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

ECM Power And Ground Circuit:

Chart A-2 No DLC Data Or MIL Always ON:

Circuit Description
There should always be a steady "Check Engine" malfunction indicator lamp when the ignition is "ON" and engine stopped. Battery positive voltage is supplied directly to the light bulb. The Engine Control Module (ECM) will turn the light "ON" by grounding VLT colored wire at the ECM.

With the diagnostic test terminal grounded, the light should flash diagnostic trouble code (DTC) 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control VLT wire, or an open in diagnostic ORN/YEL wire.


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 DTC 12. If DTC 12 does flash, be sure that the "Scan tool" is working properly on another vehicle. If the "Scan tool" is functioning properly and ORN/BLK wire is OK, the ECM, may be at fault for the NO data link connector symptom.

2. If the light goes "OFF" when the ECM connector is disconnected, then, the VLT wire is not shorted to ground. This step will check for an open diagnostic request ORN/YEL wire.

3. At this point, the "Check Engine" malfunction indicator lamp wiring is OK. The problem is a faulty ECM. If DTC 12 does not flash, the ECM should be replaced.