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Knock Sensor System Description

The knock sensor 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 knock sensor system to test for abnormal engine noise that may indicate detonation, also known as spark knock.

The knock sensor system uses a flat response sensor. The sensor uses a 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 knock sensor detects. The knock sensor is connected to the engine control module (ECM) by a signal circuit and a low reference circuit. Both knock sensor circuits are protected from electromagnetic interference by a shielding ground circuit. The shielding ground circuit is grounded through the ECM.

The ECM learns a minimum noise level, or back ground noise, at idle from the knock sensor and uses calibrated values for the rest of the engine speed range. The control module uses the minimum noise level to calculate a noise channel. A normal knock sensor signal is within the noise channel. As engine speed and load changes, the noise channel upper and lower parameters change to accommodate the normal knock sensor signal, keeping the signal within the channel. In order to determine which cylinders are knocking, the ECM only uses knock sensor signal information when each cylinder is near top dead centre of the firing stroke. If knock is present, the ECM detects that the signal is 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 always attempts to adjust back to a zero compensation level, or no spark retard. An abnormal knock sensor signal stays outside of the noise channel or will not be present. Knock sensors diagnostics are calibrated to detect faults with the knock sensor circuitry inside the ECM, the knock sensor wiring, or the knock sensor voltage output. Some diagnostics are also calibrated to detect constant noise from an outside influence such as a loose, damaged component, or excessive engine mechanical noise.