The computer in your vehicle, (ECU) makes decisions to adjust parameters based on sensor input.
One of the sensors is a speed sensor or a VSS. Some will work if speedometer fails, but most will not due to safety related issues.
The problem could be the sending unit, generally on the transmission, the connection between the VSS and speddometer or the speedometer itself.
Let me know if I can help in any way.
Thursday, December 2nd, 2010 AT 11:24 AM