- Checking the Basics
DIAGNOSTIC NOTESDiagnosing problems in modern automobiles can sometimes be very frustrating and confusing, especially when it involves computer controlled systems. The fact is, for the most part automotive electronics have been proven quite reliable, and the greatest number of problems with new cars are the same kinds of problems that older cars without computer controls have.
Before suspecting a computer problem, perform a careful visual inspection. Check under the hood for the same kinds of problems you would look for on a non-computer controlled engine. These include fluid leaks, vacuum leaks, dirty filters, overheating, oil burning, poor connections or loose wires, bad spark plug wires and/or spark plugs, restricted mufflers and exhaust systems, worn mechanical parts, exhaust leaks, and other familiar kinds of problems. Be thorough! You may save a lot of time.
All troubleshooting must begin by Checking The Basics. Certain basic faults can be undetectable by the control module self-diagnostic system and in some cases can actually interfere with the self-checking and fault memory operation. Low battery voltage, for example, can cause erroneous faults to set in control unit fault memories or can cause a system to go "Fail Safe" without setting a fault in memory. On the other hand, system fault memories are cleared whenever the control unit or the battery is disconnected. Therefore, all fault memories should be read prior to any vehicle power interruption or troubleshooting. Prior to any teardown, repair or component replacement, the following steps should always be considered:
COMPLAINT VERIFICATION
Whenever possible the repairing technician should personally verify the complaint. Having experienced the malfunction, the technician is less likely to try to repair non-existent faults.
MALFUNCTION VERIFICATION
Today's sophisticated automotive systems are easily misunderstood, which can lead to repairs that attempt to force a particular system to perform in a way that it was never intended to operate. Therefore, the troubleshooting technician should compare system operation to nominal system operation as described in Description. Furthermore, the technician is also encouraged to compare the problem vehicle system operation with a known good vehicle.
PREVIOUS REPAIRS
The vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrect components or unapproved repairs can have subtle influences on seemingly unrelated systems.
BATTERY STATE OF CHARGE
Batteries in a state of partial discharge can have a dramatic effect on the control module and related electrical components.
POSITIVE BATTERY CABLE INTEGRITY
All B+ connections must be in perfect condition for trouble-free electronic system operation. For B+ interconnects, refer to Diagrams/Electrical.
FUSIBLE LINK INTEGRITY
Fusible links are employed to prevent possible damage to electrical components and wiring harnesses. These links and their connections must be without dynamic resistance. Dynamic resistance can only be checked using the voltage drop method of testing.
NEGATIVE GROUND CONNECTIONS
As all electrical circuits are a circle, all B- connections must also be checked and verified to be in perfect condition. A poor "common" ground point will cause seemingly unrelated systems to influence one another. High current systems, which encounter a poor "common" ground, can back feed through other electrical systems causing unusual operation and perhaps inexplicable component failure. As with the B+ side of the electrical system, the ground side should be checked dynamically using the voltage drop technique.
POWER TRAIN CABLE ATTACHMENTS
Metal cables attached to the engine or transmission which appear overheated and/or discolored indicate the need to thoroughly test all ground connections.
POWER SUPPLY RELAYS
Fuel pump and main relays, as well as their plug connections, can be a source of intermittent operation which will not set a code in the control module fault memory.
FUEL DELIVERY AND FUEL PRESSURE
Fuel delivery must begin at once when cranking and pressure must be within specifications.
VACUUM AND VAPOR HOSE INTEGRITY
Vacuum and vapor hoses must be routed correctly and not leaking. Always use the underhood label for primary information and verification, refer to Diagrams/Vacuum.
Vacuum and Vapor Hose Diagrams
AIR/FUEL RATIO
Adaptive engine management systems efficiently compensate for conditions which may affect the combustion process. However, when troubleshooting idle quality or driveability complaints, it is still necessary to consider the following:
- Injector spray pattern quality
- Presence of unmetered air leaks
- Evaporative purge system
- Excessive engine oil dilution
- Substandard fuel or unapproved additives
- Carbon build-up
TESTING EQUIPMENT
High standard testing equipment is essential if accurate results are expected. The use of faulty equipment will result in erroneous test results. Use only suitable test leads, terminals and probe tips.