The knock sensor (KS) system enables the engine control module (ECM) to control the ignition timing for the best possible performance while protecting the engine from potentially damaging levels of detonation. The ECM uses the KS system to test for abnormal engine noise that may indicate detonation, also known as spark knock.

The knock sensor (KS) system uses a flat response 2-wire sensor. The sensor uses piezo-electric crystal technology that produces an AC voltage signal of varying amplitude and frequency based on the engine vibration, or noise, level. The amplitude and frequency are dependant upon the level of knock that the KS detects. The engine control module (ECM) receives the KS signal through a signal circuit. The KS ground is supplied by the ECM through a low reference circuit.

The ECM learns a minimum noise level, or background noise, at idle from the KS and uses calibrated values for the rest of the RPM range. The ECM uses the minimum noise level to calculate a noise channel. A normal KS signal will ride within the noise channel. As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the KS signal, keeping the signal within the channel. In order to determine which cylinders are knocking, the ECM only uses KS signal information when each cylinder is near top dead center (TDC) of the firing stroke. If knock is present, the signal will range outside of the noise channel.

If the ECM has determined that knock is present, it will retard the ignition timing to attempt to eliminate the knock. The ECM will always try to work back to a zero compensation level, or no spark retard. An abnormal KS signal will stay outside of the noise channel or will not be present. KS diagnostics are calibrated to detect faults with the KS circuitry inside the ECM, the KS wiring, the KS voltage output, or constant noise from an outside influence such as a loose/damaged component or excessive engine mechanical noise.