Chart A-2 No ALDL Data or Wont Flash Code 12

No Service Engine Soon/Trans Light Chart A-1:

Circuit Description:
There should always be a steady "Service Engine soon"/"Trans" light when the ignition is "ON" and engine stopped. Battery ignition voltage is supplied to the light bulb. The PCM/TCM will turn the light "ON" by grounding CKT 419/CKT 1234.

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/CKT 1234 or an open in diagnostic CKT 451.

Chart A-2 Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. If there is a problem with the PCM/TCM that causes a "Scan" tool to not read serial data, then the PCM/TCM should not flash a Code 12. 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 1061 is OK, the PROM/MEM-CAL or PCM/TCM may be at fault for the NO ALDL symptom.

2. If the light goes "OFF" when the PCM/TCM connector is disconnected, then 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"/ "Trans" light wiring is OK. The problem is a faulty PCM/TCM PROM/MEM-CAL. If Code 12 does not flash, the PCM/TCM should be replaced using the original PROM/MEM-CAL. Replace the PROM/MEM-CAL only after trying a PCM/TCM, as a defective PROM/MEM-CAL is an unlikely cause of the problem.