17 computer i


Computer Complex electronic device that will

produce programmed electrical output signals after receiving specific electrical input signals

Computers monitor and control all major systems of a modern vehicle


Cybernetics


Computer Advantages Fewer moving parts to wear and go out

of calibration Reduced fuel consumption Lower emissions Increased engine power Reduced vehicle weight


Computer Advantages On-board diagnostics Increased driver convenience Improved passenger safety Compensation for component wear and

failure to keep the car driveable


Binary Numbering System


Binary Numbering System

Binary numbers can be converted into decimal (base ten) numbers


Binary Language Single zero or one is called a bit Four bits make a nibble Eight bits make a byte or word


Digital Signal

On-off signal like that produced by a rapidly flipping switch


Analog Signal

Gradually changes in strength like the output from a dimming switch


Analog Signal

Analog “waveform” as seen onan oscilloscope


Scoping a Magnetic Sensor


Signal Frequency How fast a signal changes over time High-frequency signal

changes frequently, completing each cycle in a short time

Low-frequency signal changes more slowly, completing each

cycle in a longer period of time


Inputs and Outputs


Computer System Block

DiagramFig 18


Most vehicle sensors, or transducers, change a physical condition into an electrical signal

Transduce: to change from one form to another


Active Sensor Produces its own voltage signal

internally Signal is fed to the computer for

analysis Shielded wire may be used to block

induced voltage and interference in the signal wire(s)


Active Sensor


Passive Sensor Variable resistance sensor Voltage is fed to the sensor from the

computer Sensor resistance varies with changes

in a condition temperature, pressure, motion, etc

As sensor resistance changes, the voltage signal received by the computer changes


Passive Sensor


Reference Voltage Applied to a sensor by the computer Usually 5 volts on most vehicles Computer steps down battery voltage

so that a smooth, constant supply of dc voltage is fed to passive sensors

Sensor alters the reference voltage by changing is internal resistance


Sensor Types Common sensors used in late-model

vehicles: Intake air temperature sensor (IAT)

measures the temperature of intake air as it enters the intake manifold

Engine coolant temperature sensor (ECT) measures the temperature of engine

coolant


Sensor Types Oxygen sensors

measure the amount of oxygen in the engine’s exhaust gases

Manifold absolute pressure sensor (MAP) measures the pressure inside the engine

intake manifold Barometric pressure sensor (BARO)

measures the outside air pressure


Sensor Types Throttle position sensor (TP)

measures the opening angle of the throttle valves to detect driver power demand

Engine speed sensor measures engine rpm

Crankshaft position sensor (CKP) measures crankshaft position and speed


Sensor Types Camshaft position sensor (CMP)

measures camshaft position and rotation Mass airflow sensor (MAF)

measures the amount of intake air flowing into the engine

Knock sensor (KS) detects engine pinging, preignition, or

detonation


Sensor Types Oil level sensor

measures the amount of oil in the engine oil pan

EGR sensor measures the position of the exhaust gas

recirculation valve pintle Impact sensors

detect a collision for the air bag system


Sensor Types Vehicle speed sensors (VSS)

measures the vehicle’s road speed Fuel tank pressure sensor

measures fuel tank pressure as part of some evaporative emission control systems

Battery temperature sensor monitors battery temperature for charging

system functions


Computer Names Automobile manufacturers have many

names for their computers Here are nine of those names:

Central processing unit (CPU) Electronic control unit (ECU) Electronic control module (ECM) Engine control module (ECM)


Computer Names (continued)

Electronic control assembly (ECA) Powertrain control module (PCM) Vehicle control module (VCM) Microprocessor Logic module


Computer Locations

Computers may be located almost anywhere on the vehicle


Computer Construction

Integrated circuits are attached to a printed circuit board


32

Module Memory ROM (Read Only Memory)

Contains instructions that allows the module to perform the tasks of managing the engine, such as: Programs. Look-up tables on how the

vehicle should perform. Data and formulas used to

perform calculations. Does not need applied

power to store data.


33

Module Memory RAM (Random Access Memory)

Non-permanent memory. Information is stored temporarily. Can be read, changed or rewritten.


34

Module Memory RAM (Random

Access Memory) When power is

removed, all stored data is erased.

Used to store information: From sensors. Results of calculations. Data subject to

constant change.


35

Module Memory KAM (Keep Alive Memory)

Information can be read, written and erased from KAM.

KAM is not lost when ignition key is turned off.


36

Module Memory KAM (Keep Alive Memory)

Disconnecting Battery will erase KAM. KAM allows for adaptive strategies. Stores diagnostic codes.


37

Module Memory PROM

(Programmable Read Only Memory) Similar to ROM in that

the information is permanent.

Information is not lost when power is removed.

Data or information is specific to a particular application.


38

Module Memory PROM

(Programmable Read Only Memory) Can be

reprogrammed with new information.

Some manufacturers have a removable chip that can be replaced.


Modern automotive computer systems are designed to detect problems and indicate where they might be located

This has greatly simplified the troubleshooting of complex automotive systems


OBD I Systems Most early on-board diagnostic

systems could only check a limited number of items

Diagnostics were often unable to isolate problems within a circuit

There was little standardization each manufacturer used different

connectors, terminology, and trouble code identification


OBD II Systems Designed to more efficiently monitor the

condition of hardware and software that affect emissions

Diagnostics can detect part deterioration, not just complete failure (Sensor at low end of operating range)

OBD II standardized data link connectors, trouble codes, terminology, and scan tool capabilities

Over 500 engine performance related codes


MIL OBDII Malfunction indicator light

Steady glow Trouble is not critical but should be repaired

Flashing Critical Trouble code could damage catalytic

converter Intermittent code

Not present after 50 key cycles light goes out


Data Link Connector Multipin terminal used to link the scan

tool to the computer OBD I connectors came in various

shapes, sizes, and locations OBD II connector is a standardized, 16-

pin connector located under the dash


Scan Tool Kit

Read the tool’s operating instructions. Operating procedures vary


Scan Tool Program Cartridges

Most manufacturer’s release new cartridges every one to two years


Diagnostic Connector Location

OBD II connector located below the dash


Diagnostic Connector Location

This connector is located in thecenter console


Malfunction Indicator Lamp (MIL)

Continuous illumination means the trouble should be repaired

Flashing MIL means the trouble could damage the expensive catalytic converter

Intermittent MIL means the problem is also intermittent


Diagnostic Trouble Codes

Digital signals produced and stored by the computer when an operating parameter is exceeded

Parameters include sensor voltage range, actuator current flow, and actuator movement


Computer System Problems

Most performance problems are not caused by the computer controls


Repairing the Problem Always correct the cause of the lowest

number diagnostic trouble code first and recheck for codes fixing the cause of the lowest code may

clear other codes Troubles code do not always mean that

a certain component or sensor is bad, only that the circuit parameters have been exceeded


OBD II Diagnostic Code


Failure Types Computer system failures can be

grouped into two general types: Hard failure

problem that is always present Soft failure

problem only occurs when certain conditions are present (intermittent)

soft failure codes will be stored in memory for 30–50 ignition or warm-up cycles


Failure Types Failure types can be further broken

down into four categories: General circuit failure

circuit or component has a fixed value, no output, or an output that is out of specifications

Low-input failure voltage, current, or frequency is below

normal operating parameters


Failure Types High-input failure

signal has more voltage, more current, or a higher frequency than normal

Improper range/performance failure sensor or actuator is producing values

slightly lower or higher than normal device is still functioning, but not as well

as it should OBD II systems have this capability


Test Drive You can scan the vehicle datastream

while driving to simulate the conditions present when the trouble occurs

Some scan tools allow you to take a “snapshot” or “movie” to play back when you are safely stopped


Activating Diagnostics

Jump terminals A to B on mostGM connectors



On Ford connectors, jump the extra

pigtail to a specified terminal



Connect a test light across specified terminals in this

connector. Watch the flashes for the

trouble codes



Turning the ignition on, off, on, off, and

then on within 5 seconds will activate diagnostics on most

Chrysler cars



By pressing two buttons at the same time, the readout will display trouble codes


Check Engine Light

During diagnostics, the lamp mayflash to indicate DTC’s


Trouble Code Chart

Service manual chart that shows different trouble code numbers and possible

problems and causes


Erasing Trouble Codes There are various ways to erase

trouble codes from the computer: Use a scan tool

this is the easiest way Disconnect the battery ground cable Unplug the fuse to the ECM

both methods erase radio, seat, clock, and ECM adaptive strategy memories


Erasing Trouble Codes After clearing codes, reenergize on-

board diagnostics to verify that no codes are present

This will help to verify your repair DTC’s will be erased from memory

automatically after 30 to 50 key cycles or warm-up cycles

17 computer i

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