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" (SES) light when the ignition is "ON" and engine stopped. Battery is supplied directly to the light bulb. The Electronic Control Module (ECM) will control the light and turn it "ON" by providing a ground path through CKT 419 to the ECM.


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

1. This test will determine if the ECM has the ability to ground CKT 419.

2. If both the continuous battery supply voltages are lost at terminals "B1" and "C16" or the ignition feed to terminal "A6" is not present, the "SERVICE ENGINE SOON" light will not come "ON" with the ignition "ON".


DIAGNOSTIC AIDS:

Engine runs OK, check:

^ Faulty light bulb
^ CKT 419 open

Engine cranks but will not run:

^ Continuous battery - fusible link open
^ ECM fuse open
^ Battery CKT 340 to ECM open
^ Ignition CKT 439 to ECM open
^ Poor connection to ECM
^ Faulty ECM ground circuit(s)