Rick's Free Auto Repair Advice

Knock sensor — operation and testing

Knock Sensor — How to diagnose

How does a knock sensor work?

A knock sensor has a piezoelectric crystal inside that converts vibrations into an AC voltage. The greater the vibration, the higher the AC voltage and frequency. It’s used in engines to detect detonation (knock).

The knock sensor can’t discriminate between vibrations from cylinder detonation or vibrations caused by hitting a pothole or rumble strip. So the car’s ECM also monitors the crank sensor. During detonation, there’s a noticeable drop in that cylinder’s contribution to engine rotation. If there’s voltage from the knock sensor but no drop in crankshaft rotation speed, the ECM will ignore the knock signal.

How to test a knock sensor

Set your meter to AC volts and connect to the sensor. Strike the area around the knock sensor using a blunt chisel and hammer. Look for a small change, as little at 0.006 volts. Or, yank the fuel pump fuse, start the engine and watch your meter as it stalls. During the stall the engine will run lean as pressure drops and that can cause a knock.

How to replace a knock sensor

Before you remove the old sensor, notice it’s knock sensororientation. The new one must be installed facing the same direction. In addition, look up the torque spec before tightening and use a torque wrench.

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