KNOCK SENSOR (KS)

General description 
      In a high compression ratio engines, the optimum ignition timing (at engine speeds higher than those of idling) is very close to the occurrence of engine knocking. This proximity means that there is a possibility of knocking at some point of the engine’s operating cycle, in some cylinders. Detonation may occur at any time and the onboard controller takes care of its control. During the combustion the onboard controller identifies the exact cylinder or cylinders with detonation.

Appearance
Fig. 1 shows some types of KS sensors.

Fig. 1

Principle of operation of the KS

      KS is a piezoelectric sensor installed on the engine block and reacts to the engine‘s sound vibrations. Sensor’s signal is converted into a voltage proportional to the detonation level and is fed to the onboard computer for further processing.
      Frequency of detonation is typically in the range of 6 kHz to 15 kHz. Onboard computer analyzes each cylinder’s detonation and uses a complex algorithm to compare its level with the average noise level of the preset past periods. If the noise exceeds the average level with a certain value, the onboard computer detects a detonation.
      Initially ignition timing is based on a reference value. When detonation is being detected in some of the cylinders, the onboard controller decreases the timing with a few degrees. After the detonation disappears, timing is being increased until it reaches its base value or until the next detonation occurs. This is a continuous process, providing optimal timing for each cylinder.

Procedure for verification of KS functionality

1. Connect the probe of an inductive stroboscope to the first cylinder
2. Connect the KS terminals to an AC voltmeter.
3. Start the engine and left it idling.
4. Lightly tap on the engine cylinder block close to the first cylinder.
5. Advance should seek to delay and the voltmeter should read a small voltage (about 1V).

Oscilloscope measurements of the KS

      If you have an oscilloscope, connect the active probe to the signal terminal of the knock sensor and the ground probe - to chassis ground.
Repeat the procedures, described above. While tapping you should observe the following waveform as in fig. 2:

Fig. 2

This is a typical waveform of a properly working KS. If you do not get fluctuations in voltage while tapping, replace the sensor.