Vehicle Speed Sensors

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 ECU's 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 ECU's 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

4. Cruise control

5. Distance traveled

6. Deceleration

The PM (permanent magnet) VSS circuit can be tested using a scan tester or digital volt meter while driving the vehicle. It also can be checked for proper resistance using a digital ohmmeter. A typical resistance value would be 300 to 700 Ohms*. It can also be tested for proper output by connecting a digital volt meter across the two terminals on the VSS while turning the drive shaft. A typical specified output would be 1.5 to 6.5 volts at 20 mph*.

* Specifications vary. See service manual for correct specifications.




Another type of VSS is the optical VSS. This type is used by GM. It is located inside the speedometer head and is attached to the back of the instrument cluster. It consists of a photo cell and L.E.D. mounted in a housing, which is permanently wired to the buffer circuit (the buffer/L.E.D./photocell is an assembly). A spinning mirrored reflector with two blades completes the system.

The L.E.D. is lighted any time the ignition switch is turned on. As the speedometer cable turns, the reflector blades pass through the light beam from the L.E.D. twice each revolution. The light beam is reflected back to the photo cell. The photo cell generates an electrical signal to the buffer, which indicates the vehicle speed. The buffer then sends a digital (on/off) signal to the computer which interprets the number of pulses per mile as vehicle speed.

Caution: As simple as this system seems to be, trouble codes associated with VSS problems utilize other sensor inputs. Refer to troubleshooting guides for proper diagnosis!