Chart A-2 - ("C", "G" & "K" W/Diesel Engine)

Fig 1: CHART A-2 - MIL Circuit Schematic ("C/K" Series Diesel)

Courtesy of GENERAL MOTORS CORP.

Fig 2: CHART A-2 - MIL Circuit Schematic ("G" Series Diesel)

Courtesy of GENERAL MOTORS CORP.

MIL should be on when engine is off and ignition is on. Switched battery voltage is supplied to MIL light. The ECM turns light on by providing a ground path through circuit No. 419. With diagnostic terminal grounded, MIL should flash DTC 12, followed by any other DTC stored in memory. A steady light on indicates a short to ground in circuit No. 419, or no engine speed sensor signal.

1. If there is a problem with ECM that causes scan tester not to read serial data, then the ECM should not flash DTC 12. If DTC 12 does flash, ensure scan tester is functioning properly (on another vehicle). If scan tester is functioning properly, and circuit No. 1061 is okay, the PROM or ECM may be at fauly for no DLC data symptom.
2. If light goes off when ECM connector is disconnected, then circuit No. 419 is not shorted to ground.
3. This step checks for an open circuit No. 451.
4. At this point, MIL is working properly. The problem is a fauly PROM. If DTC 12 does not flash, the ECM should be replaced using the original PROM. Replace PROM only after trying and ECM. A defective PROM usually is an unlikely cause of the problem.

Fig 3: CHART A-2 - MIL Circuit Flowchart ("C/K" & "G" Series Diesel)

Courtesy of GENERAL MOTORS CORP.