Chart A-2 Won't Flash Code 12 Service Engine Light On

PCM Power Supply And Diagnostic Circuit Diagram:

CIRCUIT DESCRIPTION
The SES light gets power whenever the ignition is turned "ON" and illuminates when the PCM completes the circuit to ground. The light should illuminate whenever the ignition is "ON" and the engine is not running. The light should go out after the engine is started, and stay out unless a malfunction is present.

CODE 12 DISPLAY
When the diagnostic terminal of the ALDL is grounded (jumper between terminals A & B), the "SES" light should flash a Code 12, followed by any trouble codes stored in memory. Code 12 is displayed by a flash, then a short pause, then two flashes then a longer pause. This code should display three times, followed by diagnostic trouble codes. If there are no trouble codes, the light will continue flashing Code 12 until the diagnostic mode is exited (remove jumper wire from ALDL connector).

TEST DESCRIPTION

Chart A-2, No ALDL Data Or No Code 12 (SES Light "ON" Steady):

Numbers below refer to circled numbers on the diagnostic chart.

1. If there is a malfunction in the PCM that causes the system to not send serial data, the PCM should not flash a Code 12.If Code 12 is flashing, check for circuit 451 shorted to ground. If Code 12 does flash, check to see if "SCAN" tool works properly on another vehicle. If "SCAN" tool is OK and circuit 461 is OK, a MEM-CAL, or PCM malfunction may be causing the no ALDL data problem.
2. If the light stays "OFF" with the PCM disconnected, then circuit 419 is NOT shorted to ground.
3. This step checks for an open diagnostic circuit (circuit 451.)
4. At this point, the wiring for circuits 439, 419 and 451 have been verified OK. The problem is a faulty PCM or MEM-CAL. Replace the PCM first and recheck with the original MEM-CAL installed in the new PCM before replacing the MEM-CAL (as the MEM-CAL is unlikely to cause this problem.)