Operation

Knock sensors







The knock sensor produces an output voltage in proportion to mechanical vibration caused by the engine. A sensor is located in each cylinder bank between 2/4 and 3/5 cylinders. The ECM calculates if the engine is knocking due to camshaft and crankshaft sensor signals regarding the position of the engine in the cycle. The ECM can also work out exactly which cylinder is knocking and retards the ignition on that particular cylinder until the knock disappears. It then advances the ignition to find the optimum ignition timing for that cylinder. The ECM can adjust the timing of each cylinder for knock simultaneously. It is possible that all eight cylinders could have different advance angles at the same time. If the camshaft sensor fails, the knock sensor will continue to work, but as the engine may be running one revolution out of sychronisation the ECM may retard the wrong cylinder of the pair e.g. 1 instead of 6. If the knock sensor fails engine knock will not be detected and corrected. The fault is indicated by illumination of the malfunction indicator light (MIL) on North American specification vehicles.