Vehicle Speed Sensor: Description and Operation

PURPOSE
The Vehicle speed Sensor (VSS) sends a signal to the Engine Control Module (ECM), which the ECM converts to MPH. This sensor mainly controls the operation of the Torque Converter Clutch (TCC) system.

OPERATION
On models with conventional instrument panels and models with manual transmissions and digital clusters 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 (LED). As the speedometer cable mounted blade enters the LED 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 speed signal. This signal is sent to a buffer for amplification and conditioning, then to the controller. There are 2000 pulses per mile traveled.

On models with digital clusters and automatic transaxles, a transmission mounted VSS is used. The VSS is a permanent magnet generator consisting of a field coil and permanent magnet stator that produces AC voltage. The frequency of the AC signal is dependant on vehicle speed. When the vehicle is in motion a pulse is sent to the vehicle speed buffer at a rate of 4000 Pulses Per Mile (PPM). The speedometer is switched at a frequency of 4000 PPM, the ECM, cruise control module, and passive restraint module are switched at 2000 PPM.