Description of On-Board Diagnostics

Fig. 1 How to use Diagnostic Procedure:

This diagnostic procedure is only used on Electronic Engine Control (EEC-III) equipped vehicles and only when the Diagnostic Routines instruct you to perform EEC-III Diagnostics. Most of the time the Quick Test can detect total engine system malfunctions that are both EEC System related and non-EEC System related. The Quick Test accomplishes this by excercising EEC System components and looking for a corresponding engine response. If the malfunction is EEC System related, the Pinpoint testing will result with the correct malfunctioning EEC System component being replaced. However, if the malfunction is not EEC System related, the Quick Test will detect an improper engine malfunction, and the ensuing Pinpoint testing may incorrectly indicate an EEC System component as being responsible. To prevent EEC Pinpoint testing from misdiagnosing an EEC System component when a non-EEC System failure has actually occurred, attention should be paid to Diagnostic Routines. Refer to Fig. 1 for detailed information on how to use this diagnostic procedure.

To test and service the EEC-III subsystem, perform the Quick Test first, and if the vehicle passes the Quick Test without running any Pinpoint Tests, you will know the EEC-III subsystem is OK and the vehicle's problem exists somewhere else other than in the EEC-III subsystem. However, if a step of the Quick Test fails, run only the Pinpoint Tests specified by the failed step.

If any service to the vehicle is to be performed while running the Quick Test or Pinpoint Tests, do only those services that are specified in either test.

After all tests and services have been completed, repeat the entire Quick Test to make sure the EEC-III subsystem works properly.

To insure proper validation of the Electronic Engine Controls System, do not short cut this procedure, performing all of the Quick Test starting with:

TEST STEP
1 PRE-CHECK PROCEDURES AND VEHICLE PRE-START PREPARATION

THROUGH

TEST STEP
26 EEC-III SELF TEST, OK