Knock Sensor

The knock sensor is located behind the engine block near the starter motor. The knock sensor is a piezoelectric flat response (wide resonant band) device that produces an AC voltage of different amplitude and frequency based on engine mechanical vibration. The amplitude and frequency are dependent upon the level of knock the sensor detects. The PCM learns a minimum noise level at idle from the knock sensor and uses stored normal noise level calibration values for the rest of the RPM band. The knock sensor signal is only used during the combustion event of the firing cylinder. When in a combustion event, the PCM filters the knock signal and compares it to the normal calibration noise level for that RPM. If the PCM has determined that knock is present during the combustion event, it will retard timing until the knock is eliminated. The PCM will always try to work back to a zero compensation level or no spark retard.