Each wheel is equipped with a magnetic sensor mounted a fixed distance from a toothed ring which rotates with the wheel. The sensors are replaceable but not interchangeable; each must be fitted to its correct location. The toothed rings are replaceable although disassembly of the hub or axle shaft is required.
The wheel speed sensor is designed to produce a voltage directly proportional to the speed of the wheel. In other words as the speed of the wheel increases, so does the voltage. The voltage should not fluctuate when the wheel speed (speedometer) reading is steady.