How do knock sensors detect abnormal vibration in the engine and How to check and replace them on Nissan Quest?

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Knock sensors detect abnormal vibration in the engine and are part of a control system designed to reduce spark knock during heavy detonation, allowing for maximum spark advance to enhance driveability. These sensors produce AC output voltage that increases with the severity of the knock, sending a signal to the PCM to retard timing in response to severe detonation. To check a knock sensor, disconnect the KS sub-harness electrical connector located on the passenger side of the engine compartment next to the intake manifold. Use a voltmeter set to the lowest AC voltage scale, connecting the negative probe to ground and the positive probe to the sensor terminal on the sub-harness connector, typically identified by a white wire. Tap sharply on the intake manifold to simulate engine knock and observe voltage fluctuations; if none are detected, the sensor is faulty and needs replacement. To replace the sensor, remove the upper and lower intake manifold and the coolant crossover tube, unplug the electrical connector, and take out the sensor retaining bolt and the sensor from the engine block. Install the new sensor, tightening the retaining bolt to 18 to 25 ft-lbs, reconnect the electrical connector, and reinstall the intake manifold, refilling the cooling system as necessary.

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