Computer System Service 19. Computer System Service

Poor Electrical Connections Testing for wiring problems inside a harness:  disconnect the wiring at both ends of the harness  ground the suspect wire on one end  use an ohmmeter to

by

Russell Krick

 Preliminary visual inspection

 Computer system circuit problems

 Sensor and actuator problems

 Sensor service

 Actuator service

 Computer service

 Involves looking for signs of obvious

trouble:

 loose wires, leaking vacuum hoses, damaged

parts, etc.

 When a computer system malfunction

occurs, the cause is often something

simple

Contaminated Engine

Oil

 Something as simple as contaminated

oil can set a trouble code

 Fumes from the crankcase are drawn

into the intake manifold for burning

 these fuel-rich vapors can trick the

oxygen sensor into signaling a rich fuel mixture, setting a trouble code

air-Contaminated Engine

Oil

Electrical Component

Damage

 Semiconductor devices are very easily

damaged by static electricity, voltage

spikes, heat, and impact shocks

 Here are some rules to remember

when working with electronic devices

and their wiring:

 Arc welding can damage on-board

computers

Electrical Component

Damage

 Never disconnect the battery cables

while the engine is running

 Do not disconnect or connect wiring,

especially the computer wiring, with the

ignition on

 Make sure you do not reverse the

battery cable connections

Electrical Component

Damage

 Wear an anti-static wrist strap

whenever you handle static-sensitive

components

 Only use high impedance test lights

and meters when checking electronic

circuits or their wiring

 Do not disconnect a scan tool from the

data link connector with the ignition on

 Most electrical-electronic problems are

actually basic circuit problems

 increased or decreased current,

resistance, or voltage

Locating Computer

Problems

 To find the source of computer

problems, you must ask yourself the

following types of questions:

 What could be causing the specific

symptoms?

 mentally picture the circuits

 How many components are not

working?

 if several components are not working,

something close to a common power source or ground point is at fault

 Is the problem always present or is it

intermittent?

 if the problem is intermittent, the

conditions causing the fault must be simulated

Locating Computer

Problems

 Is the problem’s occurrence related to

heat or cold?

 electronic circuits are sensitive to heat

 Is the problem’s occurrence affected by

moisture?

 in most cases, moisture will not affect

sealed components, but it will affect wire connections

Locating Computer

Problems

Stress Testing

 Using heat, cold, or moisture to

simulate extreme operating conditions

of components

 moisture is often used to check the

condition of spark plug wires

 cold and heat are often used to test

electronic modules

Sensors, actuators, and their circuits can

develop opens, shorts, or abnormal

resistance or voltage values

Problem Diagnosis

 In most cases, you can use a scan tool

to find the problem circuit

 Use a digital multimeter to measure the

resistance in the circuit and the sensor

output voltage or actuator input voltage

 compare these readings to factory

specifications

Problem Diagnosis

 Wiring diagrams show the color codes

of the wires and the number of

connectors that are used in the circuit

 These diagrams or schematics are

helpful when servicing any computer

system

Poor Electrical Connections

 Common cause of electrical-related

problems in a computer system

 Caused by corroded or burned

terminals, loose terminal ends, and

chafed wires

 Always check connections when

diagnosing sensors and other

components

Poor Electrical Connections

 Testing for wiring problems inside a

harness:

 disconnect the wiring at both ends of the

harness

 ground the suspect wire on one end

 use an ohmmeter to check the wire’s

resistance

 if the resistance is high, repair or replace

the wire

Testing for wiring problems

inside a harness

Poor Electrical

Connections

Vacuum Leaks

Inspect for deteriorated, broken,

Vacuum Leaks

 Can upset the operation of the

computer system and cause a wide

range of symptoms

 Manifold absolute pressure sensor

hoses and intake manifold gaskets are

common locations of leaks

 Air leaks after the mass airflow sensor

can result in lean mixtures and stalling

 Involves testing and replacing

computer system sensors

 For testing purposes, sensors may be

classified into one of two categories:

 passive sensors

 active sensors

Passive Sensors

 Some common passive sensors

include:

 intake air temperature sensors

 coolant temperature sensors

 throttle position sensors

 transmission linkage position sensors

 EGR pintle position sensors

 manifold absolute pressure sensors

Active Sensors

 Some common active sensors include:

 oxygen sensors

 crankshaft position sensors

 camshaft position sensors

 vehicle speed sensors

 knock sensors

 solar sensors

Testing Passive

Sensors

 Passive sensors do not generate their

own voltage

 Sensors are supplied with a reference

voltage from the computer

 The sensor changes its resistance with

a change in condition or operation

 The resistance change modifies the

reference voltage

 measure sensor voltage drop with its

reference voltage applied

Testing Variable Resistance Sensors

 Sensor testing using an ohmmeter

 disconnect the sensor wires

 connect the ohmmeter test leads to the

sensor terminals

 compare the ohmmeter reading to the

manufacturer’s specifications

 if necessary, sensor temperature can be

verified by using a digital thermometer

Testing Variable

Resistance Sensors

Testing Variable Resistance Sensors

 Sensor testing using a voltmeter

 connect the voltmeter in parallel with the

sensor

 measure the voltage drop across the

passive sensor with the computer reference voltage applied

 compare the voltmeter reading to the

manufacturer’s specifications

Testing Variable

Resistance Sensors

Testing Switching

Sensors

 Use an ohmmeter to check that the

switch is opening and closing

 Connect the ohmmeter and move the

switch opened and closed

 The ohmmeter should indicate infinite

ohms and then zero ohms

Testing Switching

Sensors

Testing a switch with an ohmmeter

Testing Reference

Voltage

 Use a voltmeter to measure reference

voltage to a passive sensor

 Disconnect the sensor connector

 Connect a digital voltmeter to the

Testing Reference

Voltage

Typical reference voltage is 5 volts

Testing Reference

Voltage

 If reference voltage is not correct, test

voltage at the computer

 If voltage at the computer is correct, a

problem with the wiring harness is

indicated

Testing Reference

Voltage

Testing Active Sensors

 Active sensors produce their own

voltage

 Harness continuity is very important,

since operating voltage may be low,

often under 1 volt

 Sensors may be tested with an

ohmmeter or a voltmeter

 See the service manual for procedures

Testing a Magnetic

Sensor

 Coil resistance can be measured with

an ohmmeter

 Disconnect the sensor connector

 Connect the ohmmeter leads to the

sensor terminals

 Compare the resistance to

specifications

Testing a Magnetic

Sensor

Testing a Magnetic

Sensor

 Test sensor operation with an ac

voltmeter

 Disconnect the sensor connector

 Connect the voltmeter leads to the

sensor terminals

 Rotate the sensor by cranking the

engine, turning the wheel, etc

 Compare the voltage to specifications

Testing a Magnetic

Sensor

Replacing Sensors

 When replacing sensors, there are

several rules you should remember:

 Always use an exact sensor

replacement

 even though two sensors might look alike,

their internal resistance may be different

Replacing Sensors

These two temperature sensors look

the same but have different temperature

and resistance values

Replacing Sensors

 Release the sensor connector properly

 most connectors have positive locks that

must be released

 Use special tools as needed, such as

sensor sockets

 Use thread sealant sparingly

 Use sealants that are safe for oxygen

sensors

Replacing Sensors

 Tighten sensors to specifications

 Adjust the sensor, if needed

 some throttle position sensors require

adjustment after installation

 Scan for trouble codes and sensor

value after sensor replacement

 Involves testing the actuators for

possible electrical or mechanical

problems and replacing them if

necessary

 If an actuator fails, the computer

cannot control the engine and vehicle

systems properly

Testing Servo Motors

 If no output cycling or override is

available from the scan tool, manual

tests may be performed

 disconnect the wiring harness to the

motor

 use jumper wires to feed power to the

motor

 if the motor begins to function with an

external voltage source, harness or computer problems are indicated

Testing Servo Motors

Testing Solenoids

 If no output cycling or override is

available from the scan tool, manual

tests may be performed

 disconnect the wiring harness to the

solenoid

 use jumper wires to feed power to the

solenoid

 if the solenoid begins to function with an

external voltage source, harness or computer problems are indicated

Testing Solenoids

Testing Relays

 If no output cycling or override is

available from the scan tool, manual

tests may be performed

 verify that voltage is entering and leaving

the relay

 if there is voltage entering the relay but

no voltage leaving the relay, the computer and harness are okay but the relay points are not sending voltage out to the

controlled device

Testing Relays

Relay Locations

These relays are

located under the

rear seat on this

vehicle

Relay Connectors

This locking pin

must be removed

before disconnecting the

harness

Pinpoint Test

Locations

The service manual

will givelocations for

pinpoint tests

Replacing Actuators

 Here are a few rules to follow when

replacing an actuator:

 Do not damage the wire connectors

when releasing them

 hard plastic connectors will break easily

 Do not drop the actuator mounting

screws

 if a screw falls into an engine, major

problems can occur

Replacing Actuators

 Make sure you have the correct

replacement actuator before attempting

installation

 Check that the actuator is fully seated

before tightening the mounting screws

 Double-check actuator operation after

replacement

 Involves replacing the computer

 The computer is the last component to

be suspected of being the problem

source, only after all other potential

sources of trouble have been

eliminated

Saving Memory

 Done by connecting a small 9 volt

battery across the two battery cables

 special tools are available that plug into

the cigarette lighter or connect across the battery cables

 Provides enough power to keep the

clock, stereo, and computer from losing

the information stored in their

memories

Computer Replacement

 Scan the computer and obtain the

PROM identification number or

EEPROM calibration number

 With the ignition off, remove the battery

negative cable

 Disconnect and remove the computer

 Use the identification numbers found

on the computer to order the exact

replacement

 Many computers use a PROM to store

data for the specific vehicle make and

model

 Remove the cover over the PROM

 Using a PROM tool, remove the PROM

from the old computer

 Do not touch the PROM terminals

PROM Service

 Before installing the PROM in the

computer, use a blunt tool to push the

chip into the carrier

 Make sure the reference marks are

positioned properly so the PROM is not

installed backward

 Touching only the carrier, position the

PROM pins into the socket in the

computer

PROM Service

PROM

Service

Updated PROM

 Modified integrated circuit used to

correct a driveability problem or

improve a vehicle’s performance

 Produced to correct problems such as

surging, extended cranking periods,

excessive emissions, etc

EEPROM Programming

 Electrically Erasable Programmable

Read Only Memory (EEPROM)

 Permanently soldered to the circuit

board

 Often reprogrammed to correct

driveability and performance problems

EEPROM Programming

 Programmed using a method called

Direct Programming

 Fast and simple method of

programming

 New information is downloaded by a

shop computer directly to the data link

connector

Indirect Programming

 Vehicle information is downloaded from

a programming computer into the scan

tool

 Information is then downloaded from

the scan tool into the vehicle’s

computer

Remote Programming

 Done with the vehicle’s computer

removed from the vehicle

 Used when changes need to be made

through a direct modem connection to

a manufacturer’s database computer

 Done only at new vehicle dealerships

EEPROM Programming

Procedure

 Make sure that the battery is fully

charged

 Connect the programming computer or

scan tool to the data link connector

 The computer or scan tool may ask you

to enter the engine and vehicle type, as

well as the VIN

 Go to the programming software and

Programming a New

Computer

 If a new computer is being installed,

program only that computer

 Never attempt to program a new

computer with information from the old

computer

 An erasure may need to be performed

on a new computer before initial

programming can take place

Reprogramming

Computers

 Determine the date that the current

programming was downloaded or

check the program’s calibration number

 If the latest program has not been

downloaded, proceed with the

reprogramming sequence

Reprogramming

Computers

 If necessary, erase the existing

information from the computer

 Select the updated calibration from the

programming computer or scan tool menu

 Download the new information into the

computer

Reprogramming

Computers

 Monitor the reprogramming progress to

determine when it is complete

 When programming is complete, turn the

ignition switch to the position called for in the

program

 Disconnect the computer or scan tool

 Verify the vehicle operation

Computer Relearn

Procedures

 After being serviced, the computer system may

have to go through a relearn procedure

 period of operation during which the computer

adapts to new components and programming information

 Driving the vehicle for a short time can usually

restore idle quality and driveability

 Some scan tools may offer a forced relearn

procedure