Spark Knock Control

The PCM uses the knock sensor to determine when spark knock exists and can retard timing up to a maximum of 19°. 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 dependant 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 TDC 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 on the next firing cylinders until the knock is eliminated. The PCM will always try to work back to a zero compensation level or no spark retard.

If knock is present, the PCM will increment three counters that can be read on the Scan tool. The LOW MID and HIGH SPARK MODIFIERS represent three different RPM bands the PCM uses to store knock retard degrees. The Scan tool also displays the actual amount of spark retard degrees as SPARK RETARD CYL #1-4. If excessive spark knock is detected, the retarding of timing will cause a reduced power condition.