need some info on knock sensors

Matt82 · 6 Aug 2006 at 10:34

how is it they actually work? my car gets its timing noticably knocked back between 2-3krpm, i doubt it should be doing this at all.

can someone let me know what causes it (so that i may try to cure it)

and also, whats the principle behind a knock sensor, as i think i may have a pair of duff ones

CypherPunk · 6 Aug 2006 at 12:04

A knock sensor is normally a piezoelectric microphone; all it does is detect the sound of detonation, when this happens the timing is retarded to protect the engine. It would be very unusual for a knock sensor to fail and cause the ECU to think it's knocking because of the way the knock sensor works, a faulty knock sensor would send no signal to the ECU thus the ECU wouldn't detect a problem, so it could be that your car is actually detonating.

Clarkey · 6 Aug 2006 at 12:37

Actually a faulty det sensor will likely leave the car in 'safety' mode. Witht he 300 if there is a det sensor fault the car will run in safety boost all the time, reduced boost and retarded ignition.

Matt82 · 6 Aug 2006 at 14:25

thats what i beleived too.

i had mine off today. there are two. the one on the gearbox side is easy to get to, the one on the off side is a mission. i had to get the alt off and belt tensioner. seeing as ive done the water pump before, it was an easy job.

i did some reading to see what else i could find out. the sensors need to be torqued to 20nm.

mine were miles tighter. took them off, cleaned them, reassembled to correct torque and took it for a run.

i think it may have been cured. it has a lot more urgency when it pulls, i hear there is no way to test them without a scope tho