1. Cable-Driven Speedometer (Most Likely): 1992 E-150s likely use a mechanical speedometer driven by a cable.
* Broken Cable: This is the most common cause. The cable runs from the transmission to the speedometer. It can fray, break internally, or become disconnected at either end. Solution: Replace the speedometer cable. This involves disconnecting it at both ends (transmission and speedometer), carefully routing the new cable, and reconnecting it. You'll likely need a specialized tool to connect the cable to the transmission.
* Speed Sensor in Transmission (VSS): While less likely with a cable driven system, a faulty VSS could affect the speedometer, often causing inaccurate readings or complete failure if it's integrated into the cable drive mechanism in any way. It's worth checking if the cable spins freely when the wheels are rotated. Solution: This usually means replacing the VSS. This requires mechanical skill and often involves dropping the transmission pan, so it's usually best left to a mechanic unless you have significant experience.
* Faulty Speedometer Head Unit: The speedometer itself could be malfunctioning internally. This is less common but possible. Solution: You might be able to repair it (if you're skilled with electronics), but it's often easier and cheaper to replace the entire speedometer gauge cluster (or just the speedometer face). Finding a used part in good working order might be your best bet here.
2. Electronic Speedometer (Less Likely but Possible): Some late-model 1992 E-150s might have had an electronic speedometer.
* Speed Sensor Issues: A faulty speed sensor (VSS), located somewhere in the drivetrain (often on the transmission or rear differential), sends a signal to the electronic control module (PCM), which then controls the speedometer. Solution: Replace the speed sensor. This often requires specialized tools and knowledge of the vehicle's electrical system.
* PCM Failure: A problem with the PCM itself is less likely but possible. This is far more complex and usually requires professional diagnosis and repair.
* Wiring Issues: Broken or corroded wires in the speedometer circuit are also possibilities. Solution: Carefully inspect all wiring related to the speedometer and repair or replace any damaged sections.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the speedometer cable (if applicable). Look for obvious signs of damage, such as fraying, kinks, or disconnections.
2. Check Cable Rotation (Cable Driven): With the van on jack stands and the wheels off the ground, rotate a wheel and see if the cable spins freely. If not, the problem is likely in the cable or VSS.
3. Test the Speed Sensor (Electronic): If it's an electronic system, you might need a multimeter to test the voltage output of the speed sensor. (This requires more technical skill).
4. Check Fuses and Relays: Make sure all fuses and relays related to the speedometer are functioning correctly.
Recommendation:
Unless you have mechanical and/or electrical experience, it's best to take your van to a qualified mechanic to diagnose and repair the speedometer. The cost of replacing a cable is relatively inexpensive, while misdiagnosis and attempting complex repairs can quickly become expensive. Provide the mechanic with as much detail as possible about the symptoms. Do the symptoms include anything beyond a non-functioning speedometer, like problems with the odometer? Knowing that helps the mechanic pinpoint the issue.
