Service Transmission Lamp Diagnosis

Circuit Description
The Service Transmission Lamp (STL) should illuminate briefly when the ignition switch is on and then go out. Ignition voltage is supplied directly to the indicator bulb. The Transmission Control Module (TCM) controls the lamp by grounding the control circuit via an internal solid state device called a driver.

Diagnostic Aids
An intermittent may be caused by the following:
^ Corrosion
^ Mis-routed harness
^ Rubbed through wire insulation
^ Broken wire inside the insulation

Refer to Symptoms for Intermittents. Symptom Related Diagnostic Procedures

Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
2. This step determines whether you were sent here because of an inoperative STL, or because of a STL that is always on with no DTCs set.
5. If the other indicator lamps in the instrument cluster are functioning, the feed circuit is OK.
6. This step verifies that the STL is on with no DTCs set. If a DTC is displayed, return to the Transmission On-Board Diagnostic (OBD) System Check.
7. If the STL is on with the TCM connected and off with the TCM disconnected and no DTCs are set, there is a condition in the TCM.
12. After the engine starts, the STL should illuminate briefly and then go out. If the STL remains on, go back to the Transmission OBD System Check.