What is a Vehicle Speed Sensor?
In order for systems like anti lock brakes, traction control, and the speedometer to function properly, the ABS control module and Powertrain control module must know the speed the wheels should be spinning, as well as the speed the wheels are actually spinning. To create the data the computer needs for the speed the vehicle should be spinning, the transmission output shaft speed, for most vehicles, is measured by the vehicle speed sensor. The vehicle speed sensor will detect the speed of the output shaft many times per second, and send that data to the necessary control units for further use.
How does a Vehicle Speed Sensor work?
As the output shaft for the transmission turns, a ring on the output shaft spins very closely to the vehicle speed sensor, allowing the sensor to read how fast the ring is spinning by features on the ring. The speed of the output shaft, then, is reported back to various control modules by a digital or analog signal, whichever the control unit is designed to interpret. This is used to show the vehicle speed on the speedometer, but also has applications for the anti-lock brake system and traction control system since the information can be compared to wheel and engine speed.
What are the symptoms of a bad Vehicle Speed Sensor?
When the vehicle speed sensor fails or the wiring harness prevents signal transmission, the speedometer is the first thing that will stop functioning properly. This can be an intermittent issue where the speedometer only works occasionally, the speedometer may cease all function, or the speedometer may read erratically or incorrectly. Also, the traction control and anti lock brake warning lights specific to each vehicle may be illuminated, depending on the design of those systems. You may notice the vehicle hesitate to accelerate, cruise control will not function properly, and the vehicle may not shift properly through all of the gears.