* Vehicle Speed Sensor (VSS): This is the most common cause. The VSS is a sensor located in the transmission (or sometimes the differential) that measures the speed of the vehicle's output shaft. A faulty VSS will send no signal or an incorrect signal to the instrument cluster, resulting in a non-functioning speedometer. This sensor can fail due to wear, damage, or electrical issues.
* Speed Sensor Wiring: The wiring connecting the VSS to the instrument cluster can be damaged, corroded, or disconnected. A break in the wiring will prevent the signal from reaching the speedometer.
* Instrument Cluster (Gauge Cluster): The speedometer itself within the instrument cluster might be faulty. This is less common than a VSS issue but possible. The speedometer motor or the circuitry within the cluster could be malfunctioning.
* PCM (Powertrain Control Module): While less likely, a problem with the PCM could also cause a speedometer malfunction. The PCM receives the speed signal and might not be processing it correctly. This is less probable than the other issues.
* Gear Selector Switch: In some vehicles, a faulty gear selector switch can interfere with the speedometer signal, although this is less common.
Troubleshooting Steps:
1. Check the VSS: This should be the first place to look. A mechanic can test the sensor's output voltage using a multimeter. Replacing the VSS is relatively inexpensive and easy (depending on your mechanical skills).
2. Inspect the Wiring: Carefully examine the wiring harness leading from the VSS to the instrument cluster. Look for any breaks, corrosion, or loose connections.
3. Test the Instrument Cluster: This is more difficult to test at home. A mechanic can usually test the cluster to determine if it's receiving the signal and functioning correctly. If the cluster is faulty, it is often more cost-effective to replace the whole cluster than to repair it.
4. Check for DTCs (Diagnostic Trouble Codes): Use an OBD-II scanner to check for any stored diagnostic trouble codes. This can provide clues about the problem. However, older vehicles like the '92 Seville might not fully utilize OBD-II codes, so this step could be less informative.
Recommendation:
Unless you have significant automotive experience, it's best to take your 1992 Cadillac Seville to a qualified mechanic for diagnosis and repair. They have the tools and expertise to properly diagnose the problem and determine the most cost-effective solution. Focusing on the VSS and wiring first is a good strategy for your mechanic.
