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

Chart A-2 Circuit Diagram:

Diagnostic Chart A-2 No ALDL Data Or Will Not Flash DTC 12, MIL 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 the RED wire 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 RED wire, or an open in diagnostic ORN/YEL.

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 ORN/YEL 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, the RED wire is not shorted to ground.

3. This step will check for an open ORN/YEL.

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.