Rick's Free Auto Repair Advice

Knock sensor

What does a knock sensor do?

A knock sensor is really just a microphone that listens for abnormal engine noise caused by preignition or detonation. The sensor is tuned to a discrete frequency range so it doesn’t false alarm from normal engine noise. Most sensors use a piezoelectric material that generate a voltage when compressed.

Why does the engine need a knock sensor?

To get maximum power from each combustion event, engineers have determined that the combustion should reach maximum for at 14° after top dead center (TDC). To make sure the air/fuel mixture reaches maximum force at that point, the ignition system initiates the spark early so the flame front can grow. The timing of the spark varies depending on engine RPM, so it may occur at a few degrees before top dead center (BTDC) at idle but advance to as much as 20°  BTDC at 3,000 RPM. The car’s ECM watches the values from the knock sensors to make sure the combustion events occur as planned.

Types of knock sensors?

Knock sensors can be screw-in or bolt-on. If you’re installing a bolt on style sensor, torque is critical, so always use a torque wrench. The sensor can be mounted on the side of the engine block or in the cylinder head.

 Images of 2 types of knock sensors and where they're located on the engine

Posted on by Rick Muscoplat

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