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Knock Sensors

What It does

Mounted directly on to the side of the cylinder block, they “ listen” for engine pinking and send an oscillating voltage signal to the ECU. The ECU uses this Information to control the Ignition timing.

Pinking is a phenomenon caused by the explosion in the combustion chamber taking place too early. When this occurs the flame front within the combustion chamber collides with the still rising piston causing the characteristic pinking or pinging noise which sounds like many marbles dropping onto a steel plate. In mild cases it results in decreased engine power, in severe cases it results in major engine damage.

The more advanced the ignition timing is set on an engine the higher the theoretical power, up to a limit. The limit is generally just before the point of pinking. It is the job of the Knock Sensor, in conjunction with the ECU, to keep the Ignition timing at this peak setting.

The ECU will advance the Ignition timing until pinking is detected then retard it by, say 10 degrees, this process is repeated many time per second. Some ECUs have the capability of advancing and retarding the timing individually per cylinder, that is, a four cylinder engine may have four different ignition advance settings. The timing setting achieved is a variation on a timing “Map” within the ECU, which takes into consideration parameters such as engine speed, load and temperature.

The knock sensor can also have a secondary role. The point at which pinking occurs is also the point at which peak NOx emissions are generated. By accurately controlling the timing, the emissions of an engine can be reduced.

How They Work:

The sensing element consists of a piezo-ceramic element and a seismic mass which is clamped into place by the locating bolt. The sensor has a specific exciting frequency which is matched to the frequency vibration band within which pinking occurs.

Note:

Knock sensor tend to be very engine specific. Ensure that full vehicle Information is given when ordering a replacement.

When the sensor detects a “Noise” within this frequency it generates a small electrical signal (A damped sine wave signal of about plus and minus 2 volts). This signal is amplified and used by the engine management ECU accordingly.

The sensor is located in a position on the engine where the Knock can be detected across all cylinders. One sensor is generally used per four cylinders, so a 8 cylinder engine will have 2 sensors.

Reason For failure:

There are 3 main causes of knock sensor failure;

  • Connector wire faults between the sensor and the ECU, due to corroded connectors or wire failure.
  • Internal failure of the sensor due to the environment in which it operates.
  • Corrosion between the sensor and its mating surface causes the sensor to lose sensitivity.

Testing:

In the case of the first two examples, testing is relatively simple. Warm the engine up, monitor the timing and tap the sensor with a small metal object such as a spanner. The timing should retard, continue tapping and the timing will remain retarded, stop tapping and the timing should advance back up again.

You could perform a variation of this test by plugging the sensor into an oscilloscope and tap it with a spanner; you should be rewarded by a Blip in the output signal. This test could be carried out at the ECU end of the wiring loom; this will also check the condition of the interconnecting wires etc.

The condition of the spark plugs can also give clues to pinking. Premature wear / erosion and unusual noids / deposits over the electrodes are clues.

When a sensor fails, the driver is usually alerted by the problem by the illumination of the “Mil” lamp. Symptoms when driving the car include lack of performance and pinking noises (especially up hills with a hot engine). You should be aware that pinking can destroy an engine within seconds if severe, so ignore the warning signs at your peril.

Also Note:

The sensor is dependent upon the correct tightening torque being applied when fitting; please check the manufacturer’s workshop manual. Under no account should washers or any type of sealant be used when fitting the sensor.
For Further technical Information concerning KNOCK SENSORS Please contact our technical Help line: 01780729030.


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