Most drivers have heard about the knock sensor, but what it is and what it is intended for have a poor and approximate idea. It turns out that this sensor, when subjected to deformation, i.e., mechanical impact on it, creates an electrical voltage. Taking into account this property of the knock sensor on a running engine, it serves as a kind of controller of its operation, recording sounds unusual for its normal operation, in particular, those arising from the detonation of a car engine.
Detonation, like other noises emitted by a running engine, has its own sound frequency, which is perceived by this sensor and sent to the electronic engine control unit (ECU). The ECU, in turn, having received a signal from the knock sensor, changes the ignition timing, setting it to a later time, and this will happen until the detonation stops. This sensor also reacts to other extraneous noise, such as the crackle of grenades, vibration of the engine protection, etc. There have been cases when the knock sensor picked up noise from the CV joint and it stopped the engine.
Signs of a malfunctioning knock sensor
During operation, the knock sensor can also accumulate faults, which can be identified by the following symptoms:
- the “CHECK ENGINE” signal lights up on the instrument panel;
- errors 03, 25, 26, 27 and 28 are displayed;
- deterioration of vehicle dynamics;
- increased fuel consumption due to non-optimal ignition timing.
Often, all this is associated with oxidized contacts, both on the sensor itself and on its connector, weak tightening of the knock sensor, broken wires or contacts of the wires suitable for the sensor, or breakdown of the sensor itself.
How to check the knock sensor yourself
You can start checking the serviceability of the knock sensor by lightly tapping it with a wrench or hammer with the engine idling. In this case, the engine must somehow react to this action, for example, reduce the speed, which indicates the serviceability of the sensor and the receipt of a signal from it to the ECU.
As an addition, you can connect a scanner of the “LMQ” type and see how the ignition timing changes during the actions performed. However, this method is not suitable for all cars.
For the second method of checking the sensor you will need multimeter and the usual key or hammer. We set the device to resistance measurement mode, connect it to the sensor and measure the resistance. If it is not a Lada, then the resistance should be around 500 kOhm. Then lightly knock on the sensor, the resistance should increase briefly and quickly return to its original position.
If the resistance has increased and does not return, this means that the sensor freezes and can be considered faulty. Such sensors can be the cause of an incomprehensible dip in traction.As for the AvtoVAZ sensor with its infinite resistance and setting the resistance to 2 MOhm on the multimeter, the device shows infinite resistance.
If a mechanical impact is applied to the sensor, its resistance decreases and then becomes infinite again.
The third and most correct way to check the knock sensor is based on the use of an oscilloscope. To do this, connect the oscilloscope terminals to the knock sensor, set the necessary parameters on it, take the same wrench and lightly tap the sensor. In this case, a sample oscillogram should appear on the device screen. The pulse received at the moment of impact on the sensor should have a clearly defined single peak and smooth attenuation. If there are several peaks or it has jagged edges, then it is better to replace such a sensor with a new one.
