1. Chapter 19
Computer System
Service

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

3. 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

4. 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

5. Contaminated Engine Oil

6. 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

7. 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

8. 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

9. Computer System Circuit Problems
Most electrical-electronic problems are actually basic circuit problems
increased or decreased current, resistance, or voltage

10. 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

11. 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

12. 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

13. 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

14. Sensor and Actuator Problems
Sensors, actuators, and their circuits can develop opens, shorts, or abnormal resistance or voltage values

15. 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

16. 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

17. 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

18. 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

19. Poor Electrical Connections
Testing for wiring problems inside a harness

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

21. 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

22. 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

23. 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

24. Active Sensors Some common active sensors include: oxygen sensors
crankshaft position sensors
camshaft position sensors
vehicle speed sensors
knock sensors
solar sensors

25. 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

26. 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

27. 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

28. Testing Variable Resistance Sensors

29. 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

30. Testing Variable Resistance Sensors

31. 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

32. Testing Switching Sensors
Testing a switch with an ohmmeter

33. 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

34. Testing Reference Voltage
Typical reference voltage is 5 volts

35. 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

36. Testing Reference Voltage

37. 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

38. 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

39. Testing a Magnetic Sensor

40. 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

41. Testing a Magnetic Sensor

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

43. 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

44. Replacing Sensors
These two temperature sensors look the same but have different temperature and resistance values

45. 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

46. 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

47. 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

48. 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

49. Testing Servo Motors

50. 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

51. Testing Solenoids

52. 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

53. Testing Relays

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

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

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

57. Pinpoint Test Locations
The service manual will give locations for pinpoint tests

58. 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

59. 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

60. 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

61. 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

62. 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

63. 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

64. 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

65. PROM Service

66. 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

67. EEPROM Programming
Electrically Erasable Programmable Read Only Memory (EEPROM)
Permanently soldered to the circuit board
Often reprogrammed to correct driveability and performance problems

68. EEPROM Programming Programmed using a method called flash programming
There are three methods used to program an EEPROM:
direct programming
indirect programming
remote programming

69. Direct Programming Fast and simple method of programming
New information is downloaded by a shop computer directly to the data link connector

70. 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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. 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