The external VSSB/DRAC (Vehicle Speed Sensor Buffer / Digital Ratio Adapter Controller) module was used in 1989-1995 General Motors vehicles in many applications. Before this point a cable-drive speedometer was used, while afterwards the DRAC was incorporated into the speedometer itself, then later the vehicle computer unit.
The DRAC module is programmed to account for different tire circumferences and gear ratios. At the factory the correct external module could then be quickly installed while using identical guage clusters, speedometers, and engine control modules. It is usually located near the ECM, or behind the glove compartment, or under the dashboard. In the same fashion if gear rations or tire sizes are changed, the module may be quickly swapped or reprogrammed for the correct ratios.
The unit takes input from the transmission VSS (Vehicle Speed Sensor) to derive pulses per mile, according to the rotation of the transmission tailshaft. It then processes this information to account for the given parameters and outputs the correct adusted pulse quantity per mile to the speedometer and vehicle computer.
Programming the DRAC module will be necessary if changing tire diameter or final gear ratios. Otherwise the speedometer will not read correctly and the engine computer will not sense the correct vehicle speed.
After locating and removing the module from the vehicle, carefully open the plastic case and remove the circuit board.
The programming is accomplished by connecting or disconnecting a series of 7 resistor circuits. These 7 circuits can create any combination necessary shown the chart below. In order to determine which circuits to connect, first complete the calculation shown and then cross-reference the result in the chart. Then connect or disconnect the proper circuit pins as indicated by soldering (or unsoldering) a lead between pairs of pins.