halderman ch106 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

ABS COMPONENTS AND OPERATION

106

106 ABS COMPONENTS AND OPERATION



ObjectivesObjectives

• The student should be able to:– Prepare for the Brakes (A5) ASE

certification test content area “F” (Antilock Brake System Diagnosis and Repair).

– Explain the reason for ABS.




ObjectivesObjectives

• The student should be able to:– Describe the purpose and function of the

ABS components, such as wheel speed sensors, electrohydraulic unit, and electronic controller.

– Discuss how the ABS components control wheel slippage.




ANTILOCK BRAKING ANTILOCK BRAKING SYSTEMSYSTEM




Antilock Braking SystemAntilock Braking System

• Overview– Antilock braking systems (ABS) help

prevent the wheels from locking during sudden braking, especially on slippery surfaces

– ABS helps to eliminate lockup and minimize the danger of skidding





• Overview– ABS can improve braking when road

conditions are less than ideal– ABS monitors the relative speed of the

wheels to one another to modulate brake pressure as needed to control slippage and maintain traction when the brakes are applied





• ABS and Tire Traction– Traction is defined in terms of tire slip,

which is the difference between the actual speed and the rate at which the tire tread moves across the road





• ABS and Tire Traction– When the brakes are applied, the rotational

speed of the wheel drops, and tire slip increases, creating friction that converts kinetic energy into braking and cornering force




Figure 106-1 Maximum braking traction occurs when tire slip is between 10% and 20%. A rotating tire has 0% slip and a locked-up wheel has 100% slip.





• Tire Slip and Braking Distance– On dry or wet pavement:

• Maximum braking traction occurs when tire slip is held between approximately 15% and 30%






• On snow- or ice-covered pavement, the optimum slip range is 20% to 50%






• If tire slip increases beyond these levels, the amount of traction decreases






• A skidding tire with 100% slip provides 20% to 30% less braking traction on dry pavement






• The shortest stopping distances are obtained when the brakes are applied with just enough force to keep the tire slip in the range where traction is greatest





• Tire Slip and Braking Distance– NOTE: All antilock braking systems stop

controlling the wheel brakes when the vehicle speed drops to about 5 MPH (8 km/h). The wheel speed sensors are often not accurate that these low speeds and loss of vehicle control at this low speed is unlikely.




Figure 106-2 Traction is determined by pavement conditions and tire slip.





• Tire Slip and Vehicle Stability– When a vehicle is stopped in a straight line,

nearly all of the available traction can be used to provide braking force

– If a vehicle has to stop and turn at the same time, the available traction must be divided to provide both cornering (lateral) and braking force





• Tire Slip and Vehicle Stability– No tire can provide full cornering power

and full braking power at the same time





• Tire Slip and Vehicle Stability– When a brake is locked and the tire has

100% slip, all of the available traction is used for braking, and none is left for steering and the back end of the vehicle will tend to swing around toward the front causing a spin





• Tire Slip and Vehicle Stability– If the front brakes lock, steering control will

be lost and the vehicle will slide forward in a straight line until the brakes are released to again make traction available for steering





• ABS and Base Brakes– An antilock braking system is an “add-on”

to the existing base brake system





• ABS and Base Brakes– ABS only comes into play when traction

conditions are marginal or during sudden panic stops when the tires lose traction and begin to slip excessively





• NOTE: All vehicles sold in the United States for the 2012 model year and newer are required to be equipped with electronic stability system. These systems require that the vehicle be equipped with antilock braking system (ABS) as standard equipment.





• ABS and Base Brakes– A vehicle with ABS brakes uses the same

brake linings, calipers, wheel cylinders, and other system components as a vehicle without ABS brakes

– Should a failure occur that affects the operation of the ABS system, the system will deactivate itself and the vehicle will revert to normal braking





• ABS Limitations– There are two situations in which an

antilock brake system will not provide the shortest stopping distances

• The first involves straight stops made on smooth, dry pavement by an expert driver.

• For the average driver, or under less than ideal conditions, antilock brakes will almost always stop the vehicle in a shorter distance.







• The other situation is when braking on loose gravel or dirt, or in deep, fluffy snow.

• A locked wheel will stop the vehicle faster because loose debris builds up and forms a wedge in front of the tire that helps stop the vehicle.







• Some vehicles with antilock brakes have a switch on the instrument panel that allows the system to be deactivated when driving on these kinds of road surfaces.




Figure 106-3 A good driver can control tire slip more accurately than an ABS if the vehicle is traveling on a smooth, dry road surface.




Figure 106-4 A wedge of gravel or snow in the front of a locked wheel can help stop a vehicle faster than would occur if the wheel brakes were pulsed on and off by an antilock braking system.





• ABS and the Laws of Physics– No ABS can overcome the laws of physics– The weight and speed of a moving vehicle

give it a lot of kinetic energy, and only so much of that energy can be converted into braking or cornering force at any given time





• ABS and the Laws of Physics– A vehicle with four-wheel antilock brakes

will stop in very nearly the shortest possible distance, but this will still not prevent an accident if the brakes are applied too late to bring the vehicle to a complete stop before impact





• ABS and the Laws of Physics– If a vehicle enters a corner traveling faster

than it is physically possible to negotiate the turn, antilock brakes will not prevent the vehicle from leaving the road




Figure 106-5 Being able to steer and control the vehicle during rapid braking is one major advantage of an antilock braking system.




ABS OPERATIONABS OPERATION




ABS OperationABS Operation

• Wheel Speed Sensor Input– Wheel speed is monitored by one or more

wheel speed sensors– If one wheel starts to slow at a faster rate

than the others, or at a faster rate than that which is programmed in the antilock control module, it indicates a wheel is starting to slip and is in danger of losing traction and locking





• Wheel Speed Sensor Input– The ABS responds by momentarily reducing

hydraulic pressure to the brake on the affected wheel or wheels, allowing the wheel to regain traction





• Wheel Speed Sensor Input– As traction is regained, brake pressure is

reapplied to slow the wheel– The cycle is repeated over and over until

the vehicle stops or until the driver eases pressure on the brake pedal





• Control Valves– Electrically operated solenoid valves are

used to hold, release, and reapply hydraulic pressure to the brakes, producing a pulsating effect, which can be felt in the brake pedal during hard braking





• Control Valves– The rapid modulation of brake pressure in a

given brake circuit reduces the braking load on the affected wheel and allows it to regain traction





• Control Valves– Once the rate of deceleration for the

affected wheel catches up with the others, normal braking function and pressure resume, and antilock reverts to a passive mode




Figure 106-6 A typical stop on a slippery road surface without antilock brakes. Notice that the wheels stopped rotating and skidded until the vehicle finally came to a stop.




SYSTEM SYSTEM CONFIGURATIONSCONFIGURATIONS




System ConfigurationsSystem Configurations

• All ABS systems keep track of wheel deceleration rates with speed sensors

• ABS systems use a different number of sensors, depending on how the system is configured




Figure 106-7 ABS configuration includes four-channel, three-channel, and single-channel.





• Four-Channel ABS System– When each wheel is equipped with its own

speed sensor it is called a “four-wheel, four-channel” system since each wheel speed sensor provides input for a separate hydraulic control circuit or “channel” for that wheel





• Four-Channel ABS System– "Channel" refers to the number of separate

ABS hydraulic circuits, not the number of wheel speed sensor electrical circuits





• NOTE: For vehicle stability systems to function, there has to be four wheel speed sensors and four channels so the hydraulic control unit can pulse individual wheel brakes to help achieve vehicle stability.





• Three-Channel ABS System– Three-channel ABS systems have a

separate wheel speed sensor for each front wheel but use a common speed sensor for both rear wheels





• Three-Channel ABS System– The rear wheel speed sensor is mounted in

either the differential or the transmission and reads the combined or average speed of both rear wheels





• Three-Channel ABS System– Both rear wheels are controlled

simultaneously– This is known as the select low principle





• Three-Channel ABS System– Three-channel systems are the most

common type of ABS setup used on rear-wheel-drive applications





• Single-Channel ABS System– The single-channel rear-wheel-only ABS

system is used on many rear-wheel-drive pickups and vans





• Single-Channel ABS System– The front wheels have no speed sensors,

and only a single speed sensor mounted in the differential or transmission is used for both rear wheels





• Single-Channel ABS System– Rear-wheel antilock systems are typically

used on applications where vehicle loading can affect rear wheel traction





• Integral and Nonintegral– Integral systems combine the brake master

cylinder and ABS hydraulic modulator, electric pump, and accumulator into one assembly

– Integral systems do not have a vacuum booster for power assist





• Integral and Nonintegral– Nonintegral ABS systems, which are

sometimes referred to as “add-on” systems, have become the most common type of ABS system because of their lower cost and simplicity





• Integral and Nonintegral– Nonintegral ABS systems have a

conventional brake master cylinder and vacuum power booster with a separate hydraulic modulator unit

– Some also have an electric pump for ABS braking, but do not use the pumps for normal power assist




Figure 106-8 A typical integral ABS unit that combines the function of the master cylinder, brake booster, and antilock braking system in one assembly.




Figure 106-9 A typical nonintegral-type (remote) ABS.




ABS COMPONENTSABS COMPONENTS




ABS ComponentsABS Components

• Basic components that are common to all antilock brake systems include the following:– Wheel speed sensors– Electronic control unit





• Basic components that are common to all antilock brake systems include the following:– ABS warning lamp– Hydraulic modulator assembly with

electrically operated solenoid valves




Figure 106-10 A schematic drawing of a typical antilock braking system.





• Wheel Speed Sensor– Most wheel speed sensors, abbreviated

WSS, consist of a magnetic pickup and a toothed sensor ring





• Wheel Speed Sensor– The sensor may be mounted in the steering

knuckle, wheel hub, brake backing plate, transmission tailshaft, or differential housing

– On some applications, the sensor is an integral part of the wheel bearing and hub assembly





• Wheel Speed Sensor– The sensor rings may be mounted on the

axle hub behind the brake rotors, on the brake rotors or drums, on the outside of the outboard constant velocity joints on a front-wheel drive vehicle, on the transmission tailshaft, or inside the differential on the pinion gear shaft




Figure 106-11 Wheel speed sensors for the rear wheels may be located on the rear axle, on the transmission, or on the individual wheel knuckle.





• Wheel Speed Sensor– Sensor operation

• The sensor pickup has a magnetic core surrounded by coil windings






• As the wheel turns, teeth on the sensor ring move through the pickup’s magnetic field, which reverses its polarity and induces an alternating current (AC) voltage in the sensor windings






• The number of voltage pulses per second induced in the pickup changes frequency






• The frequency of the signal is proportional to wheel speed

• The higher the frequency, the faster the wheel is turning






• The signals are sent to the ABS control module, where the AC signal is converted into a digital signal






• The control module then monitors wheel speed by counting the pulses from each of the wheel speed sensors






• If the frequency signal from one wheel starts to change abruptly with respect to the others, it tells the module that wheel is starting to lose traction






• The module then applies antilock braking if needed to maintain traction




Figure 106-12 A schematic of a typical wheel speed sensor. The toothed ring is also called a tone ring.




Figure 106-13 Wheel speed sensors produce an alternating current (AC) signal with a frequency that varies in proportion to wheel speed.





• Wheel Speed Sensor– Sensor air gap

• The distance or air gap between the end of the sensor and its ring is critical






• A close gap is necessary to produce a strong, reliable signal






• Metal-to-metal contact between the sensor and its ring must be avoided






• The air gap must not be too wide or a weak or there could be an erratic signal (or no signal)






• The air gap on some wheel speed sensors is adjustable, and is specified by the vehicle manufacturer





• Wheel Speed Sensor– Sensor applications and precautions

• Wheel speed sensor readings are affected by the size of the wheels and tires on the vehicle






• ABS systems are calibrated to a specific tire size






• Wheel speed sensors are also magnetic






• Magnetic particles can accumulate on the end of the sensor and reduce its ability to produce an accurate signal






• Removing the sensor and cleaning the tip may be necessary if the sensor is producing a poor signal





• Wheel Speed Sensor– Digital Wheel Speed Sensors

• A conventional wheel speed sensor uses a permanent magnet with a surrounding coil of wire to produce an AC voltage signal that is proportional to wheel speed





• Wheel Speed Sensor– Digital Wheel Speed Sensors

• The voltage output and frequency are very low at slow speeds and cannot produce accurate wheel speed for ABS and traction control





• Wheel Speed Sensor– Digital wheel speed sensors

• A digital wheel speed sensor, also called an active sensor, uses either a Hall-effect or a variable-reluctance circuit to produce a square waveform where the frequency is proportional to the wheel speed






• A digital wheel speed sensor can also detect direction





• Wheel Speed Sensor– NOTE: While all GPS navigation systems

use satellites for global positioning, the vehicle uses other sensors to keep track of minor changes in speed and vehicle direction.






• The typical digital or Hall effect wheel speed sensor uses three wires:

1. A reference voltage from the controller which can be 5, 8 or 12 volts depending on the system.







2. Signal wire that sends a digital on-off signal to the controller whose frequency is proportional to the vehicle speed.







3. A ground the sensor voltage toggles between about 0.8V and 1.9V.




Figure 106-14 A digital wheel speed sensor produces a square wave output signal.




ABS CONTROL MODULEABS CONTROL MODULE




ABS Control ModuleABS Control Module

• Control Module Terms– The ABS electronic control module, which

may be referred to as an “electronic brake control module” (EBCM), “electronic brake module” (EBM), or “controller antilock brakes” (CAB) module, is a digital microprocessor that uses inputs from its various sensors to regulate hydraulic pressure during braking





• Control Module Terms– The module may be located on the

hydraulic modulator assembly or elsewhere in the vehicle, such as the trunk, passenger compartment, or under the hood





• Module Inputs– The key inputs for the ABS control module

come from the wheel speed sensors and the brake pedal switch





• Module Inputs– The brake pedal switch signals the control

module when the brakes are being applied– The wheel speed sensors provide

information about what is happening to the wheels while the brakes are being applied





• Module Inputs– NOTE: A fault with the brake switch will not

prevent ABS operation. The brake switch allows the controller to react faster to an ABS event.




Figure 106-15 Typical inputs and outputs for brake control modules.





• Module Operation– If the control module detects a difference in

the deceleration rate between one or more wheels when braking, or if the overall rate of deceleration is too fast, it triggers the ABS control module to momentarily take over





• Module Operation– The control module cycles the solenoid

valves in the modulator assembly to pulsate hydraulic pressure in the affected brake circuit (or circuits) until sensor information indicates that the deceleration rates have returned to normal and braking is under control





• Module Operation– When the brake pedal is released or when

the vehicle comes to a stop, the control module returns to a standby mode until it is again needed





• ABS Warning Lamp– Every ABS system has an amber indicator

lamp on the instrument panel– The lamp comes on when the ignition is

turned on for a bulb check, then goes out after the engine starts





• ABS Warning Lamp– If the warning light remains on or comes on

while driving, it usually dictates a fault in the ABS system

– The ABS system usually disables if the ABS warning light comes on and remains on





• ABS Warning Lamp– The ABS warning light is also used for

diagnostic purposes when retrieving flash codes (trouble codes) from the ABS module




HYDRAULIC HYDRAULIC MODULATORMODULATORASSEMBLYASSEMBLY




Hydraulic Modulator AssemblyHydraulic Modulator Assembly

• Purpose and Function– The modulator valve body is part of the

master cylinder assembly in nonintegral antilock systems but separate in nonintegral systems





• Purpose and Function– It contains solenoid valves for each brake

unit– The exact number of valves per circuit

depends on the ABS system and the application





• Purpose and Function– Some use a pair of on-off solenoid valves

for each brake circuit while others use a single valve that can operate in more than one position





• ABS Solenoid– ABS solenoid consists of a wire coil with a

movable core and a return spring– When the coil is energized, it pulls on the

movable core





• ABS Solenoid– This may open or close a valve that is

attached to the movable core, depending on its design

– When the control current is shut off, the solenoid snaps back to its normal or rest position





• ABS Solenoid– Some solenoids pull a valve to an

intermediate position when a certain level of current is applied to the coil, then pull the valve to a third position when additional current is provided





• ABS Solenoid– This design allows a single solenoid to

perform the same functions as two or even three single-position solenoids





• ABS Solenoid– The solenoids in the hydraulic modulator

assembly are used to open and close passageways between the master cylinder and the individual brake circuits





• ABS Solenoid– By opening or closing the modulator

valves, brake pressure within a circuit can be held, released, and reapplied to prevent lockup during hard braking




Figure 106-16 An ABS three-way solenoid can increase, maintain, or decrease brake pressure to a given brake circuit.





• ABS Control Pressure Stages– The standard ABS control strategy is a

three-step cycle:• The first step is to hold or isolate the

pressure in a given brake circuit by closing an isolation solenoid in the modulator assembly.

• This solenoid is normally electrically and hydraulically open.






three-step cycle:• When the solenoid is electrically closed, it

becomes hydraulically closed, which blocks off the line and prevents any further pressure from the master cylinder reaching the brake.

• This is called the pressure holding stage.






three-step cycle:• If the wheel speed sensor continues to

indicate the wheel is slowing too quickly and is starting to lock, the same solenoid or a second release solenoid is energized to open a vent port that releases pressure from the brake circuit.






three-step cycle:• Releasing pressure in the brake circuit

allows the brake to loosen its grip so the wheel can speed up and regain traction.

• This is called pressure reduction, pressure release, pressure decay, or pressure dump stage.

• The pressure reduction solenoid is normally hydraulically closed and electrically opened.






three-step cycle:• The release and/or isolation solenoid(s) are

then closed and/or the additional solenoid energized so pressure can be reapplied to the brake from the master cylinder or accumulator to reapply the brake.

• This is called the pressure increase stage.






three-step cycle:• During the pressure increase stages, the

isolation solenoid is electrically and hydraulically opened.

• The pressure reduction solenoid is electrically opened and hydraulically closed.





• ABS Control Pressure Stages– The hold-release-reapply cycle repeats as

many times as needed until the vehicle either comes to a halt or the driver releases the brake pedal

– The speed at which this occurs depends on the particular ABS system that is on the vehicle, but can range from a few times per second up to dozens of times per second




Figure 106-17 The isolation or hold phase of an ABS on a Bosch 2 system.




Figure 106-18 During the pressure reduction stage, pressure is vented from the brake circuit so the tire can speed up and regain traction.




Figure 106-19 The control module reapplies pressure to the affected brake circuit once the tire achieves traction so that normal braking can continue.





• Pump Motor and Accumulator– A high-pressure electric pump is used in

some ABS systems to generate power assist for normal braking as well as the reapplication of brake pressure during ABS braking





• Pump Motor and Accumulator– The pump motor is energized by a relay,

which is switched on and off by the ABS control module

– The fluid pressure generated by the pump is stored in the accumulator





• Pump Motor and Accumulator– The accumulator on ABS systems consists

of a pressure storage chamber filled with nitrogen gas

– A thick rubber diaphragm forms a barrier between the nitrogen gas and brake fluid





• Pump Motor and Accumulator– As fluid is pumped into the accumulator, it

compresses the gas and stores pressure– When the brake pedal is depressed,

pressure from the accumulator flows to the master cylinder to provide power assist





• Pump Motor and Accumulator– A pair of pressure switches mounted in the

accumulator circuit signals the ABS control module to energize the pump and shut it off once pressure is built back up





• Pump Motor and Accumulator– If the pump fails, there is usually enough

reserve pressure in the accumulator for 10 to 20 power-assisted stops

– After that, there is no power assist– The brakes still work, but with greatly

increased effort




Figure 106-20 An integral ABS unit with a pump motor to provide power assist during all phases of braking and brake pressure during ABS stops.





• Accumulator Precautions– A fully charged accumulator can store up to

2,700 PSI (19,000 kPa) of pressure for power-assist





• Accumulator Precautions– The accumulator should be depressurized

prior to doing any type of brake service work by pumping the brake pedal 25 to 40 times with the ignition key off





• Accumulator Precautions– In nonintegral ABS systems the

accumulator consists of a spring-loaded diaphragm

– This type of accumulator does not have to be depressurized prior to performing brake service




BRAKE PEDAL BRAKE PEDAL FEEDBACKFEEDBACK




Brake Pedal FeedbackBrake Pedal Feedback

• Many ABS units force brake fluid back into the master cylinder under pressure during an ABS stop, causing the brake pedal to pulsate

• Some manufacturers use the pulsation of the brake pedal to inform the driver that the wheels are tending toward lockup and that the ABS is pulsing the brakes





• Some manufacturers use an isolation valve that prevents brake pedal pulsation even during an ABS stop





• NOTE: A pulsating brake pedal may be normal only during an ABS stop. It is not normal for a vehicle with ABS to have a pulsating pedal during normal braking when the ABS system is not functioning. If the brake pedal is pulsating during a non-ABS stop, the brake drums or rotor may be warped.




BRAKE PEDAL TRAVELBRAKE PEDAL TRAVELSWITCH (SENSOR)SWITCH (SENSOR)




Brake Pedal Travel Switch (Sensor)Brake Pedal Travel Switch (Sensor)

• Purpose and Function– Some ABS systems use a brake pedal

travel switch (sensor) to turn on the hydraulic pump when the brake pedal has been depressed to 40% of its travel





• Purpose and Function– The pump runs and pumps brake fluid back

into the master cylinder, which raises the brake pedal until the switch closes again, turning off the pump





• Purpose and Function– When the brakes are applied, the electronic

controller “gets ready” to act if ABS needs to “initialize” the starting sequence of events





• CAUTION: If the driver pumps the brakes during an ABS event, the controller will reset and reinitialization starts over again. This resetting process can disrupt normal ABS operation. The driver need only depress and hold the brake pedal down during a stop for best operation.




TIRE PRESSURE TIRE PRESSURE MONITORINGMONITORING

SYSTEM (TPMS)SYSTEM (TPMS)




Tire Pressure Monitoring System Tire Pressure Monitoring System (TPMS)(TPMS)

• Tire pressure monitoring systems (TPMS) are required on all new vehicles

• A tire that is underinflated will have a slightly smaller rolling radius than one that is properly inflated, creating a difference in the wheel speed sensor reading if the difference in inflation pressure is 12 PSI or more





• The ABS controller will then turn on the low tire pressure warning lamp

• To help compensate for speed variation during cornering, an indirect tire pressure monitoring system checks the rotating speeds of diagonally opposed wheels





• The system adds the speeds of the right front and left rear and then subtracts that value from the sum of the left front and right rear tires

• If the total is less than or equal to a threshold value, no warning is given





• However, if the total is greater, the TPMS warning light is lit until air is added to the tire and the ignition is cycled off and on





• NOTE: This system cannot detect if all of the tires are underinflated, only if one tire is underinflated and this indirect system is not used in 2008 or newer vehicles because it does not directly measure tire inflation pressure as required by the Tread Act.




WHEEL SPEED SENSOR 1 A tone ring and a wheel speed sensor on the rear of a Dodge Caravan.




WHEEL SPEED SENSOR 2 The wiring from the wheel speed sensor should be inspected for damage.




WHEEL SPEED SENSOR 3 To test a wheel speed sensor, disconnect the sensor connector to gain access to the terminals.




WHEEL SPEED SENSOR 4 Pulling down the rubber seal reveals the connector.




WHEEL SPEED SENSOR 5 The ABS controller (computer) on this vehicle supplies a 2.5-volt reference signal to the wheel speed sensors. Set the meter to read DC volts and test at the computer end of the connector for voltage.




WHEEL SPEED SENSOR 6 The meter reads about 2.4 volts, indicating that the ABS controller is supplying the voltage to the wheel speed sensor.




WHEEL SPEED SENSOR 7 Set meter to read Ohms. The test probes are touched to the terminals leading to the wheel speed sensor and the resistance is 1.1032 k ohms or 1,103.2 ohms.




WHEEL SPEED SENSOR 8 With one lead connected to the sensor terminal and the other to a ground, The meter should (and does) read “OL,” indicating that the wheel speed sensor and pigtail wiring is not shorted to ground.




WHEEL SPEED SENSOR 9 To measure the output of the wheel speed sensor, select AC volts on the digital multimeter.




WHEEL SPEED SENSOR 10 Rotate the wheel and tire assembly by hand while observing the AC voltage output on the digital mul timeter.




WHEEL SPEED SENSOR 11 A good wheel speed sensor should be able to produce at least 100 mV (0.1 V) when the wheel is spun by hand.




WHEEL SPEED SENSOR 12 After testing, carefully reinstall the wiring connector into the body and under the rubber grommet.

halderman ch106 lecture

Documents

antilock braking system

braking force

sudden braking

tire traction traction

maximum braking traction

terms of tire slip

braking distance note

skidding tire