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How do knock sensors fail?

How do knock sensors fail?

What is a knock sensor?

The knock sensor is used to detect engine noise. In general, knock occurs when:

  • The engine compression ratio is too high;
  • The intake temperature is rising;
  • Self-ignition makes the spark plugs too hot;
  • The engine is running out of fuel due to excess air.

Knock overheats the engine and clogs the cooling system. It also leads to increased temperatures in the fuel injection system, intake pipes, or carburetor.

Effects of knock

Knock promotes premature wear of engine components. The parts most susceptible to knock are

  • Pistons;
  • Valves;
  • Cylinder head;
  • Cylinder head gasket.

What are the symptoms?

You may notice the following symptoms when the knock sensor is in poor condition:

  • The engine light on the instrument panel illuminates. You must take this warning seriously. Have your vehicle diagnosed and stop driving.
  • You hear loud noises while driving. Normally, the engine makes a deafening noise. The noise will get louder if you don't fix the problem. As a result, the ignition of the air-fuel mixture in the combustion chamber increases.
  • Fuel consumption is abnormal. There are many reasons for increased fuel consumption. It's best to call a professional to determine the source of the problem.
  • The car doesn't accelerate normally. Even when the pedal is depressed, acceleration is not rapid. In fact, the knock sensor is preventing it from moving forward.
  • Engine performance decreases due to damage to certain parts inside the engine. Your car starts to shake and slide. You also smell something burning inside your car.

Causes of a faulty knock sensor

The causes of failure can be as follows:

  • Internal short circuits
  • Open circuits
  • Lead short circuit
  • Mechanical damage
  • Poor mounting
  • Corrosion

To test the knock sensor

Testing with a multimeter

Testing a knock sensor is possible, but complicated. Its piezoelectric technology makes it an "active" sensor. You can start by checking the signal continuity with a multimeter to rule out any open circuit problems that could be caused by a break in the sensor itself or a wiring harness/connection problem.

Step 1: Connect the ohmmeter between the knock sensor connector and the disconnected ECU connector.

Step 2: With the ohmmeter connected and the ECU connector disconnected, check for the absence of DC to ground on each pin of the harness connector. Goal: 30 MOhm minimum.

Hot engine oscilloscope test

Step 1: Connect oscilloscope test probes between the ECU knock sensor pin and ground.

Step 2: The oscilloscope should provide a signal with a significant increase in amplitude.

Step 3: If the signal is not discernible, tap lightly on the engine block near the sensor.

Step 4: If the click is not detected, this indicates a faulty sensor or circuit.

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