Rick's Free Auto Repair Advice

Test knock sensor

How to test a knock sensor

The testing procedure for knock sensor depends on knock sensorthe type of knock sensor used by the carmaker.

There are 3 types of knock sensors

• Inductive resonant knock sensor
• piezoelectric non-resonant sensors.
• Piezoelectric resonant knock sensor

Initial knock sensor testing

Test the knock sensor resistance

Using a multimeter, touch one probe to the connector on the knock sensor and the other to a metal portion of the body of the sensor. Read the resistance and compare to carmaker specs. If the resistance is out of spec or the test shows an open or short to ground, replace the sensor.

How to test a knock sensor

• Connect a multimeter probe to the knock sensor terminal and the other probe to a metal section of the body of the sensor.
• Using a large hammer or maul, exert strong taps on the engine block near the knock sensor (NEVER directly on the knock sensor itself). Monitor the voltage created by the hammer blow and compare to carmaker specifications.

Know the difference between the three types

Inductive and piezoelectric resonant sensors

Inductive and piezoelectric resonant sensors contain a vibration plate that responds to the same resonant frequency as a knock normally produces. When knocking occurs, the vibration plate converts the vibration into an electric signal through either an electromagnetic inductive effect or a piezoelectric effect.

Resonant sensors can only detect a particular frequency of engine knock due to their narrow-band response characteristics. So they’re referred to as narrow-band knock sensors.

Piezoelectric non-resonant knock sensor

A piezoelectric non-resonant knock sensor contains a spring-mass system to measure the vibration directly. So it has a much wider frequency bandwidth (5 kHz – 15 kHz). Carmakers use this type of knock sensor because the knock frequency can vary with the engine speed, so non-resonant sensors tend to perform better than resonant ones.

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