What Causes A Knock Sensor To Go Bad? (Testing Ways)

The electronic control of the gasoline engine involves a group of sensors that constantly transmit information to the controller. But there is one additional element that is in sleep mode and gives an alarm at a certain moment when the air-fuel mixture knocks in the combustion chambers. This phenomenon greatly damages engine parts, significantly accelerating the wear of pistons and cylinder heads. Therefore, it is advisable for drivers who repair cars with their own hands to know what causes a knock sensor to go bad and how to test the knock sensor when characteristic symptoms of a malfunction appear.

What Causes A Knock Sensor To Go Bad? (Testing Ways)

What is Engine knock?

Gasoline engines are designed for a certain rate of combustion of the air-fuel mixture in the cylinders, corresponding to the degree of compression. When the fuel burns too slowly, the engine loses power significantly. If the burning rate in the chambers exceeds the calculated one, knock combustion occurs as a microexplosion, accompanied by a sharp release of a large amount of energy in a closed space.

Symptoms of engine knock are vibration and ringing of the piston pins caused by the shock wave from the combustion chamber. The reason is the use of gasoline with a lower octane rating than that provided by the engine design. How does the engine knocking occurs:

  1. A low-octane combustible mixture, designed for combustion at a pressure of 8–9 bar (conditionally), is compressed by a piston up to 12 bar.
  2. From the spark, the mixture ignites and begins to burn at high speed. A shock wave is formed, accelerating combustion, which, in turn, amplifies this wave. Knock occurs.
  3. The result is a strong blow to the piston crown, which is transmitted to the connecting rod and crankshaft. All moving parts of the motor are exposed to destructive vibration.

Important! Knock combustion is often confused with the “pseudo-diesel” effect – spontaneous ignition of fuel without a spark at the spark plug electrodes.

The latter occurs due to the formation of additional sources of high temperature in the chambers a hot layer of soot. Then the mixture ignites itself – from compression and contact with this source. The engine continues to run even with the ignition off.

How does knock sensor work?

The engine is unable to constantly work in knock mode the cylinder-piston group will collapse very quickly. Therefore, the knock sensor is responsible for detecting the primary shock wave in the chambers and alerting the electronic control unit, which takes further measures to prevent micro-explosions.

How does knock sensor work?

There are three ways to reduce the burning rate of the mixture:

  • lower the compression ratio (previously, additional gaskets were placed under the cylinder head);
  • organize a flash a fraction of a second later, that is, reduce the ignition timing;
  • do not fill in gasoline with a low octane number and low knock resistance.

Of the listed methods, only one is available to the controller – adjusting the lead angle. When the owner of the car fills in unsuitable or low-quality fuel, a shock wave is formed in the chambers from burning too quickly. The knock sensor is located outside on the wall of the cylinder block, registers the emerging microexplosions, and supplies a voltage pulse to the control unit. The latter reacts and reduces the lead angle making the ignition late.

The combined action of the controller and the knock sensor saves engine parts, but degrades driving performance. After refueling, the car starts to “stupid” and spend more gasoline – these are not symptoms of a faulty sensor, but the result of pouring low-octane fuel and a sign of the correct operation of the meter. The problem goes away by itself when you use bad fuel and refuel with good quality.

Types of knock sensors used in engines

Gasoline engines of modern cars are equipped with two types of knock sensor resonant and broadband. The first, more common type, is configured to register narrow-range shock waves, that is, each element is designed for a specific engine model. Broadband devices are versatile but more expensive and less common.

Knock sensor operate on the same principle and consist of the following elements:

  • housing with a hole or stud for attachment to the cylinder block;
  • vibration plate;
  • piezoelectric element;
  • insulating layer;
  • ballast resistance (on broadband devices).

The sensor operation algorithm is simple: when a knock occurs, the plate vibrates and acts on the piezoelectric element. It generates a high-voltage pulse that is transmitted to the controller. The stronger the explosion of the fuel mixture in the chamber, the higher the pulse potential. The control unit makes the ignition later and the car accelerates more slowly, no matter how hard the driver presses the accelerator pedal.

Conversely, a faulty knock sensor causes a loud clatter of the piston pins because the controller does not correct the advance angle for low-quality fuel. In this case, the health of the motor depends only on the motorist and how sharply will press the gas.

The meter fits snugly to the cylinder block and does not respond to other engine vibrations, but only to detonation processes. The sensor is considered a very reliable element and sometimes lasts longer than the power unit itself. Many motorists who have been operating cars for decades have not even heard of their existence.

what causes a knock sensor to go bad?

It is rather difficult to clearly identify all the causes of a knock sensor to go bad due to the “sleep” mode of operation of the element. If you constantly fill the tank with high-quality fuel and carefully operate the car, then you may not find out about a failed device soon.

The main symptom of a malfunction is the appearance of a knock as such. If, when you sharply press the accelerator, a ringing sound of piston fingers is heard from the motor, then the electronic control unit no longer corrects the ignition. Most likely the problem is with the sensor.

Reference. It often happens that the failure of the knock meter cannot be determined by the specialists of the service station. This is where the qualifications of the craftsmen come into play.

test knock sensor manually by ears

The first diagnostic method is performed “by ear” on a running warm engine in the following sequence:

  1. Provide access to the element so that it can be touched by hand. If necessary remove the air filter and a branch pipe.
  2. Start the engine and let it run at idle speed of the crankshaft.
  3. Take a small metal object and gently tap on the sensor body, listening to the power unit.
  4. If knocking on the device causes a change in engine operation, the element is probably working. Otherwise, the device must be dismantled to check further.

Sometimes a completely working sensor is unscrewed as a result of vibration and does not fit snugly against the cylinder block, and therefore is unable to detect knock. Make sure that the device is securely fastened before knocking on the case and drawing conclusions.

test knock sensor with multimeter

A more accurate diagnosis is to check the knock sensor with a multimeter. The method is based on the principle of operation of the device the generation of an electrical impulse by a piezoelectric element from the impact of a vibrating plate. 

Steps to test knock sensor with multimeter:

  1. Disconnect the wiring harness and remove the part from the cylinder block.
  2. Set the multimeter to measure the maximum voltage of 200 mV and connect the clamps to the sensor leads.
  3. Take a small open-end wrench and tap the cell body (without fanaticism) while observing the display readings.
  4. Ideally, voltage surges measured in tens of millivolts should be observed. The stronger the impact, the greater the value will show the working sensor. There is no reaction – buy a new spare part.

In the process of diagnostics, do not forget about the serviceability of the transmission link contacts and wiring. If the knock sensor is operational, but the sound of fingers is still heard when the accelerator is pressed, ring the supply cable and clean the oxidized contacts.

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