Engine Control Module - ECM

The control module system has a diagnostic system built into the ECM to indicate a failed circuit. An amber Malfunction Indicator Lamp (MIL) "Service Engine Soon" on the instrument panel will illuminate if a problem has been detected when the engine and vehicle are running. This lamp is also used for a bulb and system check.

The "On-Board Diagnostic System Check" Testing and Inspection is the starting point for the diagnostic procedures or an emissions test failure. The diagnostic charts are related to the ECM and will determine if the ECM is working properly. This section diagnoses the fuel system controlled by the ECM and has charts to diagnose a circuit when the ECM has displayed a DTC.

The system requires a scan tool, test light, digital voltmeter with 10 megohms impedance (J 39200), vacuum gage and jumper wires for diagnosis.
With the ignition "ON" and engine not running, the MIL should illuminate, which indicates that the ECM has completed the circuit to turn "ON" the lamp.

If the MIL is not "ON," refer to CHART A-1 Related Tests, Information and Procedures for diagnosis.
When the engine is started, the lamp will turn "OFF." If the lamp remains "ON," refer to "On-Board Diagnostic System Check."


ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here.

The circuit check is performed through the twelve terminal assembly line Data Link Connector (DLC) under the instrument panel in the passenger compartment.