Speed Sensor Buffer

The Vehicle Speed Sensor Buffer (DRAC) processes inputs from the Vehicle Speed Sensor (VSS) and outputs signals to the Speedometer, Electronic Control Module (ECM), Cruise Control Module and the Rear Wheel Antilock Brake (RWAL) Module.

The VSS is made up of a 40-tooth rotor and a coil mounted on the transmission. As each rotor tooth nears the coil, the coil produces an AC voltage pulse. Each revolution of the rotor produces 40 AC voltage pulses. As the vehicle speed increases, the number of AC voltage pulses per second increases.

The Vehicle Speed Sensor Buffer (DRAC) takes the voltage pulses from the VSS and uses them to open and close four solid-state output switches to ground at a rate proportional to vehicle speed.

The Speedometer switch opens and closes at the same rate as the VSS pulses. The ECM and Cruise Control switches open and close at half the rate of the VSS pulses. The RWAL switch opens and closes at a rate seven times the VSS pulses.

The Vehicle Speed Sensor Buffer (DRAC) is matched to the vehicle based on transmission, final drive ratio and tire size. It is important to ensure that the correct Vehicle Speed Sensor Buffer (DRAC) is installed in the vehicle if replacement is necessary.