Chart A-2 No ALDL Data or Won't Flash Code 12

Chart A-2 - No ALDL Data Or Won't Flash Code 12:

Wiring Diagram for "Service Engine Soon" Light:

CHART A-2 - NO ALDL DATA OR WON'T FLASH CODE 12

Circuit Description:


There should always be a steady "Service Engine Soon" (SES) light when the ignition is "ON" and engine stopped. Battery ignition voltage is supplied to the light bulb. The Electronic Control Module (ECM) will turn the light "ON" by grounding CKT 419 at the ECM.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control CKT 419, or an open in diagnostic CKT 451.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. If there is a problem with the ECM that causes a "Scan" tool to not read serial data, the ECM should not flash a Code 12. If Code 12 is flashing check for CKT 451 short to ground. If Code 12 does flash be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" is functioning properly and CKT 461 is OK, the Programmable Read Only Memory (PROM) or ECM may be at fault for the No Assembly Line Data Link (ALDL) symptom.


2. If the light goes "OFF" when the ECM connector is disconnected, CKT 419 is not shorted to ground.

3. This step will check for an open diagnostic CKT 451.

4. At this point the "service engine soon" (SES) light wiring is OK. The problem is a faulty ECM or PROM. If Code 12 does not flash, the ECM should be replaced using the original PROM. Replace the PROM only after trying an ECM, as a defective PROM is an unlikely cause of the problem.