Common Causes:
* Speed Sensor (Vehicle Speed Sensor - VSS): This is the most common culprit. The VSS measures the speed of the vehicle's transmission output shaft and sends this information to the instrument cluster. A faulty VSS can result in an inaccurate or non-functioning speedometer, as well as other problems like the cruise control not working and incorrect odometer readings. This sensor is usually located on the transmission.
* Wiring/Connectors: Damaged, corroded, or loose wiring between the VSS and the instrument cluster can interrupt the signal. Check the wiring harness for breaks, chafing, or corrosion, especially near the transmission and the instrument cluster itself. Loose connectors are also a common problem.
* Instrument Cluster (Speedometer Gauge): The speedometer itself could be faulty. This is less common than a VSS issue, but a malfunctioning gauge can cause the needle to not move. Repairing or replacing the entire instrument cluster is usually expensive.
* PCM (Powertrain Control Module): Less likely, but possible. The PCM receives the speed signal and processes it. A faulty PCM could prevent the signal from reaching the instrument cluster. Diagnosing a PCM issue usually requires specialized tools.
* Fuse: While less frequent, a blown fuse related to the instrument cluster could be the problem. Check your owner's manual for the correct fuse and test it.
Less Common Causes:
* Transmission Issues: Severe transmission problems can sometimes affect the VSS's ability to accurately measure speed.
Troubleshooting Steps (in order of easiest to hardest):
1. Check the fuses: This is the simplest and quickest check.
2. Inspect the wiring and connectors: Look for damage, corrosion, or loose connections between the VSS and the instrument cluster.
3. Check the Vehicle Speed Sensor (VSS): This is likely the most common cause. Often you can test its output with a multimeter, but this requires some automotive electrical knowledge. Replacing it is relatively inexpensive.
4. Check the instrument cluster: This usually requires professional diagnostic tools to test if the gauge is functioning correctly.
5. Inspect the PCM: This is the most complex and requires specialized diagnostic equipment.
Recommendation:
Start with the simplest checks (fuses and wiring). If those are fine, the VSS is the most likely suspect. If you're not comfortable working with car electronics, take your Concorde to a qualified mechanic for diagnosis and repair. They have the tools and expertise to pinpoint the exact problem quickly and efficiently. Remember to mention the symptoms clearly – a completely non-functioning speedometer, as opposed to just an inaccurate one.
