Every wheel has a speed sensor that sends to the control module a small AC electrical signal that is crucial for the proper working of the ABS. To get accurate signal generation all the wheels and tires needs to be of the same size and kind; any difference produces the improper signal. The contactless sensor, which can be used to measure the wheel's rotating speed, contains a permanent magnet and a coil; gear pulsars (Tone Rings) are installed on all rotating parts. These pulsars rotate and change the magnetic flux through the coil and produce voltage that follows the frequency and is taken by the ABS control to measure the speed of every wheel. As with other parts of the car, power should be off when uninstalling or installing the sensor, special care should be taken when dealing with the airbag system wiring harness to avoid accidental deployment. They include lifting the car off the ground, disconnecting the sensor's wires, sometimes unbolting some brackets or other parts, and then pulling out the sensor. Aligning the sensor requires careful torque settings of the mounting bolts, proper harness routing without twists and crimps and after that the harness must be connected tightly. After folding the wiring, the use of the ABS checker will allow the signal of the wheel speed sensor to be checked. For inspection, a non-metallic feeler gauge is used to check the gap between the sensor and the pulser; the gap is to be passed at various points to be between 0.020"-0.040" (0.5-1.0mm).