Chart A-1 No Service Engine Soon Light

"Service Engine Soon" Light:

CHART A-1, NO "CHECK ENGINE" LIGHT

There should always be a steady "Check Engine" Light when the ignition is "on" and engine stopped. Battery voltage is supplied directly to the light bulb. The Eletronic Control Module (ECM) will turn the light on by grounding circuit 419 at the ECM.
Engine runs ok, check:
a. Faulty light bulb.
b. CKT 419 open.
c. Gage fuse blown. This will result in no stop lights, oil or generator lights, seat belt reminder, etc.

Engine cranks but will not run.
a. Continous battery - fuse or fusible link open.
b. ECM ignition fuse open.
c. Battery CKT 340 to ECM open.
d. Ignition CKT 439 to ECM open.
e. Poor connection to ECM.

Chart A-1 - No "Service Engine Soon" Light:

1. If both the continous battery supply voltages are lost at terminals B1 and C16 or the ignition feed to terminal A6 is not present, the check engine light will not come on with the ignition on.
2. Solenoids are turned "ON" or "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 can damage any other driver within the set.

Solenoids coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check the coil resistance of all solenoids or relays shown in the ECM wiring diagrams.