Vehicle Speed Sensor: Description and Operation

On models with conventional instrument panels, 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 speed signal. This signal is sent to a buffer for amplification and conditioning, then to the controller.
On models with electronic speedometers or electronic instrumentation, a transmission mounted Vehicle Speed Sensor (VSS) is used. This sensor utilizes a permanent magnet (PM) to generate vehicle speed information to the controller. Some models use a buffer amplifier to modify the sensor output. The vehicle speed sensor (VSS) is located on the transmission. The VSS is a permanent magnet generator consisting of a field coil and permanent magnet stator that produces AC voltage. When the vehicle is in motion an impulse is sent to the vehicle speed buffer. The information is then sent to the speedometer, engine control module, cruise control module, and passive restraint module at a rate of 4004 or 2002 pulses per minute.