Wheel Speed Sensor: Description and Operation
Wheel sensors and pulse wheel
On the front wheels, the pulse wheel is pressed onto the outer driveshaft and the wheel sensor is located in the spindle casing radially relative to the
On the rear wheels, the pulse wheel is pressed onto the hub and the wheel sensor is located axially relative to the pulse wheel. The rear pulse wheels
can be exchanged.Both the front and rear pulse wheels have 48 teeth.
The wheel sensor's task is to provide the control module with information on the wheel rotation speed. When a wheel rotates, the teeth on the pulse
wheel induce a voltage in the wheel sensor coil. This generates an AC voltage signal whose frequency and voltage depend on the number of teeth
passing per second. The frequency and voltage increase as the speed of rotation increases. The voltage induced is normally approximately 300mV AC
when the wheel is rotating at 1 turn per second. By recording the frequency, the control module can calculate the wheel's acceleration and retardation.
If there is a fault in any wheel sensor, ABS and TRACS will be disconnected or not engaged in the first place. However, EBD (Electric Brakeforce
Distribution) remains on the whole time. Using the signals from the wheel sensors, the control module calculates the car's road speed.