The speed sensor in a vehicle controls a number of different systems and can noticeably affect the way a car will run. The speed sensor can be on the transmission, differential, transaxle or speedometer head. A bad or malfunctioning speed sensor on a vehicle can cause a wide variety of problems ranging from an inconsistent speedometer to what appear to be transmission problems.
Service Engine Light
If the speed sensor has gone out on your vehicle, the check engine light, or service engine light, will come on, indicating there is a problem. Depending on your vehicle, it may trigger other service lights.
A primary function of the speed sensor is to tell how fast the vehicle is travelling. A vehicle with a broken or malfunctioning speed sensor will also probably be experiencing a malfunctioning speedometer. The speedometer may "jump," or jerk back and forth, even though the vehicle's speed does not change, and may stop working all together.
Speedometer problems also commonly lead to problems with the cruise control in a number of vehicles. If the vehicle's computer, or brain, cannot tell how fast it is travelling it will be unable to regulate the cruise control.
A malfunctioning speed sensor may cause problems with a vehicle's automatic transmission, depending on how it is malfunctioning.
If the speed sensor is sending incorrect information to the vehicle's computer about how fast the vehicle is travelling, the vehicle may switch gears inappropriately, causing jerking and other assorted issues. The issues will depend on whether the vehicle is adapting for going faster or slower than it actually is. Either way, a bad speed sensor can wear out or damage a transmission if left unrepaired.
A vehicle with a completely dead speed sensor may stall because the vehicle is not sending the right signals to the computer and does not adjust properly to idle.