DTC P0500: Vehicle Speed Sensor (ALLISON A/T): Description
WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The Vehicle Speed Sensor (VSS) monitors the rotation speed of the transmission output shaft. The VSS assembly is a permanent magnet generator. It produces an AC voltage as the rotor teeth on the output shaft of the transmission, 2WD, or transfer case, 4WD, pass through the sensor's magnetic field. The AC voltage frequency and amplitude increase as the vehicle speed increases. The TCM will send a controlled duty cycle voltage to the Engine Control Module (ECM) that operates in a range of -4.5 volts or +5.0 volts. The ECM converts this signal into vehicle speed. As vehicle speed increases, the frequency of the duty cycle will increase.