GM 30020773 Sensor,Vehicle Speed

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The Vehicle Speed Sensor (VSS) is an important component in vehicles from 1992 through 1998 and 1999 and later models. In the earlier models, the VSS is integrated into the speedometer and consists of a reed switch and magnet. The magnet's rotation with the speedometer cable causes the reed switch to turn on and off, generating a pulsing voltage signal that is sent to the PCM to calculate vehicle speed. If a failure occurs, a code 24 is set. In the later models, the VSS is located on the side of the transmission or transfer case and consists of a Hall effect element, magnet, and gear. As the sensor gear is turned by a drive gear, the signal voltage switches back and forth in a square-wave form, with the frequency increasing in proportion to vehicle speed. This signal is sent to the PCM to control various devices, and a P0500 diagnostic trouble code is set if there is a problem in the VSS circuit. To check the VSS, specific steps are followed depending on the model year. For 1992 through 1998 models, an ohmmeter is used to test the resistance between certain terminals while turning the speedometer. If the VSS doesn't operate as described, the instrument cluster is replaced. For 1999 and later models, a voltmeter is used to test the voltage between terminals while rotating the right rear wheel. If the VSS doesn't operate as described, it is replaced. The replacement process also differs between the two model ranges. In 1992 through 1998 models, the instrument cluster is replaced as a whole, while in 1999 and later models, the VSS is replaced by disconnecting the negative battery cable, raising the vehicle, unplugging the electrical connector, removing the VSS and old O-ring, and installing a new O-ring and tightening the VSS retaining bolt.