The Vehicle Speed Sensor

The PM (permanent magnet) VSS is usually located on the transmission case in the speedometer cable opening. It will either replace the cable (for electronic speedometers) or mount on the transmission in series with the cable (for mechanical speedometers).
The permanent magnet vehicle speed sensor (VSS) consists of a permanent magnet generator, which produces a pulsing (ac) voltage when it spins. Three miles per hour is approximately the speed at which the ECU recognizes its output. The voltage level and number of pulses increase with vehicle speed.
Since ECUs cannot read ac voltage easily, a buffer module is often used to amplify and convert it to a digital (on/off) signal which can be read easily by the ECU. The buffer module also sends the signal to the electronic cruise control and electronic speedometer systems.
Newer GM, Ford and Chrysler ECUs amplify and convert the ac signal to digital internally and no buffer module is needed.
The computer uses VSS input to operate the following:
1. The idle air control valve
2. The canister purge system
3. The torque converter clutch

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