Q: My speedometer does not work

The speedometer is controlled by the PCM (Powertrain Control Module). The PCM receives data from the ABS module then sends it to the instrument cluster. In all, there are three modules, and a VSS (Vehicle Speed Sensor) involved with the operation of the speedometer. This sort of design requires a professional level scanner to access all the modules in order to pinpoint the failure.

The data from each module needs to be observed to give us a clue to what is going on here. In addition, there is usually a code associated with systems that utilizes modules in this fashion. This code will give us a diagnostic path to begin with. It may not be the correct path, but often times a process of elimination needs to be employed to diagnose a vehicle.

The possible problems are:

• Wiring harness issues between the modules and or the VSS.
• PCM
• Instrument Cluster. If the speedometer gauge itself is faulty, the instrument cluster as a whole will need to be removed and repaired. The instrument cluster should have a test procedure that can be initiated with a shop level scanner.

I would have included the ABS module and the VSS, but it appears you have already replaced those. It is possible that the parts you replaced are faulty, particularly if they are used. For this reason I highly recommend good quality parts.

If this is to daunting, as it will be for most, try booking an appointment with one of our certified technicians for a speedometer inspection.

Good Luck!