1. The Obvious First Steps:
* Fuses: Check the fuses related to the instrument cluster and speedometer. Your owner's manual will show you the fuse box diagram.
* Wiring: Inspect the wiring harness leading to the speedometer. Look for broken wires, loose connections, or corrosion, especially near the connections at the instrument cluster and the vehicle speed sensor (VSS). Pay particular attention to areas that might experience vibration or flexing.
2. The Speed Sensor (VSS):
* Location: This is the most common culprit. The VSS is a magnetic sensor usually located on the transmission. Its exact location varies slightly depending on the transmission type (e.g., NV4500, Getrag 360). You'll need to consult a repair manual or online resources specific to your transmission to find its location.
* Testing: You can test the VSS with a multimeter. The procedure involves checking for continuity and sometimes voltage depending on the sensor design. A repair manual is highly recommended for this, as incorrect testing can damage the sensor or other components.
* Replacement: If faulty, the VSS needs replacement. Be sure to get the correct part for your transmission.
3. The Instrument Cluster:
* Internal Failure: The speedometer itself within the instrument cluster might be faulty. This is less common than a VSS problem but can occur.
* Testing: Testing the instrument cluster is more complex and usually requires specialized equipment or an understanding of electronics. Often, it's easier and cheaper to replace the cluster (if you can find a good used one).
* Repair: Some instrument clusters can be repaired, but this often involves soldering and requires experience with electronics.
4. Gear Driven Speedometer (Less likely on a '92):
While '92 Cummins trucks generally use electronic speedometers, there's a slight chance yours uses a cable-driven system. If so, the cable itself could be broken, stretched, or disconnected.
5. PCM (Powertrain Control Module):
In some cases, a faulty PCM (the truck's computer) can affect the speedometer signal. This is less likely, but possible. Diagnosing a PCM problem usually requires specialized equipment.
Recommendations:
* Repair Manual: A repair manual specific to your 1992 Dodge Cummins is your best friend. It will give you diagrams, location information, and testing procedures.
* Online Forums: Search online forums dedicated to Dodge Cummins trucks. Others have likely encountered the same problem. Posting details about your truck (transmission type, etc.) will help you get specific advice.
* Professional Mechanic: If you're not comfortable working on your truck's electrical system, take it to a qualified mechanic.
Start with the simplest checks (fuses and wiring). If those are good, the VSS is the most probable next cause. Always disconnect the negative battery terminal before working on any electrical components.
