Rick's Free Auto Repair Advice

What is a knock sensor — How It Works And Why It’s Used

Carmakers install a knock sensor to detect pre-ignition and detonation knock

A knock sensor is built with a piezoelectric crystal inside that converts engine knock vibrations into an AC voltage. The greater the vibration, the higher the AC voltage and frequency. It’s usually bolted on the side of the engine block or in the valley of a V-style engine.

Why detecting vibration is important

Pre-ignition can damage pistons

Engines can develop carbon buildup in the combustion chamber

preignitiuon damage

Piston damaged caused by the high heat of pre-ignition

and that carbon can glow hot enough to ignite the air/fuel mixture before the spark plug fires. Then, when the spark plug first, you wind up with multiple flame fronts right as the piston is reaching top dead center. This causes a knocking sound referred to as “ping.” Extended ping operation can damage the pistons by burning a hole right in the top of the piston.

Pre-ignition can also occur in high compression engines that require high octane fuel but were filled with regular fuel. The high heat generated during the compression stroke can cause regular fuel to self combust before the spark plug first, again causing multiple flame fronts.

So carmakers monitor the voltage from the knock sensor, compare when the knock voltage occurred to when it commanded the spark. Then it adjusts spark timing or air/fuel mixtures to prevent multiple flame fronts from forming.

Detonation can damage the engine

detonation damge to pistons

Examples of detonation damage

Detonation is different than pre-ignition. Detonation occurs after the spark has fired and while the piston is on the way down during the power stroke. The rapidly expanding flame front causes an equally rapid pressure buildup that ignites still unburned pockets of fuel, causing multiple flame fronts to collide. The flame front collisions cause a knocking sound.

The ECM uses the knock sensor to detect pre-ignition and detonation. Based on when the signal occurs, the ECM can change spark timing, air/fuel mixture, or both.

How the ECM detects a bad sensor

On startup, the ECM sends a bias voltage to the knock sensor. Since it know the resistance value of the wiring and sensor, it expects to see a certain return voltage. If the voltage doesn’t match the specifications, the ECM will set a knock sensor code.

A knock sensor is used to notify the computer of pre-ignition and detonation (knock) events.

The 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 sensor but no drop in crankshaft rotation speed, the ECM will ignore the knock signal.

How often do the sensors go bad?

Rarely. When you get a sensor trouble code, it’s usually caused by a bad connection at the sensor or in the wiring harness. You can damage a sensor by dropping it before installation, but they rarely fail once installed.

knock sensor

A typical knock sensor.

When people replace the knock sensor and the trouble code goes away, it’s usually because they’ve removed and reinstalled the electrical connector and the sensor now has good contact.

For more information on how to test a knock sensor, see this post.

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Posted on by Rick Muscoplat

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