* Vehicle Speed Sensor (VSS): This is the most common cause. The VSS is a sensor that measures the speed of the vehicle's transmission output shaft. It sends this information to various systems, including the speedometer and the transmission control module (TCM). A faulty VSS will prevent the speedometer from working and can lead to erratic or incorrect shifting because the TCM relies on this data to determine the appropriate gear.
* VSS Wiring Harness: Damaged, corroded, or loose wiring to the VSS can prevent a signal from reaching the speedometer and TCM. Check for broken wires, chafing against other components, and corrosion, especially near the transmission.
* Transmission Control Module (TCM): The TCM receives the speed signal from the VSS and controls the shifting. A faulty TCM can lead to both speedometer failure and shifting problems, even if the VSS is working correctly. This is a more expensive and complex issue to diagnose and resolve.
* Speedometer Cable (if equipped): While less likely in a 1995 Grand Prix (which likely uses electronic signals), some older vehicles still used a mechanical cable to drive the speedometer. A broken or damaged cable would cause a speedometer failure, but wouldn't directly affect shifting.
* PCM (Powertrain Control Module): While less likely to cause both problems directly, a failing PCM can affect various systems, and if it's malfunctioning in a way that interferes with the communication between the VSS and the TCM, it could lead to both symptoms.
* Other less likely causes: While less probable, a faulty speedometer itself could be the issue (though this is less likely if the transmission is also exhibiting problems). A severely low transmission fluid level could also affect shifting but wouldn't usually cause the speedometer to stop working, unless it damaged the VSS in the process.
Troubleshooting Steps:
1. Check the VSS: This should be your first step. Locate the VSS (usually on the transmission) and inspect it for damage. You might also need a multimeter to test the sensor's output signal.
2. Inspect the Wiring Harness: Carefully examine the wiring harness connecting the VSS to the TCM and the instrument cluster for any signs of damage.
3. Check Transmission Fluid: Ensure the transmission fluid level is correct and the fluid is clean. Low or dirty fluid can contribute to shifting problems.
4. OBD-II Scanner: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the vehicle's computer. These codes can help pinpoint the source of the problem.
If you're not comfortable working on your vehicle's electrical system or transmission, it's best to take it to a qualified mechanic for diagnosis and repair. Replacing the VSS is often relatively inexpensive, but a faulty TCM or PCM can be significantly more costly.
