GENERAL SYSTEM DIAGNOSTICS SERVICE MANUAL NUMBER 23
Page 5E-52 90-861326--1 MARCH 1999
CIRCUIT DESCRIPTION:
The on-board diagnostic system check must be the starting point for any driveability com-
plaint diagnosis. Before using this procedure, you should perform a careful visual/physical
check of the ECM and engine grounds for being clean and tight.
The on-board diagnostic system check is an organized approach to identifying a problem
created by an electronic engine control system malfunction.
DIAGNOSTIC AIDS:
An intermittent may be caused by a poor connection, rubbed through wire insulation or a
wire broken inside the insulation. Check for the following items:
• Inspect the ECM harness and connectors for improper mating, broken locks, improperly
formed or damaged terminals poor terminal to wire connection, and damaged harness.
TEST DESCRIPTION:
Number(s) below refer to the Step number(s) on the Diagnostic Table:
1. The MIL should be “ON” steady with the ignition “ON,” engine “OFF.” If not, chart A-1
should be used to isolate the malfunction.
2. Checks for grounded CKT 451 and ensures that the MIL driver circuit is not shorted to
ground.
3. This test ensures that the ECM is capable of controlling the MIL
4. If the engine will not start, chart A-3 should be used to diagnose the condition.
5. Refer to “ECM Diagnostic Trouble Code” chart for a list of valid Diagnostic Trouble
Codes (DTC). An invalid DTC may be the result of a faulty MDTC tool, EEPROM, or
ECM.
6. If the customer complaint or driveability problem does not currently exist, refer to “Diag-
nostic Aids” to check for an intermittent problem.
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