1. Speed Sensor Issues (Most Common):
* Vehicle Speed Sensor (VSS): This sensor, often located on the transmission or transfer case, measures the vehicle's speed and sends a signal to the speedometer. A faulty VSS is the most frequent cause of speedometer failure. Check the sensor for damage, corrosion, or loose wiring. You may need a multimeter to test its output.
* VSS Wiring: Examine the wiring harness connecting the VSS to the instrument cluster for broken wires, loose connections, or corrosion. A break anywhere in the circuit will prevent a signal from reaching the speedometer.
2. Instrument Cluster Problems:
* Speedometer Cable (If equipped): Older vehicles sometimes use a cable-driven speedometer. If your Wrangler has a cable, it could be broken, frayed, or improperly connected. Inspect its entire length for damage. (Less likely on a '91, as they generally transitioned away from mechanical systems).
* Instrument Cluster Failure: The speedometer itself could be faulty. This is less common but possible. Sometimes a simple repair might be possible, but often requires replacing the entire instrument cluster.
* Fuse/Circuit Breaker: A blown fuse or tripped circuit breaker supplying power to the instrument cluster could also be the culprit. Check your owner's manual for the relevant fuse or breaker.
3. Other Potential Issues:
* Gearbox Problems (Less Likely): Severe issues within the transmission or transfer case *could* affect the VSS signal, but this is less likely to be the sole cause of speedometer failure.
* PCM/ECU Problems (Less Likely): The Powertrain Control Module (PCM) or Engine Control Unit (ECU) processes signals from the VSS. A faulty PCM is a less likely cause but could contribute if it's not properly interpreting the VSS signal.
Troubleshooting Steps:
1. Check the fuses and circuit breakers.
2. Locate the Vehicle Speed Sensor (VSS). Consult a repair manual or online resources to find its location for your specific model. Visually inspect it for damage.
3. Inspect the VSS wiring harness for breaks, corrosion, or loose connections.
4. If you have a multimeter, test the VSS for proper output. (You'll need a repair manual or online resource to find the proper testing procedure).
5. If the VSS and wiring check out, investigate the instrument cluster itself.
Important Note: A repair manual specific to your 1991 Jeep Wrangler will be invaluable in diagnosing this problem. It will show you exactly where the VSS is located, how to test it, and the wiring diagrams. Without a manual, troubleshooting will be much more difficult.
