Possible Culprits and Sensor Locations (general, as exact location varies slightly):
* Transmission Range Sensor (PRNDL Sensor): This sensor tells the transmission control module (TCM) what gear the shifter is in. A faulty sensor or wiring could lead to incorrect gear selection. Location: Typically on the transmission itself, near where the shift linkage connects. It's a fairly small sensor with several electrical connectors.
* Vehicle Speed Sensor (VSS): This sensor is crucial. It tells the TCM how fast the vehicle is moving. If it's malfunctioning or providing incorrect information, the transmission's shifting logic will be flawed. Location: Could be on the transmission itself or on the differential (rear axle). It's usually a single connector.
* Input/Output Speed Sensors: These sensors, within the transmission, monitor the rotational speed of the input and output shafts. Malfunctions can result in improper shifting. Location: These are *inside* the transmission, making them much harder to access and test without significant transmission work.
* Transmission Control Module (TCM): While not a sensor, a faulty TCM can cause a multitude of transmission issues. It receives input from all sensors and controls the transmission's operation. Location: Usually located under the dash or near the transmission (varies by year).
* Wiring Harness: As you mentioned, damaged wiring in the harness connecting these sensors to the TCM is a very real possibility. Wires can chafe, short, or become disconnected over time, especially in high-vibration areas like the engine bay and transmission area. This is often harder to diagnose without proper wiring diagrams.
Why reverse works but low gear doesn't:
The fact that reverse works correctly suggests the problem isn't a completely catastrophic transmission failure. Reverse often uses a different set of internal mechanisms than forward gears, so it can function even if other parts of the transmission are malfunctioning.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the wiring harness connected to the transmission. Look for any obvious damage, frayed wires, or loose connections. Pay close attention to the area around the transmission range sensor and vehicle speed sensor.
2. Check Transmission Fluid: Ensure the transmission fluid is at the correct level and is clean. Low or dirty fluid can cause shifting problems.
3. Professional Diagnosis: Given the complexity, the best course of action is to take the car to a qualified mechanic specializing in automatic transmissions. They have the tools (scanners) to read diagnostic trouble codes (DTCs) from the TCM. These codes can pinpoint the source of the problem much faster than guesswork. They can also properly test the sensors and wiring.
Don't attempt major transmission repairs yourself unless you have extensive mechanical experience. The internals are delicate, and incorrect repairs can lead to even more significant problems.
