There are at least two distinct speed sensors on a vehicle. The first is a magnetic signal generator that, when spun, generates an electrical pulse that is used by the computer. The speed sensor on a conventional rear-wheel-drive transmission is located in the rear tail housing of the transmission. It is driven by a gear located in the tail housing. On a front-wheel-drive vehicle, the speed sensor is located on top of the differential housing on the transmission, close to the drive axle.
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Transmission Speed Sensor
The speed sensor is used to indicate the speed of the vehicle and for various other handling and braking systems within the vehicle. It also, in a wide variety of vehicles, controls the transmission shift points.
ABS Speed Sensors
The second type of speed sensor is incorporated on all vehicles that have ABS brakes. These ABS systems have a computer delegated to the ABS, a set of four speed sensors located near the wheels and an actuator downstream from the master cylinder that controls the circuit. The wheel sensors are a hall-effect type sensor mounted to the spindle. The rotor or driveshaft has a star wheel attached to it that rotates on the inside of the wheel sensor, which gives off an oscillating signal that is sent to the ABS computer. The signal gives the speed of each individual wheel.
The computer, in response to any difference in signal from a particular wheel in a hard braking situation, will engage or disengage the brakes many times a second to prevent a skid or uncontrollable spin. It does this through the use of an ABS module. The ABS module has valves to each of the wheels. When the ABS system is in operation on braking, you may feel the pedal shake or bounce. The ABS system is an electrically driven system, and should a light become illuminated on the dash indicating a failure in this system, the fuses should be checked first. The fluid level is also important and should be checked. If both are in order, a wheel speed sensor is at fault. They must each be checked individually or with a scan tool that shows the wheel speed on each individual wheel and any significant difference between them.