Knock Sensor - Block Surface Preparation

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I will be installing a new knock sensor on my 98 Expedition 5.4 4x4 while I have my intake off.
My question is what would be the best way to prepare the block surface where the sensor mounts?
I am thinking there would be an opportunity for corrosion between the parts over time, affecting the future operation, but I am not sure if this part actually works with conduction.
Should it be clean and free of paint to make a good conductive mating surface between parts?
Should I use a dielectric grease between the surfaces to prevent future corrosion or just a dry connection?
 
Sure, the surfaces should be clean. Dielectric grease will have no effect on the operation of the sensor at the frequencies it needs to detect.
 
Just wanted to clarify, when I said clean, I was referring to a bare metal surface and not a painted surface.
Again, not sure if it is a conductive connection or if it just senses vibration. Part of me wants to paint the top of the block with a high temp paint to prevent corrosion.
I am leaning towards a bare metal connection with dielectric grease at the knock sensor, but I hope to not see this connection for another 10 years.
 
The knock sensors I was familiar with 25 or so years ago were piezoelectric and converted the mechanic energy of vibrations in the block to electrical energy that was carried to an interface circuit differentially (two wires).
This didn't require an electrical connection to the block and I doubt that would be necessary or desired in any knock sensor, but I haven't kept up on things and there may be systems that work nothing like what I was familiar with.
I guess I would worry about corrosion in the mechanical connection to the block more than anything, you want to make sure the sensor is vibrating in unison with the block...
 
Paint shouldn't matter because it's so thin and the knocking sound is not going to be influenced by it. A thin layer of grease won't either. You won't have any ability to adjust the level of grease because it will all get squeezed out as you tighten the sensor to its proper torque spec. In fact you should be worrying less about grease or paint and more about the proper torque spec, as in significant as that could be perceived.

If you use a mechanic's stethoscope, the likelihood of paint or grease affecting the output of it is extremely unlikely, I don't think the knock sensor will have any issue.

If it's a two prong electrical connector on the knock sensor it probably doesn't use the housing of the sensor to ground either, like oil pressure sensors/senders usually have one prong connectors since the threaded housing itself grounds to the block.
 
Looks like the connector on the one I took out has 3 prongs on the female connection.
 
To make sure that there's no issue with paint or grease causing a ground to be lost, you could also try a multimeter to check for continuity between the metal parts of the housing and the block. I would suspect neither will impact it since a bolt is going to run through the housing and into the block. Unless someone painted the threads, some portion of metal to metal contact is going to occur.
 
Knock sensors are piezoelectric or MEMS-based. They aren't grounded. Just make sure the mounting boss is clean, torque the bolt or sensor to spec and that the wiring harness is in good shape. The OEMs will use shielded coaxial cable to ensure the knock sensor signal is free of interference.
 
Originally Posted by nthach
Knock sensors are piezoelectric or MEMS-based. They aren't grounded. Just make sure the mounting boss is clean, torque the bolt or sensor to spec and that the wiring harness is in good shape. The OEMs will use shielded coaxial cable to ensure the knock sensor signal is free of interference.


Both Resonant(Narrow Band) & Flat Response(Wide Band) Knock Sensors require a ground.
Piezoelectric Resonant type sensors generally get their ground from the body of the sensor thats grounded to the block (Case Grounded).
Piezoelectric Flat Response type sensors get their ground from a cleaner source.....Sensor Ground from the PCM/ECM.

The "shield ground" when used is a separate circuit......Generally only used with Flat Response type & is a Case Ground.
 
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