Chapter 19 Computer System Service

Contents Preliminary visual inspection Computer system circuit problems Sensor and actuator problems Sensor service Actuator service Computer service

Preliminary Visual Inspection 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

Gasoline 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 air-fuel mixture, setting a trouble code

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

Computer System Circuit Problems 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

Locating Computer Problems 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

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

Sensor and Actuator Problems 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

Poor Electrical Connections Testing for wiring problems inside a harness

Inspect for deteriorated, broken, or loose vacuum hoses Vacuum Leaks Inspect for deteriorated, broken, or loose vacuum hoses

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

Sensor Service 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

Testing Passive Sensors Passive sensors may be tested in one of two ways: measure sensor resistance with an ohmmeter 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 connector terminals Turn the ignition on and note the voltage Compare the readings to specifications

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

Sensor Function Check Measuring the frequency signal from a manifold absolute pressure 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

Actuator Service 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

These relays are located under the rear seat on this vehicle Relay Locations These relays are located under the rear seat on this vehicle

This locking pin must be removed before disconnecting the harness Relay Connectors This locking pin must be removed before disconnecting the harness

Make sure the replacement part is the same as the original Relay Replacement Make sure the replacement part is the same as the original

Pinpoint Test Locations The service manual will give locations 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

Computer Service 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

PROM Service 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 Install the cover over the PROM

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, and excessive emissions

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 flash programming There are three methods used to program an EEPROM: direct programming indirect programming remote programming

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 follow the prompts

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