S15/T15 2WD P/U V6-262 4.3L VIN Z (1992)
Vehicle Speed Sensor Signal Buffer: Description and Operation
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.
