For 1990-2009 cars

## Service Transmission Lamp Diagnosis Early Production

Automatic Transmission Components
Automatic Transmission Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### 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 in Engine Controls for Intermittents.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. 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.

2. If the other indicator lamps in the instrument cluster are functioning, the feed circuit is OK.

3. 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.

4. 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.

5. 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.

#### Service Transmission Lamp (STL) Diagnosis

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the On-Board Diagnostic System Check performed? | -- | Go to Step 2 | Go to On-Board Diagnostic System Check
2 | Turn the ignition switch to the RUN position. Does the STL illuminate? | -- | Go to Step 6 | Go to Step 3
3 | Check the fuse for the instrument cluster ignition feed circuit. Is the fuse OK? | -- | Go to Step 4 | Go to Step 9
4 |

1. Turn the ignition switch to the OFF position.
2. Disconnect the TCM.
3. Turn the ignition switch to the RUN position.
4. Using a fused jumper wire connect TCM harness Pin 25 to ground.

Is the STL on?

--

Go to Step 10

Go to Step 5

5 | Check the STL circuit for the following conditions:

• | Damaged bulb.

• | Open ignition feed to the bulb.

• | Control circuit open or shorted to B+.

Refer to General Electrical Diagnosis in Wiring Systems.

Is the action complete?

--

Go to Step 12

--

6 | Use the Scan Tool in order to verify that no DTCs are set. Are there any DTCs displayed? | -- | Go to On-Board Diagnostic System Check | Go to Step 7
7 |

1. Turn the ignition switch to the OFF position.
2. Disconnect the TCM.
3. Turn the ignition switch to the RUN position.

Does the STL illuminate?

--

Go to Step 8

Go to Step 11

8 | Repair the short to ground in the STL control circuit. Is the action complete? | -- | Go to Step 12 | --
9 | Repair the short to ground in the feed circuit to the instrument panel indicator lamps. Is the action complete? | -- | Go to Step 12 | --
10 | Check the TCM harness connector for a poor connection or poor terminal tension. Refer to General Electrical Diagnosis in Wiring Systems. Was a condition found and corrected? | -- | Go to Step 12 | Go to Step 11
11 | Replace the TCM. Refer to Transmission Control Module Replacement . Is the action complete? | -- | Go to Step 12 | --
12 | Start the engine and observe the STL. Does the STL illuminate briefly and then go out? | -- | System OK | Go to Step 1