A-2 ~ Data Link Connector (DLC) Diagnosis and MIL "ON"

Chart A-2 Circuit Diagram:

Chart A-2 No ALDL Data Or Won't Flash Code 12 (Light On Steady):

CIRCUIT DESCRIPTION:

There should always be a steady "CHECK ENGINE" light when the ignition switch is "ON" and engine stopped. Battery voltage is supplied directly to the light bulb. The Electronic Control Module (ECM) will turn the light "ON" by grounding CKT 419 at the ECM. With diagnostic connector terminal grounded the "CHECK ENGINE" light should flash a Code 12, followed by any trouble codes stored in memory. A steady "CHECK ENGINE" light suggests a short to ground in the light control CKT 419, or an open in diagnostic terminal 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, then the ECM 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 tool is functioning properly and CKT 461 is OK, the ECM, may be at fault for the NO ALDL symptom.

2. If the light goes "OFF" when the ECM 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 "CHECK ENGINE" light circuit is OK. The problem is a faulty ECM. If Code 12 does not flash, the ECM should be replaced.