Chart A-2, No Data Link Connector (DLC) Data, Will Not Display DTC 12 Or Mil On Steady - "G" & "P" Series With TCM

Fig 1: Chart A-2 Circuit Diagram (TCM - 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. The TCM turns light on by providing a ground path through the MIL (SERVICE ENGINE SOON) control circuit. 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 the MIL (SERVICE ENGINE SOON) control circuit or an open in the diagnostic request circuit.

1. If there is a problem with TCM that causes scan tester not to read serial data, TCM 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 serial data circuit is okay, PROM or TCM may be at fault for the NO DLC symptom.
2. If light goes off when TCM connector is disconnected, MIL (SERVICE ENGINE SOON) control circuit is not shorted to ground.
3. This step checks for an open in the diagnostic request circuit.
4. At this point, MIL wiring is okay. Problem is a faulty PROM. If DTC 12 does not flash, TCM should be replaced using original PROM. Replace PROM only after trying a TCM. A defective PROM usually is an unlikely cause of problem.

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

Courtesy of GENERAL MOTORS CORP.