Diagnostic Circuit Check

Diagnostic Circuit Check:

DIAGNOSTIC CIRCUIT CHECK

The Computer Command Control system has a diagnostic system built into the ECM to indicate a failed circuit. An Service Engine Soon" light on the instrument panel will illuminate if a problem has been detected when the engine and vehicle are running. This light is also used for a bulb and system check.


The System Check 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 code.

The system requires an ALDL read-out "Scan" tool, tachometer, test light, ohmmeter, digital voltmeter with 10 megohms impedance (J-34029A), vacuum gage and jumper wires for diagnosis.

BULB CHECK

With the ignition "ON" and engine not running, the lamp should illuminate, which indicates that the ECM has completed the circuit to turn "ON" the light.

If the "Service Engine Soon" light is not "ON", refer to CHART A-1 for diagnosis.

When the engine is started, the light will turn "OFF". If the light remains "ON" refer to system check.


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 system check is performed through the twelve terminal assembly line diagnostic link (ALDL) connector (Figure 3-2) under the instrument panel in the passenger compartment.

The Computer Command Control System Check is a procedure that determines the following:

1. Bulb Check - to check SES light circuit and that the ECM can complete the circuit.

2. Diagnostic Mode - this indicates if the diagnostic code system is working.

3. "Scan" Data - this determines if the ECM is supplying input and output visual data.

4. Engine Start - this step is done after it has been determined that the ECM will display codes and data.

5. Other Codes - Proceed to applicable chart if a code is displayed. Scanning the data for typical value may indicate a problem area if they are incorrect. If all systems appear to be functioning, see Diagnosis By Symptoms - No Trouble Codes Stored.

The system check starts with a bulb check. If there is no "Service Engine Soon" light, refer to CHART A-1.