Entering Diagnostic Mode
- To enter diagnostic mode, turn ignition switch on, but do not start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Diagnostic Link (ALDL) connector attached to ECM wiring harness under instrument panel. Insert jumper wire across diagnostic mode "test" terminal "B" and ground terminal "A". See Fig 1.CAUTION: Inserting jumper wire in terminals of ALDL connector completes test circuit. DO NOT ground ALDL connector until after ignition switch is on. Inserting jumper wire in terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of the SERVICE ENGINE SOON light will not indicate codes if this is done.
- SERVICE ENGINE SOON light should flash Code 12 (FLASH, pause, FLASH, FLASH) followed by a longer pause. Trouble Code 12 will be repeated 3 more times, then if any trouble codes are stored in the ECM memory, they will be displayed in the same manner.
- Trouble codes will be displayed from lowest to highest numbered codes (3 times each) and be repeated as long as the test terminal of the ALDL connector is grounded.
- To exit diagnostic mode, remove jumper wire from ALDL connector and turn ignition switch off.
NOTE:
If Code 12 is not flashed when the ALDL diagnostic test terminal is grounded, information received by a scan tester on the serial data line may not be accurate. For complete testing of the ECM diagnostic circuit, refer to appropriate article in the ENGINE PERFORMANCE section.