Knock sensor

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Channel A

  • Plug a test lead or BNC test lead into Channel A on the PicoScope.
  • Fit a black clip on the test lead's black (negative) connector.
  • Fit a  multimeter probe on the test lead's red (positive) connector.
  • Place the black clip on the battery negative terminal.
  • Probe the knock sensor's output terminal with the multimeter probe. (The sensor has two terminals: earth return and output.)

Alternative connections can be made using the TA012 two-pin test lead adapter, with both terminals connected directly to the BNC test lead. The connections to the knock sensor are illustrated in Figure 1.

  • Test the sensor by gently tapping the unit's body. This 'excites' the crystal and produces a small voltage.

It may help to remove the sensor and monitor its output on the workbench. When testing in this way it may be necessary to fix short fly leads to the sensor. Connect two small clips to the BNC test lead and clip them on the two wires. If you see an inverted picture, reverse the two connections.

Note: When refitting the sensor, tighten to the correct torque setting as overtightening can damage the sensor.

Example waveform

Knock sensor waveform notes

As the response of the sensor is very fast, the scope must be set to an appropriate time base. In the example waveform, this is 50 ms per division (a total of 500 ms across the screen). The voltage range should be set to −5 to +5 volts. The best way to test a knock sensor is to remove the it from the engine and tap it with a small spanner. The resultant waveform should be similar to the example shown.

knock sensor

Figure 2 - A typical knock sensor

Technical information

A typical engine in a modern motor vehicle is expected to produce a good power output with minimal fuel consumption and exhaust emissions. Given these factors, it is important that the mapping of the ignition advance curve is as near to detonation (pinking) as possible. As the optimal point at which the spark plug ignites the air/fuel mixture is therefore just before pinking occurs, it is inevitable that at certain times and under certain conditions knock will occur. The frequency of the vibration caused by knocking (pinking) is about 15 kHz.

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 Unit's (ECU's) internal knock control system processor, can identify the 15 kHz signal associated with knock and retard the ignition timing accordingly.

The ECM compensates for knock by retarding the ignition timing (causing it to fire later). The knock sensor then listens for knock on subsequent engine rotations and gradually releases the timing retardation until the ignition timing is back at its original mapped setting.

Knock 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

AT021-3

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

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £7.00

  • Small Crocodile/Gator Clips

    £5.00

  • PicoScope Battery Clip

    £5.00

  • Large Dolphin/Gator Clips

    £10.00

  • 2 Pin AMP connector breakout lead

    £42.00

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

    £48.00

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

    £179.00

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Guided test: Knock