Knock Sensor Operation

the knock sensor is basically a

microphone connected to the engine block

the knock sensor is also tuned to the

very specific frequency of an engine

pinging inside the knock sensor is a

crystal and supporting contacts knock

sensors may have one or two terminals

with only one terminal the sensor is

grounded at the body itself with two

terminals one is ground and the other is

the signal wire itself the ACM expects

to see a normal signal output with a

normally running engine if no output is

seen then the ECM issues Nov d2 code P 0

3 2 4 through P 0 3 3 4 so a totally

quiet knock sensor equals a dead or

defective sensor but instead if the ECM

sees the normally operating signal it

then looks for pinging waveform

signature and when it does finds it

tries to adjust ignition timing

accordingly the timing adjustment is

done until the pinging stops if it does

then the ECM assumes a mechanical fault

with the engine on some older systems a

faulty knock sensor may set erroneous

OVD 2 codes for engine mechanical

problems when in fact it is the knock

sensor finally whenever faced with a

knock sensor OVD 2 code makes sure the

sensor wire is not broken disconnect and

perform a continuity test between the

sensor and the ECM sensor pin a sensor

range or performance code means that the

ECM has not detected the normally

running engine signal a circuit hike

means that only the bias diagnostic

voltage output by the ECM is seen or a

possible broken wire a circuit low means

that the sensor is shorted to ground

some newer vehicles may have to knock

sensors to better detect opening engine

the best way to test a knock sensor is

with the ADP scope one which will show

you the signal itself