Chart A-2 - No ALDL Data Or Will Not Flash Code 12 (Check Engine Light On Steady)
There should always be a steady CHECK ENGINE light when ignition is on and engine is off. Battery voltage is supplied to the bulb by the ECM, which activates the light by grounding Violet wire. With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble codes stored in memory. A steady light could be a short to ground in the light control circuit No. 419 (Violet wire), or an open in diagnostic circuit No. 451 (Orange/Yellow wire).
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- If a problem exists with the ECM that causes a scan tester not to read serial data, ECM should NOT flash a Code 12. If Code 12 does flash, ensure scan tester is working properly on another vehicle. If scan tester is functioning properly and circuit No. 461 (serial data) is okay, the ECM may be at fault.
- If light goes off when ECM connector is disconnected, circuit No. 419 is NOT shorted to ground.
- This tests for an open in circuit No. 451 (diagnostic enable).
- At this point, CHECK ENGINE light wiring is okay. The problem is a faulty ECM. If Code 12 does not flash, replace ECM.