Chart A-1 No Service Engine Soon Light

Chart A-1:

Wiring Diagram For Service Engine Soon Light:

CIRCUIT DESCRIPTION:

There should always be a steady "SERVICE ENGINE SOON" light when the ignition is "ON" and engine stopped. Battery ignition 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 check determines if there is a fault in the "SERVICE ENGINE SOON" light circuit or in the ECM.
2. If circuits 440 and 439 have voltage, the ECM grounds or the ECM is at fault.
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 COMPUTER TERMINAL VOLTAGE VALUES under ELECTRICAL AND ELECTRONIC SPECIFICATIONS.

CAUTION: Not all SCAN testers are compatible with every system. A SCAN tool that produces faulty readings should NOT be used, and the problem should be reported to the tool manufacturer. Use of a faulty SCAN tool can result in improper diagnosis and unnecessary component replacement. Follow the proper instructions, located in the owners manual, for the specific SCAN tool being used.


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, use:
^ CHART A-3.