Engine Control Module - ECM

Chart A-2:

DLC Connector Circuit:

CIRCUIT DESCRIPTION
There should always be a steady Malfunction Indicator Lamp (MIL) when the ignition is "ON" and engine stopped. Battery voltage is supplied to the lamp. The ECM will turn the lamp "ON" by grounding MIL control circuit at the ECM. With the diagnostic terminal grounded, the lamp should flash a DTC 12, followed by any DTC(s) stored in memory. A steady lamp suggests a short to ground in the MIL control circuit, or an open in diagnostic test circuit.

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. If there is a problem with the ECM that causes a Tech 1 scan tool not to read serial data, then the ECM should not flash a DTC 12. If DTC 12 does flash, be sure the scan tool is working properly on another vehicle. If the scan is functioning properly and the serial data circuit is OK, the PROM or ECM may be at fault for the No DLC symptom.

2. If the lamp goes "OFF" when the DLC is disconnected, then the MIL control circuit is not shorted to ground.

3. This step will check for an open diagnostic test circuit.

4. At this point the MIL wiring is OK. The problem is a faulty ECM or PROM (CAL-PAK). If DTC 12 does not flash, the ECM should be replaced using the original PROM (CAL-PAK). Replace the PROM (CAL-PAK) only after trying an ECM, as a defective PROM (CAL-PAK) is an unlikely cause of the problem.