If the car moves forward when the TCM connector is unplugged but not when it's plugged in, it strongly suggests a problem within the transmission control system itself, rather than a purely mechanical issue within the transmission. Here are the most likely culprits:
* Faulty TCM: The TCM itself might be malfunctioning. It could be internally shorted, have a failing processor, or suffer from damaged internal components.
* Faulty PCM/ECM Communication: The communication link between the PCM/ECM (the main car computer) and the TCM might be broken. This could be due to a bad connection, a wiring fault in the harness, or a problem with the PCM/ECM itself.
* Faulty Solenoids or Valves within the Transmission: The TCM sends signals to solenoids and valves inside the transmission to control shifting. If one or more of these components are faulty (stuck open, stuck closed, or internally shorted), it can cause erratic behavior, and plugging in the TCM might make the problem apparent because the TCM is then trying to control them (and failing).
* Low Transmission Fluid: While less likely given the symptom, critically low transmission fluid *could* cause the transmission to behave erratically. Check the transmission fluid level and condition. It should be red and not smell burnt.
* Incorrect TCM Programming/Calibration: Although less common, an improperly programmed or calibrated TCM could lead to this issue. This is more likely if the TCM has recently been replaced or reprogrammed.
Why it might move in "limp mode" (unplugged): When the TCM connector is unplugged, the transmission often goes into a default "limp mode." This mode usually allows only a limited range of gear operation (often only first gear, allowing forward movement), prioritizing safety over functionality. This is a failsafe to prevent further damage to the transmission.
Troubleshooting Steps:
1. Check the transmission fluid: Make sure it's at the correct level and in good condition.
2. Inspect the TCM connector: Look for any corrosion, bent pins, or broken wires. Clean the connector with contact cleaner if necessary.
3. Check the wiring harness: Visually inspect the wiring harness between the TCM and the PCM/ECM for any damage.
4. Have the transmission control system scanned with a professional-grade OBD-II scanner: This will retrieve any diagnostic trouble codes (DTCs) stored in the PCM/ECM and TCM, which will help pinpoint the problem. A basic code reader may not be sufficient. A professional scan tool can also perform more in-depth testing of the transmission control system.
5. Professional diagnosis: If the problem persists after checking the above, it's best to take the vehicle to a qualified transmission specialist or mechanic for a proper diagnosis and repair. The internal workings of the transmission are complex, and incorrect repair attempts can lead to more damage.
Do not attempt to drive the vehicle extensively while experiencing this issue, as further damage to the transmission is possible.
