Entering Diagnostic Mode
- Turn ignition switch to "ON" position. "CHECK ENGINE" light should come on. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness under instrument panel. Connector is located under center of instrument panel on "S" series trucks, and on left side of instrument panel on all other models. See Fig 1 . Insert jumper wire from terminal "A" to "B" (12-pin ALDL) or terminal "D" to "E" (5-pin ALDL).
- "CHECK ENGINE" light should flash code "12". Code "12" consists of a "FLASH", pause, "FLASH", "FLASH". A longer pause will follow, then code "12" will repeat 2 more times. This check indicates that the self-diagnostic system is working. After code "12" displays 3 times, any other trouble codes stored in the ECM memory will be displayed.
- If more than one code is stored, they will be displayed from the lowest numbered code to the highest. Each code will flash 3 times. After all codes have been displayed, the complete cycle will repeat. Repetition of cycle continues as long as the "TEST" terminal of ALDL connector is grounded.
CAUTION:
DO NOT
start engine or turn ignition switch "OFF" without first removing ground from "TEST" terminal.