Initial Inspection and Diagnostic Overview
All troubleshooting must begin by "CHECKING THE BASICS." Certain basic faults can be undetectable by the ECU self-diagnostic system and in some cases can actually interfere with self-checking and fault memory operation. For example, low battery voltage can cause erroneous faults to set in ECU memory or can cause a system to go "Fail-Safe" without setting a fault code. However, if the ECU or battery are disconnected, fault codes are cleared from memory. Fault codes should be read prior to any power interruption or troubleshooting. Before any teardown, repair or component replacement, the following should always be considered:
COMPLAINT VERIFICATION
Whenever possible, the technician should personally verify complaints. Having experienced the malfunction, the technician is less likely to attempt repair of non-existent problems.
MALFUNCTION VERIFICATION
Today's sophisticated automotive systems are easily misunderstood, leading to repairs that attempt to force a particular system to perform as it was never intended. Therefore, the troubleshooting technician should compare actual system operation to nominal system operation described in DESCRIPTION AND OPERATION. The technician is also encouraged to compare problem vehicle system operation with a known good vehicle.
PREVIOUS REPAIRS
Vehicle repair history can provide explanations for unusual complaints which seem to elude normal troubleshooting procedures. 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 dramatic effects on electronic control units and related components.
POSITIVE BATTERY CABLE INTEGRITY
All battery positive connections must be in perfect condition for trouble-free electronic system operation. Refer to ELECTRICAL AND ELECTRONIC DIAGRAMS for battery positive interconnects.
FUSIBLE LINK INTEGRITY
Fusible links are used 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
Because all electrical circuits are a circle, battery negative 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. Battery negative (ground) side of the electrical system should be checked dynamically using the voltage drop method.
POWER TRAIN CABLE ATTACHMENTS
Metal cables attached to 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 problems which will not set an ECU malfunction code.
HIGH TENSION COIL AND SECONDARY COMPONENTS WIRING
The secondary ignition system should be checked visibly as well as with a suitable engine analyzer and scope. All components should be examined for tight connections and freedom from carbon tracking, moisture and corrosion.
FUEL DELIVERY AND FUEL RAIL PRESSURE (FI VEHICLES)
Fuel delivery must begin at once when cranking and fuel rail pressure must be within specifications. Fuel pressure must be retained between fuel pump outlet and pressure regulator after engine shutdown.
SPARK PLUG CLEARANCE
If out of specification, spark plug gap can significantly impair engine performance. Spark plug type, condition and gap must be verified according to specifications.
VACUUM AND VAPOR HOSE INTEGRITY
Vacuum and vapor hoses must be routed correctly and not leaking. Always use underhood labels for primary information and refer to VACUUM AND VAPOR DIAGRAMS for verification.
AIR/FUEL RATIO
Electronic 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 quality testing equipment is essential if accurate results are expected. The use of faulty or incompatible equipment will result in erroneous test results. Use only suitable test leads, terminals and probe tips.