Fix It

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.

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.

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.

AC compressors can vary in location depending on if its for an auto or residential application. An auto AC compressor is connected to a vehicles engine. A residential AC compressors is usually located outside the home encased within a sealed unit.

An auto AC compressor has two sides which are vital to how it functions. A suction side allows gases produced from refrigerant to be pulled in. The compressor then compresses the gases and uses its discharge side to transfer them for use to other parts of the system.

A residential AC compressor is a pump which brings in refrigerant gas as well. In a residential compressor, this refrigerant gas is brought in through a large suction line. The compressor then uses compression to change the refrigerant into a highly pressurized gas which is then sent to be used in other parts of the system.