1. Vehicle Speed Sensor (VSS): This is the most common cause. The VSS sends a signal to the instrument cluster based on wheel rotation. A failing VSS can send erratic signals, causing the bouncing needle.
* Location: The VSS is typically located on the transmission, often near the tailshaft housing. Its exact location may vary slightly depending on the transmission type. Consult a repair manual specific to your truck's transmission (e.g., Getrag, NV4500).
* Testing: You'll need a multimeter to test the VSS. The repair manual will give you specifics on testing the signal voltage and resistance. A faulty VSS usually shows intermittent or incorrect readings.
* Replacement: Relatively inexpensive and easy to replace if you're mechanically inclined. Make sure to get the correct VSS for your transmission.
2. Wiring and Connectors: Corrosion, broken wires, or loose connectors in the wiring harness between the VSS and the instrument cluster can interrupt the signal, leading to erratic speedometer readings.
* Inspection: Carefully inspect the wiring harness for any damage, corrosion, or loose connections, paying particular attention to the area around the VSS and the instrument cluster.
* Repair: Repair or replace damaged wiring and connectors. Use dielectric grease on connectors to prevent future corrosion.
3. Instrument Cluster: The problem might reside within the speedometer itself in the instrument cluster. A faulty component inside the cluster could cause the erratic reading.
* Testing: This is more difficult. You'd ideally need access to a diagnostic tool to check the signal reaching the instrument cluster. Simply swapping the cluster with a known good one (if you have access to one) would be the most efficient test.
* Repair: Repairing an instrument cluster is often expensive and may require specialized tools and knowledge. Replacement might be more practical if testing confirms a faulty cluster.
4. Speedometer Cable (if applicable): While less likely on a 1990 Cummins (which likely uses an electronic VSS, not a cable), it's worth a quick check if there's a mechanical speedometer cable. A frayed or damaged cable can produce erratic readings.
5. PCM (Powertrain Control Module): While less probable, a faulty PCM can also contribute to speedometer issues. However, this is generally indicated by other problems as well. Diagnosis typically requires professional-level diagnostic equipment.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the VSS and its wiring harness for obvious damage.
2. VSS Testing: Use a multimeter to test the VSS according to your vehicle's repair manual.
3. Wiring Inspection: Carefully inspect the wiring harness for breaks, corrosion, and loose connections.
4. Instrument Cluster Check: If the VSS and wiring check out, suspect the instrument cluster. Consider a replacement or professional diagnostic testing.
Important Notes:
* Repair Manual: Obtain a repair manual specific to your 1990 Dodge Power Ram with the Cummins engine. This will provide invaluable information on component locations, testing procedures, and specifications.
* Safety: Disconnect the negative battery terminal before working on any electrical components.
* Professional Help: If you're not comfortable working on your vehicle's electrical system, take it to a qualified mechanic.
The bouncing needle strongly suggests a problem with the signal itself (VSS and/or wiring) rather than a purely mechanical issue. Start with the simplest and most likely cause (VSS) and work your way through the list.
