Knock Sensor: Description and Operation

Knock Sensor Construction:

The knock sensor is mounted to the engine block, just below the cylinder head. The sensor is comprised of a piezo-ceramic crystal sandwiched between two electrical contacts with a fixed weight on top. When a knock occurs in the engine, the resulting vibrations cause the crystal to be compressed. When compressed, the internal structure of the crystal is stressed. This stress on the crystal produces a low voltage signal which is read by the knock control unit. The knock control unit then filters the signal and passes to the DME control unit. The DME unit then retards timing according to built in "maps" and gradually steps advance back to the point where knocking occurred.