How Do Speed Sensors Work?

by Don Bowman


Speed sensors take the place of the speedometer gear in conventional speedometers. A conventional speedometer was mechanically driven by a cable in a housing that was spun by a gear in the tail shaft or rear of the transmission. This driven gear could be replaced with several different sizes to compensate for different size tires or a change in the final drive gear in the rear axle housing.


The speed sensor gives off an electrical signal when rotated that is used by the computer to calculate speed. This signal transferred to the speedometer to illustrate speed. It to can be re-calibrated to compensate for differences in tires and final gears. The speed sensor is situated in the tail housing of a conventional rear wheel drive vehicle. In front wheel drive vehicles, it is situated on the top-side of the transmission on the differential housing near the drive axles. It has an electrical connector.

What it Does

A vehicle speed sensor output is used in a fuel-injected engine for a variety of things. It will give the computer information as to speed and is also used by the computer for adjusting the shift levels or speeds on the transmission. It is used for cruise control and for distance measurement used in the odometer which indicates the elapsed mileage. The speed sensor is a signal generator of the magnetic type and produces a pulsing voltage at three miles per hour and higher. The input to the computer from the sensor is also used by the computer to determine when to lock the doors on some vehicles and is sensory input for various new stabilizing and handling technology.


When a speed sensor fails, the computer will turn the check engine light on. The speedometer will obviously not work and there will be a noticeable difference in the shift patterns in the transmission. The transmission shifting may not be affected on certain vehicles because some vehicles have separate speed sensors on the side of the transmission just for this purpose.


Replacing a speed sensor requires a 10mm wrench to loosen the hold down bracket. It can be seen on the topside of the transmission close to the drive axle. If it can not be seen from the top because other parts are blocking your view, you should be able to see it from under the vehicle. Remove the electrical connector and move it out of the way. Remove the 10mm bolt from the bracket holding it into the transmission. Remove the speed sensor by rotating slightly and lifting straight up. Replace it in reverse order of removal.

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).

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