Saturday, March 1, 2014
What Are the Functions of a Speed Sensor on a Car
Unlike the first mass-produced cars of a century ago, cars today have advanced control systems that are computer dependent. Being computer dependent, more and more information about how the car is functioning goes to the on-board computer. The vehicle speed sensors are the gatherers of this information.
Vehicle Speed Sensor Description
Cars manufactured today are dependent on on-board computers. The computer receives the information it needs to have a car operate properly from strategically located vehicle speed sensors. Most vehicle speed sensors are of the permanent magnet type. They function similarly to a cam shaft or crankshaft sensor. The installed sensors are either in the transmission case or rear differential assembly. When a speed sensor fails, the cars check engine light comes on and a code goes to the computer and can be read at a service station.
Locating Speed Sensors
This code indicates which sensor has failed and what the problem is. Occasionally, a sensor will fail without sending a code. If the driveability of the car is suspect, say the power steering isnt working and the ABS brakes are stuttering, a mechanic can check the vehicles manual and quickly find if the same speed sensor is controlling both modules.
Speed Sensors Control Many Functions
Modules or functions that use information from vehicle speed sensors include the variable assist power steering unit where the information from a speed sensor regulates the power steering pressure with higher pressure at low speeds and vice versa. This makes things like parallel parking maneuvers easier.
A cars anti-lock braking system (ABS) also uses information from a speed sensor to decide when a wheel has locked up and thus commands the ABS to cut pressure in order for the driver to maintain control of the car. While some cars use separate sensors for the ABS, it is common that the computer uses the information from the wheel sensors to act as a vehicle speed sensor. For example, many newer Ford vehicles use a rear axle mounted speed sensor for ABS and vehicle speed functions that are computer related.
Both traditional and hybrid cars use information from vehicle speed sensors for the speedometer display.
Many new cars have developed suspension systems that use vehicle speed sensor information to control the height of the ride. They adjust the ride down for better control at higher speeds for improved maneuverability.
Automatic transmissions must have the information from a vehicle speed sensor to decide shift strategy.
Additionally, cruise control depends on speed sensor information to know when to engage and when to disengage, as well as when to cruise at a set speed.
All Electronic Engine Control (EEC) systems use some speed sensor information to perform their tasks.
