Q: What is the purpose of the Vehicle Speed Sensor and how can it be checked and replaced on Nissan Frontier?A: The Vehicle Speed Sensor is a permanent magnet generator mounted on the transmission case. It produces an AC voltage sine wave, the frequency of which is proportional to vehicle speed. The PCM uses the sensor input signal for several different engine and transmission control functions. The Vehicle Speed Sensor signal also drives the speedometer on the instrument panel. A defective Vehicle Speed Sensor can cause various driveabilily and transmission problems. The PCM can detect sensor problems and set trouble codes to indicate specific faults. To check, remove the Vehicle Speed Sensor from the transmission. Connect a voltmeter to the two terminals of the Vehicle Speed Sensor, set the meter on the AC scale and spin the sensor drive gear by hand. The sensor should generate approximately 0.5 volts AC. If no AC voltage signal is produced, replace the sensor. If the Vehicle Speed Sensor is good, check for continuity between the sensor connector and the instrument cluster. If the wiring is good, have the instrument cluster and PCM diagnosed by a dealer service department or other qualified repair facility. To replace, raise the vehicle and support it securely on jackstands. Disconnect the electrical connector from the Vehicle Speed Sensor. Remove the hold-down bolt and clamp and remove the Vehicle Speed Sensor from the transmission. Inspect the 0-ring on the sensor and replace it if damaged. If you are installing a new sensor, use a new 0-ring. Installation is the reverse of removal.