Chart A-2 No ALDL or Code 12 SES Light on Steady

Chart A-2:

No Service Engine Soon Light 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 Mem-Cal or ECM may be at fault for the no Assembly Line Data Link symptom.

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

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

4. At this point the "SERVICE ENGINE SOON" light wiring is OK. The problem is a faulty ECM or Mem-Cal. If Code 12 does not flash, the ECM should be replace using the original Mem-Cal. Replace the Mem-Cal only after trying an ECM, as a defective Mem-Cal is an unlikely cause of the problem.