Wheel Speed Sensors
Wheel speed sensors are a necessary ABS component and sensor input. It is used to inform the ABS control module of rotational wheel speed. There are two types of sensors found on today's vehicles, the passive and the active. The conventional passive sensor consists of a magnetic pin with fine copper wire wrapped around to create a magnetic field. This sensor changes polarity as a toothed metal ring, called a tone ring passes through its field. This tone ring is attached to a part rotating at the same speed as the wheel, such as an outboard CV joint of a front wheel drive or the axle or differential of a rear wheel drive vehicle.
When the signal is affected by a weak sensor, damaged tone ring, or debris a false cycling problem will result. False cycling is unwarranted activation of the ABS system, usually at slow speeds and at the very end of a stop cycle. This traditional type of PM sensor, similar to those implemented on crankshaft and camshaft systems had a few pitfalls. These include limited operation at low speeds and the inability to operate in reverse.
A passive sensor creates an AC signal that changes frequency as the wheel changes speed. The ABS control module converts this AC signal to a digital signal for interpretation. It monitors all wheel speed inputs. If one wheel starts to rotate slower than the others, the system will reduce hydraulic pressure to that wheel until it regains traction.
The newer active sensor creates a digital signal. This type of digital wheel speed sensor uses a hall effect or a variable reluctance signal with a square wave pattern. The sensor may have two or three wires. One of which is used as a voltage power supply from the ABS control module. A digital WSS is more accurate at slow speeds. It's also capable of detecting which direction the wheel is turning.