Knock Sensor: Description and Operation

The functioning element of the knock sensor is a piezoelectric crystal. Detonation (knocking) causes vibrations in the engine which cause the crystal to generate small voltage signals. This signal is used by the ignition control computer to correct the ignition timing.
When knocking is detected, the ignition control computer will retard the timing. If the knocking stops, the timing will be advanced in .5° steps until it reaches its normal programmed value. If the knocking continues, ignition timing can be retarded as much as 12°.