Chart A-1 No Service Engine Soon Light

Chart A-1:

Check Engine Light Circuit:

CIRCUIT DESCRIPTION:
There should always be a steady "CHECK ENGINE" light when the ignition is "ON" and engine stopped. Battery voltage is supplied to the light bulb. The ECM will control the light and turn it "ON" by providing a ground path through circuit 419 to the ECM.

TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

1. This test will provide a ground path for the "CHECK ENGINE" light circuit. If the "CHECK ENGINE" light is "ON," then the external circuit is OK.

2. If circuits 439 and 2 have voltage, the ECM grounds or the ECM are faulty.

3. Using a test light connected to 12 volts, probe each of the system ground circuits to be sure a good ground is present. See ECM Pinout Connectors under Diagrams.

4. If fusible link or fuse is blown, be sure to consider branch circuits and splices to other components.

DIAGNOSTIC AIDS:
If the engine runs OK, check:
^ Faulty light bulb.
^ Circuit 419 open.
^ Gauge fuse blown. This will result in no oil, generator, or seat belt reminder lights.

If the engine cranks but will not run, check:
^ Continuous battery-fuse or fusible link open.
^ ECM ignition fuse open.
^ Battery circuit 2 to ECM open.
^ Ignition circuit 439 to ECM open.
^ Poor connection to ECM.