Knock Sensor: Description and Operation

Fig 15 Knock Sensor:

KNOCK SENSOR-PCM INPUT
Engine uses a single knock sensor. The sensor monitors both the right and left bank of cylinders. The sensor mounts into the side of the engine block. When the knock sensor detects a knock in one of the cylinders, it sends an input signal to the PCM. In response, the PCM retards ignition timing by a scheduled amount.

Knock sensors contain a crystal which constantly vibrates and sends an input voltage (signal) to the PCM while the engine operates. As the intensity of the crystal's vibration increase, the knock sensor output voltage also increases.