The speedometer receives signals from the Vehicle Speed Sensor located on top of the differential trans axle.
Quite often the VSS fails causing erratic speedometer needle funtions. However since the problem also occurs even at idling, apart from the VSS, it is possible the ground circuit or meter cluster is faulty.
Testings would have to be done to determine the cause. To avoid complicated testings etc, get a replacement VSS and install to test. If it is the cause, you would save yourself a lot of time doing the diagnostics.
Wednesday, January 27th, 2010 AT 11:43 AM