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How to Check Transmission Speed Sensors

by Jeremy Holt

A vehicle speed sensor is a permanent magnet generator attached to the transmission under the vehicle. The sensor monitors several different engine and transmission functions and sends the information to the on-board computer. It is triggered by the turning of the transmission shaft, sending a signal which increases or decreases in frequency with the vehicle's speed. This information is then relayed to the speedometer. If you suspect you have a problem with the speed sensor in your vehicle, you can check its operation with the help of a voltmeter.

Determine Whether the Speed Sensor is Receiving an Input from the On-Board Computer

Raise the front of the vehicle and then set it down to rest securely on jack stands. Chock the rear wheel to prevent movement. Locate the speed sensor attached to the rear section of the transmission.

Follow the wire from the sensor and disconnect it from the wiring harness where it is attached to the frame.

With the ignition on but the engine not running, push the probes of a voltmeter into the reference wires in the connector. If no voltage is recorded, there may be a problem with the input signal from the on-board computer. Have the vehicle checked by a dealer service department.

Determine Whether the Speed Sensor is Defective

Reconnect the harness and turn the ignition off. Remove the electrical connector from the speed sensor, and then remove the bolt that secures the sensor to the transmission housing. Gently withdraw the sensor from the transmission.

Place the sensor on a bench and check the pulsing AC voltage with the voltmeter as you slowly turn the gear by hand. If there is no voltage, the sensor may be defective.

Install the new sensor into the transmission and replace the retaining bolt. Connect the electrical terminal and then test the new speed sensor by starting the engine. If the sensor is operating correctly the "Check Engine" light should no longer be illuminated.

Items you will need

About the Author

Based in Middletown, Delaware, Jeremy Holt has worked as a writer, copy-editor and proofreader for MBNA Corporation, Rahli Inc. and Kreativz.com. since 1998. Born and educated in the United Kingdom, he holds a Bachelor of Science degree in psychology from the University of Glamorgan.

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