Knock sensor

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

How to perform the test

View connection guidance notes.

  1. Use manufacturer's data to identify the knock sensor output circuit.
  2. Connect PicoScope channel A to the Knock Sensor output circuit.
  3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  4. Start the scope to see live data.
  5. Simulate an engine knock by tapping lightly on the engine block with a metallic object and observe your screen.
  6. With your waveform on screen stop the scope.
  7. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

Knock sensors may have one, two or three connection terminals. One is the output the second a separate earth terminal and a third terminal for cable shielding.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • An initial spike as the engine block is tapped, followed by reduced oscillations as the ringing and output from the piezo crystal dissipates.

Waveform Library

Go to the drop down menu bar at the lower left corner of the Waveform Library window and select, Knock sensor.

Further guidance

Automotive knock sensors fall into two type categories;

  • Resonant – one terminal, earth via engine block or cylinder head.
  • Flat response – two terminals, separate earth terminal usually at ECM.

Both are piezo crystal devices and both can be tested by PicoScope in the same way. They convert vibration/sound via pressure on the piezo crystal into a voltage.

Resonant sensors are the earlier development of knock sensor and are mechanically tuned to a particular and usually narrow frequency. Primarily based on cylinder diameter. They have a slightly higher spike voltage when tested. As the name suggests they continually resonate with the tuned peaks causing ECM reaction.

Flat response knock sensors have evolved with the development of electronics within manufacturer ECM’s. The flat response sensor is able to detect a wider range of vibration from the engine, dependent on ECM tuning. As a result of monitoring the sensor output rather than having to react to the sensor an ECM is able to exercise much closer control of ignition. This type of sensor necessitates the use of an open circuit detection resistor which is built into the sensor. Some sensors, dependent on application, may be found with three terminals. Terminal three carries a shielding cable for circuits one and two.

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.

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

Should the ECM detect engine knock it will retard the ignition timing until, with the resonant sensor the knock stops, or with the flat response sensor the vibration returns to within tuned parameters.

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. It is also vitally important that knock sensors are installed following the specific manufacturer torque specifications.

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.

Diagnostic trouble codes

Selection of Diagnostic Trouble Codes (DTCs):

P0324 - Knock Sensor (KS) Module Performance

P0325 - PCM Knock Sensor Circuit

P0326 - Knock Sensor Circuit Excessive Spark Retard

P0327 - Knock Sensor Circuit Low Voltage

P0328 - Knock Sensor 1 Circuit High Input (Bank 1 or Single Sensor)

P0329 - Knock Sensor 1 Circuit Intermittent (Bank 1 or Single Sensor)

P0330 - Knock Sensor (KS) Circuit Bank 2

P0331 - Knock Sensor 2 Circuit Range/Performance (Bank 2)

P0332 - Knock Sensor 2 Circuit Low Input (Bank 2)

P0333 - Knock Sensor 2 Circuit High Input (Bank 2)

P0334 - Knock Sensor 2 Circuit Intermittent (Bank 2)

P1602 - Knock Sensor (KS) Module Performance

GT021-5

Disclaimer
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

  • Premium 6-way breakout lead set

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  • Back-pinning Probe Set

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  • Flexible Back-pinning Probe

    £3.00

  • Large Dolphin/Gator Clips

    £10.00

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

    £47.00

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Guided test: Knock sensor