What Does a Knock Sensor Do?

Written by michael davidson
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What Does a Knock Sensor Do?
(Image by Flickr.com, courtesy of Nathan)

Automobile engines are complicated machines. They require a multitude of belts, fluids, moving parts and chain reactions all occurring in the correct timing and rhythm for your car to run efficiently. If one device fails or if a part doesn't move at the precise time it needs to, the car may not run correctly or the engine may be damaged. One of the components that is vital to your car running properly is the knock sensor.

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Function

The knock sensor is a listening device located inside the engine near the intake manifold or the pistons that is designed to detect pressure inside the engine. The vibrations that it records as a result of this pressure are converted into voltage and sent to the car's engine control unit (ECU). This is to prevent detonation in the engine when the fuel/air mixture explodes too quickly and causes engine knock or "pinging." The ECU takes the information provided by the knock sensor and adjusts the ignition timing to the engine.

Components

The knock sensor is made of a piezoelectric element that has a coil around it. A wire connects the sensor directly to the ECU. When the element vibrates, it sends electric current through the wire to the ECU, which then interprets the reading. There is generally one knock sensor for every four cylinders in the engine.

Benefits

"Pinging" or engine knock happens when there is an early explosion in the combustion chamber of the engine. The flame collides with the piston which is still rising (or the spinning rotor in the case of a rotary engine) which results in engine vibration and engine knock. In minor cases, this results in a decrease in engine power. If it gets severe, major engine damage could result which would require up to thousands of dollars to repair. The knock sensor is the key instrument that prevents this from happening and keeps the timing of the engine running smoothly and safely. In some models, the knock sensor serves a dual purpose of also increasing the ignition timing to the level just below engine knock to achieve optimal power and performance.

Symptoms of Failure

If the knock sensor is starting to go bad, your car will start to experience acceleration problems and a declining fuel economy. Frequently, the car will start to become sluggish. The sensor rarely fails outright and instead will start registering engine knock when there isn't any and the ECU will dial down the engine performance and slow the ignition timing in response. Reasons for failure include corroded connectors in the wire between the sensor and the EPU or internal failure of the sensor itself since it is very sensitive to vibrations.

Replacement

Depending on the make and model of your car, replacing a knock sensor can be very expensive. It is cheaper to replace it yourself but is very time-consuming and only recommended if you have a knowledge about automobiles. Some models have the knock sensor stored inside the engine block, in which case the radiator coolant would need to be drained. The sensor must also be torqued to exact specifications or it will be too sensitive to vibration and harm engine performance or not work at all. Refer to your owner's manual for the specifics on your engine.

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