Sensor, Knock: Operation
WARNING: This page is about a different car, the 2017 Jeep Cherokee. However, it is still accessible from the selected car via links, so may be relevant.
The molecules of a quartz crystal are affected by electrical polarization. In rest conditions (A) the molecules are not arranged in a particular way. When the crystal is subjected to pressure or to an impact (B), the higher the pressure, the more marked their arrangement. This arrangement produces a voltage at the ends of the crystal.