Vehicle Speed Sensor: Description and Operation

PURPOSE
The Vehicle Speed Sensor(VSS) signal is used by the Powertrain Control Module (PCM) to calculate vehicle speed in mph.


OPERATION
The VSS is a variable reluctance sensor mounted in the rear axle housing. It monitors rear axle speed by sensing teeth on an exciter ring mounted on the ring gear. As teeth pass the sensor pole piece, an AC voltage with a frequency proportional to average rear wheel speed is induced. This signal is sent to the Programmable Speedometer/Odometer Module (PSOM). The PSOM processes this input signal information and relays it to the powertrain control module and the speed control module. This signal tells the powertrain control module the vehicle speed in miles per hour. The PCM uses this signal as an input in determining shift scheduling and Electronic Pressure control.




SYMPTOMS:
^ Harsh engagements
^ firm shift feel.
^ abnormal shift schedule; unexpected downshifts may occur at closed throttle.
^ abnormal torque converter clutch operation or torque converter clutch engages only at wide-open throttle.
^ May flash transmission control indicator lamp.

DIAGNOSTIC TROUBLE CODES:
- 29 (two digit)
- 452 (three digit).