Diagnostic Notes

Prior to any teardown, repair or component replacement, the following steps should be considered.

NOTE: It is essential that the engine has been properly warmed up and is at operating temperature, except when attempting to diagnose a cold driveability problem. The catalytic converter and the oxygen sensor will not function as designed if this is not done. Failure to do so will result in inaccurate test results.

COMPLAINT VERIFICATION

Whenever possible the repairing technician should personally verify the complaint. Having experienced the malfunction, the technician is less likely to make unnecessary repairs.

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 the system operation to the nominal system operation as described in the section DESCRIPTION AND OPERATION. Furthermore, the technician is also encouraged to compare the problem vehicle system operation with a known good vehicle if at all possible.

PREVIOUS REPAIRS

The vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrectly performed repairs can have subtle influences on seemingly unrelated systems.

NOTE: Always inquire about accident history. Vehicles that have been involved in even minor accidents may have other damage not previously detected.

BATTERY STATE OF CHARGE

If the battery is not fully charged, it can have a dramatic effect on control units and related components.

BATTERY CONNECTIONS, CABLES AND WIRING

A poor connection in either positive or ground circuits can cause unusual problems or system failures. A poor "common" ground connection can 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/or component failure. Battery and other electrical connections must be clean and tight, and cables and wiring must be in good condition for trouble-free electronic system operation. When checking for poor connections in high current circuits, use the voltage drop method of testing. A poor connection can appear OK (when tested with only the current from a multimeter) but not be able to handle the normal current flow. Refer to ELECTRICAL AND ELECTRONIC DIAGRAMS for voltage source and ground locations.

FUSIBLE LINK INTEGRITY

Fusible links are employed to prevent 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.

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.

CRANKSHAFT AND CAMSHAFT POSITION SENSOR

Monolithic sensors should be checked statically and dynamically. A sensor may have the correct internal resistance and not generate a stable signal. Physical mounting and condition of the sensor and segment wheel must also be checked.

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. Components should be inspected for good connections and be free of carbon tracking, moisture and corrosion.

GENERAL ENGINE CONDITION

Verify that the engine meets minimum mechanical requirements. Low compression, vacuum leaks, worn valve train, etc. must be corrected.

VALVE CLEARANCE AND SPARK PLUGS

These two service items, if out of specification, can significantly impair engine performance. Valve clearance adjustment procedures should be strictly observed. Verify correct spark plug type, and condition.

AIR/FUEL RATIO

When troubleshooting idle quality or driveability complaints, it is necessary to consider the following:

^ INJECTOR SPRAY PATTERN
^ PRESENCE OF UNMETERED AIR LEAKS
^ EVAPORATIVE PURGE SYSTEM
^ EXCESSIVE ENGINE OIL DILUTION
^ SUBSTANDARD FUEL OR UNAPPROVED ADDITIVES
^ CARBON BUILD-UP