Powertrain Diagnosis

FOR ADDITIONAL INFORMATION NOT INCLUDED HERE, REFER TO TSB # 00-T-28 DATED Jan 7, 2003.

Schematic:

This DTC indicates that a transmission related OBD II failure has occurred. The Transmission Control Module (TCM) controls a dedicated Service Transmission Lamp (STL) which illuminates when a fail is reported by certain non-emissions related TCM diagnostics. The TCM has no direct control of the engine MIL, but if a transmission fault occurs that is emissions related, the engine MIL must illuminate. A MIL request circuit between the ECM and the TCM provides a means for illuminating the MIL, even though the fault was detected by the TCM. This circuit is pulled up to B+ within the ECM. In order to illuminate the MIL, the TCM pulls the circuit low. The ECM detects this and DTC P1780 is set, which illuminates the MIL. DTC P1780 indicates that the ECM has a detected a problem with the MIL Request Circuit.

DTC PARAMETERS
For this code to set, the engine has to be cranking, and the MIL request circuit has to be shorted to ground, open, or shorted to battery voltage. The condition has to be set for 2.5 seconds.

DTC P1780 is a (type B) DTC.

DIAGNOSTIC AIDS
When attempting to diagnose an intermittent problem, use the Scan tool to review freeze frame/failure record diagnostic information. This supplemental data can be used to duplicate a problem.

Check for poor connections at the ECM or the TCM and for a damaged harness.