my tech told me knock sensors are for on board diagnostics, the techs at my work say that it can affect the performance of the vehicle. but if a sensor is for internal diagnostics, it shouldn’t have any affect on the performance of the vehicle.
i haven’t done a knock sensor before, so i am just wondering if someone could confirm which statement above is true.
thanks
I’m not sure how the knock sensor can be used for diagnostics… that’s a new one.
All the knock sensor does is detects “knock” in the motor 
What it really does is detects pinging caused by too much ignition timing advance or too low a grade of fuel or a combination of the two. Like any sensor this signal just goes back to the ECU. Based on the logic in the ECU it does the right thing, or maybe doesn’t do the wrong thing. The “right thing” when detecting knock is usually to retard the timing. The right thing might also be to keep advancing timing if knock isn’t detected and it’s wide open throttle. Not doing teh wrong thing would be to retard the timing and not advance it any more even if it’s WOT.
So how does a knock sensor effect performance? Well that depends on the logic of the computer that is reading the signal, but it’s pretty safe to assume that a bad knock sensor will always have a detrimental effect on performance either because a) the system is running in limp mode bacause there is no signal at all. or b) it’s not advancing the ignition timing when it should or c) it’s not retarding it when it should (“c” usually causes melted pistons and broken ring lands etc).
I don’t know if that really answers your question, but I’d pretty much say “your tech’s” have very little, if any, idea what function a knock sensor performs.
-Martin.
I wouldn’t say they have no idea…
They gave the correct answer, just super simplified.
I’m assuming they didn’t mean tech diagnostics but ECU monitoring the engine.
And yes a bad sensor will screw with performance. One of the main complaints about the new SE-Rs is that the knock sensor could pick up the knock in other cars on the road 
A while back SCC dyno’d the SE-R and couldn’t get the same number twice, when they mufled the sensor with a resistor they then claimed that it had the most rock solid consistent dyno runs they had seen.
If the knock sensor isn’t grounded properly it will generate false “knocks” since really a knock sensor is a microphone and retard the timing… atleast in NEW nissan engines… I have no idea if it does anything at all in the old ones.
I think what your tech is tryin to get at is if your pinging or “knocking” then that sensor will pick it up retard your timing which intern means yes less preformance at that instant.
thanks, that does clear it up, either way i will replace it, since i gotta pull off my intake manifold to get at my other sensors.
i figured the knock sensor may act like the iat sensor, used for on board diagnostics and nothing else, but wasn’t sure, and had conflicting answers, thats why i asked.
my “tech” had just finished re-manufacturing a 2000 jag type s motor, as well as my buddies q45 motor. so to say he is incompetant, is a little ignorant (no flame intended).
thanks for the help though
I’m not sure how the knock sensor can be used for diagnostics… that’s a new one.
All the knock sensor does is detects “knock” in the motor What it really does is detects pinging caused by too much ignition timing advance or too low a grade of fuel or a combination of the two. Like any sensor this signal just goes back to the ECU. Based on the logic in the ECU it does the right thing, or maybe doesn’t do the wrong thing. The “right thing” when detecting knock is usually to retard the timing. The right thing might also be to keep advancing timing if knock isn’t detected and it’s wide open throttle. Not doing teh wrong thing would be to retard the timing and not advance it any more even if it’s WOT.
So how does a knock sensor effect performance? Well that depends on the logic of the computer that is reading the signal, but it’s pretty safe to assume that a bad knock sensor will always have a detrimental effect on performance either because a) the system is running in limp mode bacause there is no signal at all. or b) it’s not advancing the ignition timing when it should or c) it’s not retarding it when it should (“c” usually causes melted pistons and broken ring lands etc).
I don’t know if that really answers your question, but I’d pretty much say “your tech’s” have very little, if any, idea what function a knock sensor performs.
-Martin.[/quote]
Couldn’t have said it any better myself!
^^^^whoa, thanks smoke, i don’t know what i would have done without your input!
:roll:
just got off the phone with an advisor at Scarborough Nissan, who confirmed that a knock sensor is used for on board diagnostics, and will have no affect on the vehicle’s performance (it is used to detect knock. as Martin stated, but it shouldn’t trigger the ses light to come on.
The knock sensor ITSELF does not affect performance. It sends data to the ECM when it detects a knock. The ECM takes it from there, and might retard the timing if it is deemed neccessary to do so.
So it does have an affect on performance. It can activate the SES light if the knock is bad enough that it constantly imbalances normal engine operation parameters.
I don’t know if that really answers your question, but I’d pretty much say “your tech’s” have very little, if any, idea what function a knock sensor performs.
Here’s what I said - where did I say your tech was incompetant?
I’m not in the habit of being ignorant, and don’t appreciate being called that.
-Martin.
i think he just posted a general comment about his tech’s competence… he never said anything about you stating that
If there is one thing I’ve learned it this: Mechanics don’t know squat about electronics. Simple really. If you don’t believe me then just ask your mechanic to explain the function of some simple automotive electronic part. They can’t because they were never trained as electricians or mechanical/electrical engineers.