. If the parking brakes are released, as sensed by the secondary brake pressure switchon the parking brake control valve, and parking brake pressure is below 3445 kPa (500 psi), aparking brake dragging event will be logged in the Brake ECM. The event can be viewed withCat ET.

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This illustration shows the TCS valve with the engine running and the brakes RELEASED.

With the engine running, oil flows from the brake charging pump to the parking brake valve.When the operator moves the transmission lever out of the PARK position, the Brake ECMenergizes the parking brake solenoid which directs oil flow to the TCS valve.

In the TCS valve, oil flows through a screen and orifices to the selector solenoid and the brakereducing valves. When the TCS is not activated, the oil is blocked at the selector solenoid. Oilpressure moves the brake reducing solenoids to the left and oil from the brake charging pump isdirected to the parking brakes. The parking brakes are RELEASED.


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This illustration shows the TCS valve with the engine running and the left brake ENGAGED.

When signals from the sensors indicate that the left wheel is spinning 60% faster than the rightwheel, the Brake ECM sends a signal to the selector solenoid valve and the proportionalsolenoid valve. The selector solenoid valve shifts up to open a passage between the right endof the left brake pressure reducing valve and the proportional solenoid valve.

The torque converter lockup pump oil provides signal oil to the drain ball check which allowsoil from the TCS valve to return to the tank.

The proportional solenoid valve opens a passage from the selector solenoid valve to drainthrough the drain ball check. The proportional solenoid valve also controls the rate at whichthe oil is allowed to drain. Control circuit oil drains through the selector valve and enters theproportional valve.


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The reducing valve spool for the left parking brake shifts and blocks oil flow to the parkingbrake. Oil in the left parking brake control circuit begins to drain and the left parking brakebegins to ENGAGE. The left brake orifice restricts the flow of oil from the parking brakevalve.

When the signals from the sensors indicate that the left wheel is no longer spinning, the BrakeECM stops sending signals to the selector solenoid and the proportional solenoid. The selectorsolenoid valve and proportional solenoid valve block the passage to drain and allow the controlcircuit pressure to increase.

The left brake reducing valve spool shifts to the left and blocks the passage to drain. Parkingbrake oil is directed to the left parking brake and the brake is RELEASED.


CONCLUSION

This presentation has provided a basic introduction to the Caterpillar 777F Off-highway Truck.All the major component locations were identified and the major systems were discussed.When used in conjunction with the service manual, the information in this package shouldpermit the technician to analyze problems in any of the major systems on these trucks.

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777f mg sistemas

Documents

electronic control system

converter hydraulic

retarder control system

power train components

ecpc transmission

highway truck power

power flows

main power traincomponents