Chart A-1 No Service Engine Soon Light

Service Engine Soon Lamp Circuit Chart A-1:

Service Engine Soon Lamp Circuit:

Circuit Description:
There should always be a steady "Service Engine Soon" 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 CKT 419 to the ECM.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step will isolate the problem in CKT 419.

2. If CKT's 440 and 439 have voltage, the ECM connections, grounds, or ECM is 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. Refer to ECM terminal end view in this section for ECM pin locations of ground circuits.

4. If the fuse/fusible link is blown, refer to ECM wiring diagram for complete circuit.


Diagnostic Aids:
If the engine runs 0K, check:

^ Faulty light bulb
^ CKT 419 open, shorted to ground or blown fuse

If the engine cranks but will not run, check:

^ Continuous battery-fuse or fusible link open
^ ECM ignition fuse open
^ Battery CKT 440 to ECM open
^ Ignition CKT 439 to ECM open
^ Poor connection to ECM