Chart A-1 No Service Engine Soon Light

Chart A-1:

Wiring Diagram For SES 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. Battery feed circuit 240 is protected by a fusible link at the battery.
2. If circuits 439 and 240 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 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, check:
^ Continuous battery-fuse or fusible link open.
^ ECM ignition fuse open.
^ Battery circuit 240 to ECM open.
^ Ignition circuit 439 to ECM open.
^ Poor connection to ECM.