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    Vehicle Speed Sensor


    A speed sensor. Courtesy of General Motors Corporation, Service Technology Group.

    A defective vehicle speed sensor may cause different problems depending on the computer output control functions. A defective vehicle speed sensor (VSS) may cause improper converter clutch lockup, improper cruise control operation, and inaccurate speedometer operation.

    • Prior to VSS diagnosis, the vehicle should be lifted on a hoist so the drive wheels are free to rotate.
    • Backprobe the vss output wire and connect the voltmeter leads from this wire to ground.
    • Select the 20-volt AC scale on the voltmeter.
    • Then start the engine.
    • Place the transaxle in drive and allow the drive wheels to rotate. If the VSS voltage signal is not 0.5 volt, or more, replace the sensor.
    • When the VSS provides the specified voltage signal, backprobe the VSS terminal at the PCM and repeat the voltage signal test with the drive wheels rotating.
    • If 0.5 volt is available at this terminal, the trouble may be in the PCM (powertrain control module).
    • When 0.5 volt is not available at this terminal, turn the ignition switch off and disconnect the wire from the VSS to the PCM.
    • Connect the ohmmeter leads across the wire. the meter should read zero ohm.
    • Repeat the test with the ohmmeter leads connected to the VSS ground terminal and the PCM ground terminal.
    • This wire should also have zero ohm resistance. If the resistance in these wires is more than specified, repair the wires.
    • The condition of a speed sensor can be checked by going through the diagnostic routines for the system.
    • If this test indicates that a sensor is faulty, the sensor should be replaced.
    • Speed sensors can also be checked with an ohmmeter. Most manufacturers list a resistance specification.
    • The resistance of the sensor is measured across the sensor's terminals.
    • The typical range for a good sensor is 800 to 1400 ohms of resistance.

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