Knock sensor

The purpose of this test is to evaluate the correct operation of an engine piezoelectric knock sensor when subjected to a simulated engine knock.

How to perform the test

  1. Connect PicoScope to the Knock Sensor.
    Note; connection at the control unit, if access permits, checks out the circuit for you.
  2. Minimise the help screen and with the Example Waveform on your screen PicoScope has already selected suitable scales for you to capture your waveform.
  3. Select GO on your screen or press the space bar to see live data.
  4. Simulate an engine knock by tapping lightly on the engine block with a metallic object and observe your screen.

    Note; if access is difficult this simulation can be carried out by tapping on the sensor with the sensor removed from the engine.

Example waveform

Waveform notes

The Example Waveform shows an initial spike as the sensor is tapped, followed by short oscillations as the output from the piezo crystal dissipates.

Waveform Library

There are 8 examples in the Waveform Library. Go to the drop down menu bar at the lower left corner of the Waveform Library window and select knock Sensor.

knock sensor

Figure 2 - A typical knock sensor

Technical information

A motor vehicle petrol engine is expected to produce a good power output with minimal fuel consumption and exhaust emissions.  It is therefore important that the mapping of the ignition advance curve is as near to detonation, Knocking/pinking as possible. The optimal point at which the spark plug ignites the air/fuel mixture is just before detonation occurs. It is inevitable that at certain times and under certain conditions detonation will occur. The frequency of the vibration caused by this is about 15 kHz.

Detonation is damaging to the engine and if unchecked it will cause hot spots within the combustion chamber which will lead to component failure.

To avoid such situations, a knock sensor is fitted to some management systems. The sensor is small piezoelectric device that, when coupled with the Electronic Control Module (ECM) internal detonation control system processor, can identify the 15 kHz signal associated with knock and retard the ignition timing accordingly.

The ECM control circuit continues to monitor the frequency delivered by the sensor on subsequent engine rotations and gradually releases the timing retardation until the ignition timing is back at its original mapped setting.

An engine running on a retarded ignition setting will have a noticeable lack of power and over an extended period of time may also cause overheating in the exhaust system.

It is also a possibility that mechanical faults will produce vibrations that can be interpreted by the ECM as detonation.

Detonation occurs if any of the following are present.

  • Very high combustion temperatures.
  • Over-advanced ignition timing.
  • Lean air/fuel ratio causing high temperature.
  • Carbon deposits pre-igniting the air/fuel mixture.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Small Crocodile/Gator Clips


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • 2 Pin AMP connector breakout lead


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Knock