Powertrain Control Module - PCM

Chart A-2:

Data Link 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 PCM will turn the lamp "ON" by grounding MIL control circuit at the PCM. With the diagnostic terminal grounded, the MIL should flash a DTC 12, followed by any Diagnostic Trouble Code(s) [DTC(s)] stored in memory. A steady MIL suggests a short to ground in the MIL control circuit, or an open in diagnostic circuit.

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

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

2. If the lamp goes "OFF" when the PCM connector is disconnected, then 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 (Service Engine Soon) wiring is OK. The problem is a faulty PCM or PROM (MEM-CAL). If DTC 12 does not flash, the PCM should be replaced using the original PROM (MEM-CAL). Replace the PROM (MEM-CAL) only after trying a new PCM, because a defective PROM (MEM-CAL) is an unlikely cause of the problem.