Most Likely Culprits:
* Poor Connections: The most common cause of intermittent electrical issues is loose, corroded, or damaged connections. Vibration from hitting the dashboard might temporarily improve a bad connection, explaining why the speedometer sometimes works. Check the connections at the instrument cluster itself (located behind the dashboard), as well as the connectors for the vehicle speed sensor (VSS) and the fuel level sender. These connections can be prone to corrosion and wear over time.
* Instrument Cluster Failure: The instrument cluster itself might be failing. Internal components like the speedometer stepper motor or the circuit boards can become damaged, leading to intermittent readings. This is a more significant repair, potentially requiring replacement or repair of the instrument cluster.
* Vehicle Speed Sensor (VSS): A faulty VSS is a common cause of speedometer problems. It provides the signal to the instrument cluster to show the vehicle's speed. A failing VSS may provide an intermittent or incorrect signal. This sensor is usually located in the transmission.
* Wiring Harness Issues: Damage or breakage within the wiring harness leading to the instrument cluster can also cause intermittent problems. This could be from wear, rodent damage, or previous repairs.
Troubleshooting Steps:
1. Check Connections: Start by disconnecting and carefully inspecting all the connections mentioned above (instrument cluster, VSS, fuel level sender). Look for corrosion, broken wires, or loose terminals. Clean any corrosion with a wire brush and electrical contact cleaner. Reconnect securely.
2. Inspect the Wiring Harness: Visually inspect the wiring harness leading to the instrument cluster for any signs of damage, such as cuts, fraying, or exposed wires.
3. Test the Vehicle Speed Sensor (VSS): This might require a multimeter and some knowledge of automotive electrical systems. The VSS sends a specific signal based on vehicle speed; an incorrect or absent signal indicates a problem. A mechanic or auto parts store can usually test this sensor.
4. Check for Error Codes (OBD-II): Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the vehicle's computer. This might provide clues to the underlying problem.
5. Instrument Cluster Testing: Testing the instrument cluster is more complex and might require professional tools or replacement. If the problem persists after checking connections and sensors, this is likely the next place to investigate.
Important Considerations:
* Safety: A malfunctioning speedometer is a safety concern. Avoid driving extensively with a non-functional speedometer.
* Professional Help: If you're not comfortable working on car electrical systems, it's best to take the vehicle to a qualified mechanic for diagnosis and repair. Incorrect repairs can lead to more serious problems.
The intermittent nature of the problem makes pinpointing the exact cause difficult without direct inspection. Start with the simpler steps (checking connections) before considering more advanced diagnostics or replacement parts.
