Types of Wheel Speed Sensors
Wheel speed sensors are a necessary ABS component and sensor input used to inform the ABS control module of wheel rotational speed. There are two "types" of sensors found on today's vehicles. The conventional passive WSS consists of a magnetic pin with fine copper wire wrapped around to create an alternating magnetic field. This sensor changes
polarity as a toothed metal ring called a tone ring passes through its magnetic field. This tone ring is attached to a part rotating at the same speed as the wheel, such as an outboard CV joint or steering knuckle on a front wheel drive vehicle or the axle or differential of a rear wheel drive vehicle. It depends on the type of vehicle.
When the signal is affected by a weak sensor, damaged tone ring, or debris, a false cycling problem may 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.
The 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. The ABS control module monitors all wheel speed inputs. If one wheel starts to rotate slower than the others the system will reduce hydraulic pressure to that wheel allowing it to regain its traction with the road surface.
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. The digital WSS is also more accurate at slow speeds. This type of sensor is capable of detecting the direction the wheel is turning. These newer braking systems offer the controller hill holding and hill descent control.
