Vehicle Speed Sensor: Description and Operation

VSS Buffer Amplifier
On vehicles with conventional instrumentation, a vehicle speed sensor (VSS) buffer amplifier is used to supply the vehicle speed signal to the cruise control module. On this type system, a speedometer frame mounted optic head is used to pick up light reflected by a speedometer cable mounted blade. The reflected light is produced by a light emitting diode (L.E.D.). As the speedometer cable mounted blade enters the L.E.D. light beam, the light is reflected into the optic head. From there, the light enters a photocell in the optic head and produces a low power signal. This signal is sent to a buffer for amplification and conditioning, then to the controller.

Digital Ratio Adapter Controller
On vehicles with electronic instrumentation, a digital ratio adapter controller (DRAC) is used to supply the vehicle speed signal to the cruise control module. On this type system, the DRAC receives vehicle speed signals from a transmission or transfer case mounted vehicle speed sensor. The DRAC is calibrated according to axle ratio and tire combination, and provides 40 electronic pulses per revolution of the transmission or transfer case output shaft.