There are two forms of wheel speed sensors currently used: magnetic inductive, additionally known as passive sensors and magneto resistive, or active wheel speed sensors. Both of sensors do a similar job, however they function in completely different ways. Both function effectively within their own ways, but the more modern active wheel sensor is normally regarded as the more reliable.

Passive sensors have been around since the early days of the anti-lock brake system (ABS). These sensors function on the generator principle. The speed sensors use the toothed tone wheels to monitor and offer the anti-lock brake module (ABM) with wheel speed information. The actuator is a toothed tone wheel that rotates with the individual wheel. Each tooth on the tone wheel acts as an actuator for the wheel speed sensor. As the tone wheel rotates, one’s teeth go in and from the proximity of the sensor. The effect is an alternating current (AC) voltage that’s generated in the sensor coil by magnetic lines of force fluctuating as the tone passes by the magnetic sensor.

The output of the wheel speed sensor can vary from vehicle to vehicle due to: winding type, air gap, magnetic strength of sensor, metal properties of the tone wheel and wheel speed. Unfortunately these kind of passive systems have been prone to false cycling, which is the word used to describe an ABS cycle despite the fact that the road conditions do not dictate the need because of this cycle. This condition is quite susceptible to happen at slower speeds. This will happen due to wheel damage or rust build-up on the tone wheel. wind sensor An incorrect sensor air gap may cause this condition, together with debris on the wheel speed sensor tip.

Active wheel sensors have been around in use since 1999. This style of speed sensor helps to increase performance, durability and low speed accuracy. Active sensors do not appear to have the same false cycle problem of passive sensors do. Most vehicles with active sensors still use a toothed tone wheel which acts because the trigger mechanism for the sensor. Some vehicles work with a magnetic encoder in place of a normal tone wheel. The encoders have north/south pole magnets imbedded into the ring. The ring is then pressed on the axle shaft as being a tone wheel. In any case the result is really a digital square wave signal.

With this system, the ABM sends battery voltage to the speed sensor to power it up. The sensor, subsequently, supplies the ABM a continuing 7 milliamp (mA) signal on a signal return circuit. Depending on the tone ring or magnetic encoder position, this 7 mA signal is fired up or off. The output of the sensor delivered to the ABM is a Direct Current (DC) voltage signal with changing voltage and current levels. The ABM monitors the changing digital signal from each wheel speed sensor and is interpreted as wheel speed.