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Knock Sensor Diagnostics

The optimal point at which the High Tension (HT) ignites the Air/Fuel mixture will be just before pinking occurs, it would appear inevitable that at certain times and under certain conditions knock (detonation) will occur in a std production map.

A knock sensor is fitted to some management systems, the sensor being a small piezo-electrical device that, when coupled with the Electronic Control Modules (ECM), can identify when knock occurs and retard the ignition timing accordingly.

The frequency of knocking (pinking) is approximately 15 KHz. The response of the sensor is very fast. The best way to test a knock sensor is to remove the knock sensor from the engine and to tap it with a small spanner.

NOTE :- when refitting the sensor tighten to the correct torque setting as over tightening can damage the sensor.

Technical Information

A typical engine found in today's modern motor vehicle will be expected to produce a good power output with minimum fuel consumption and exhaust emissions. Given these factors, it is important that the mapping of the ignition advance curve is as near to detonation (pinking) as possible. At the optimal point at which the spark plug ignites the Air/Fuel mixture will therefore be just before pinking occurs, it would appear inevitable that at certain times and under certain conditions knock (detonation) will occur.

To avoid such situations happening a knock sensor is fitted to some management systems, this sensor being a small piezo-electrical device that, when coupled with the Electronic Control Module's (ECM) internal knock control system processor, can identify the frequency at which knock occurs and retard the ignition timing accordingly.


The frequency of knocking (pinking) is approximately 15KHz. When the ECM compensates for knock the ignition timing as previously mentioned will be retarded (firing later), the knock sensor will then "listen" for knock on subsequent engine rotations and gradually release the timing retardation until the ignition timing is back at its original mapped setting.

A typical knock sensor.

Knock will occur if any or a combination if the following is present :-

Very high combustion temperatures

Over advanced ignition timing

Lean Air/Fuel ratio (causing high temperature)

Carbon deposits pre-igniting the Air/Fuel mixture

Testing of the knock sensor is quite simple using either a stroboscopic timing light or an oscilloscope. To test using the timing light, simply shine the stroke onto the ignition timing marks and tap the block/cylinder head close to the sensor; at this point the timing mark will move, indicating a response to the operators actions. Alternatively, the output voltage can be seen on an oscilloscope by connecting to the two electrical terminals and tapping the component.