Simple they are piezoelectric microphones, which are gated on [alternate banks] for approximately 20 degrees of rotation every 90 degrees.........to listen for abnormal sounds which occur with spark knock [spark fires at 20-40 degrees before TDC and max pressure occurs at 17 degrees AFTER TDC.

A sound of ~~ 2-5KHz that occurs in the -15 to _+5 degree ATC position of EACH cylinder] is a knock. The frequency spectrum [from the microphones] is shaped and filtered by software to ignore other frequencies.

The sounds are accumulated as a voltage rise on 2 separate capacitors which continuously discharge [at a rate based on rpm].........knock counts per second.

This voltage is used by ecu to subtract ignation advance from a maximum number.......no knocks no subtraction and ignition advance is maximum preprogrammed value.

The are 2 x 2 accumulators [long term ones for those using regular fuel to sense all those extra knocks] and reset maximum to 2,3,4 less degrees vs short term to sense extra that average knocks.


Tricky to decide average vs peak vs rpm vs load and the knock sensors/ecu run out of calculation time as rpm increases with old style non obd2 ecu [90-95 models] so it gives up and just retards -3.........but this is set into software for WOT with RPM above 5,000.

So the knock sensors don't offer any protection above 4,000 rpm.

As engine ages noises increase from wear and knock sensors may get fooled.

Why you study WOT advance numbers to look for anomolies. Does the 6,000 rpm advance get to -28 degrees is the clue.

As KS age the sensitvity vs frequency curve changes from heat cracks etc.

As new knock control requires as new KS.......why they get changed every 100k.

A better more expensive way is to measure each cylinders peak pressure during combustion......what's used on race cars to set individual cylinder timing [ignition advance for each individual spark plug firing].