Malfunction Indicator Lamp: Testing and Inspection

Chart A-1:

Wiring Diagram:

CIRCUIT DESCRIPTION:

There should always be a steady "SERVICE ENGINE SOON" light when the ignition is "ON" and engine stopped. Ignition voltage is supplied to the light bulb. The ECM will control the light and turn it "ON" by providing a ground path through CKT 419.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to SCHEMATIC DIAGRAMS/ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS/ECM CONNECTOR IDENTIFICATION for correct color code identification of circuit.

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

1. "SERVICE ENGINE SOON" light should be "ON."

2. 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 CONNECTOR IDENTIFICATION in ELECTRICAL AND ELECTRONIC DIAGRAMS for ECM pin locations of ground circuits.

DIAGNOSTIC AIDS:

If the engine runs OK, check:

^ Faulty light bulb.
^ CKT 419 open.
^ 10 Amp indicator fuse is bad.

If the engine cranks but will not run, check:

^ Circuit 440, battery fuse or fusible link open.
^ ECM ignition fuse open.
^ Ignition CKT 439 to ECM open.
^ Poor connection to ECM.