Variable reluctance (VR) sensors, also known as mag (magnetic) sensors are built with a wire wound around a permanent magnet (pole) similar to a solenoid or DC motor. They do not require external power (considered passive) and have two leads, a signal and ground.

As the ferrous tooth approaches the sensor, changing magnetic flux induces a current which produces a voltage. When the tooth leaves the area, the magnetic field rapidly collapses producing an opposite voltage. This cycle produces an alternating current as seen by the blue line in Figure 2.

The ECU conditions this signal into a digital signal shown by the red line in Figure 2. This one cycle is interpreted as one tooth count. The magnitude of the voltage produced is directly proportional to how fast the magnetic field changes, therefore, the faster the engine spins the higher the peak voltage.