Chart A-1, No Mil (Service Engine Soon Light) - "G" & "P" Series With TCM: Notes

Fig 1: Chart A-1 Circuit Diagram (TCM - Diesel)

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MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL. The PCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit.

NOTE: Test numbers refer to numbers on diagnostic chart.

If ECM-BAT fuse is blown, see WIRING DIAGRAMS for complete circuit.
Using a test light connected to 12 volts, probe each of the system ground circuits to ensure a good ground is present.

Fig 2: Chart A-1 Troubleshooting Chart (TCM - Diesel)

Courtesy of GENERAL MOTORS CORP.