Malfunction Indicator Lamp: Testing and Inspection

Chart A-1:

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. 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 CKT 419.


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

1. If the fuse in holder is blown, refer to wiring diagram above for complete circuit of Code 54.

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

DIAGNOSTIC AIDS:

If the engine runs OK, check:

^ Faulty light bulb.
^ CKT 419 open.
^ Gage fuse blown. This will result in no brake warning light, oil or generator lights, seat belt reminder, etc.

If the engine cranks but will not run, check:

^ Continuous battery-fuse or fusible link open.
^ ECM ignition fuse open.
^ Battery CKT 340 to ECM open.
^ nition CKT 439 to ECM open.
^ Poor connection to ECM.

Solenoids and relays are turned "ON and OFF" by the ECM, using internal electronic switches called "drivers". Each driver is part of a group of four called quad-drivers. Failure of one driver can damage any other driver in the set. Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "driver". Before replacing ECM, be sure to cheek the coil resistance of all solenoids and relays controlled by the ECM. See ECM wiring diagram for the solenoid(s) and relay(s) and the coil terminal identfication.