Knock senor. How it works

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smdk2500

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I have been reading the thread about knock counts and was wondering how the sensor actually works. I looked at a wiring diagram and it looks like it gets a 5 volt reference from the ecm and then it grounds through the sensor to the block. How does it know when knock is happening? Is it when there is knock the ground is broken and that tells the ECM that there is knock and then the ecm does its thing to get rid of it? Then when the ground is reestablished it stops doing its thing?
 

delta_p

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There are a lot of really good youtube video on how the knock sensor works.

It's basically a microphone. Not all of them require a reference voltage but i believe the ones on the c/k's is. In the manual it says the 5v from the VCM is pulled down to 1.3 volts in a no knock condition, so that implies there is a shunt resistor in there to ground around the sensor.

They use a material that has piezoelectric characteristics. usually a quarts crystal in the form of a diaphragm. When the material is stretched or compressed it generates an extremely small electrostatic charge (aka a voltage) positive and negative (aka alternating). As the diaphragm moves back and forth under a vibration, the voltage produced would add and subtract to the 1.3 volts and computer would read it and decide if it is a knock or not.

The ones on the c/k's are wide banded just meaning they are not tuned to resonate at a specific vibration frequency and the computer will decide the condition. Some knock sensors are tuned meaning the crystal doesn't really get stimulated to move around except at exactly a narrow range of vibration frequency that is the knock.
 
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