exactly what does a knock sensor do?

[Alex]

hey guys,

im after a proper explanation of the purpose of a knock sensor. Does it actually detect knock and piston slap etc from a dead set of bearings?

Im also aware if constantly monitors the engine and adjusts fuel ratios and stuff to prevent pinging?

If you get a knock sensor code, does it mean its detected pinging, or does the code only come up when the sensor is faulty and needs replacement?

Basically im curious as to if the knock sensor tell you that you have a major problem? Or just that the sensor needs replacement.

thanks!

alex

[Gannon]

The knock sensor is basically a piezo microphone that is tuned to specific frequency range so that it only picks up detonation like sounds. Its possible that piston slap and litter tick could effect it.
The ecu operates in closed loop and uses feedback from it to constantly fine tune the ignition timing. The check engine light should only come on if its faulty.

The cel in my outback comes on occasionally and there is a crack in my sensor. I have bought a cheap sensor off ebay ($ 8.99) and will report if its any good.

Sent from my new touchy phone thingy

[Alex]

The knock sensor is basically a piezo microphone that is tuned to specific frequency range so that it only picks up detonation like sounds. Its possible that piston slap and litter tick could effect it.
The ecu operates in closed loop and uses feedback from it to constantly fine tune the ignition timing. The check engine light should only come on if its faulty.

The cel in my outback comes on occasionally and there is a crack in my sensor. I have bought a cheap sensor off ebay ($8) and will report if its any good.

Sent from my new touchy phone thingy

thanks mate, perfect answer.

I also bought a cheapish one from ebay afew years ago when the one on my liberty died. Still going strong. I also replaced mine with a resistor for a little while which tricked the car into thinking something was there which worked but obviously isnt the right solution.

This question is basically in relation to my VW which has chucked on the engine light and retarded the power output considerably. Going in for diagnostics tomorrow. (i have a suspicion its the knock sensors, as the VW has two if them)

alex

[steptoe]

hey guys,

im after a proper explanation of the purpose of a knock sensor. Does it actually detect knock and piston slap etc from a dead set of bearings?

Im also aware if constantly monitors the engine and adjusts fuel ratios and stuff to prevent pinging?



alex

I had a feeling some time in the past that it sounded like you had the knock sensor and O2 sensor functions confused, from some things you had written.

The knock sensor activity does not trim the fuel mix, that is the job of the O2 sensor.

My L Series EA82T 3 plugger ECU is a great learning tool along with the diagram in the manual. Firstly, it is a separate knock control box , not within the ECU like the 4 plug ECU and later models.

It is tested with a timing light, a DMM and a knock making device such as a 1/2" socket extension bar.

Hook up to the sensor output test connector with engine running reads 1.7V - all is happy. Next is to test the function by sharp tap on the block and watch for the 1.7V reading advance incrementally to a max of 4.5V. At 4.5V I have seen the timing retarded by watching the marks with a timing light as the 4.5V is reached. The timing pulled back 10 degress or so then gradually reset back to the set timing spec.

I bet you wondered if you should tell us your VW was playing up ??? Hope it i$ a $imple fix. The VW must be like many other V configurations and use one knock sensor per bank

[Alex]

I bet you wondered if you should tell us your VW was playing up ??? Hope it i$ a $imple fix. The VW must be like many other V configurations and use one knock sensor per bank

yeah i was punting on whether to say it or not haha. The poor VW has had afew problems since ive owned it which is a shame because its such a well built car and a pleasure to drive!

Having two knock sensors makes sense with the V6, im still learning how these things work (i'll know tomorrow exactly what the problem is)

however i didnt start this thread to talk about the VW, it can serve as a guide and info session on knock sensors as its a very common part to fail on alot of different cars and gets question pretty often.

alex

[steptoe]

and to answer your question it's not there to detect piston slap or crappy bearing wear/tolerances - more precisely the moment the spark ignites so not to get detonation from incorrect spark timing. The other part of your question - it may be that if the ECU has set off a CEL and code thrown is for knock sensor may be due to not getting the "I'm alright " voltage such as the 1.7V mine displays while engine is running without any retardation by the knock sensors module .

[TOONGA]

Alex just how cheap was the knock sensor on ebay? I see them advertised from $8.99 right up to $140.

I washed my EJ22 engine in the vortex yesterday and it threw a knock sensor code (22) when I started it. The knock sensor is second hand and I think it only works part time.

TOONGA

[Alex]

Toonga I paid about $100 I think. Could have been a bit less.

Probably wouldn't get a very cheap one..

Sent from my Motorola RAZR HD

[Alex]

so the only code worth 'worrying about' was an ABS code apparently.

They have done a proper clear of all codes and if anything faults again they said to go back to them asap.

I have a question (gannon) about the OBD2 system...

the car was getting the CEL come on along with the EPC light and not letting it rev past about 2000rpm.

I was disconnecting the battery to get rid of the light and putting everything back to normal, until the warning lights came back on again (i did this 3 times)

Does the OBD2 system remember old codes? Say if i was to pick up some bad fuel and the car went into limp mode because of it, but then i disconnect the battery and reset the system (lets say that all the bad fuel is gone) will the car still log the bad fuel in its system and throw up the code at a later date until the ECU is cleared of codes properly by a scanning tool?

hope this makes sense! and i know i should have taken the car to get checked straight away, im only getting the time to do it now.

thanks!

[taza]

The cel in my outback comes on occasionally and there is a crack in my sensor. I have bought a cheap sensor off ebay ($ 8.99) and will report if its any good.

The one in my Forester was like this and then in the end the CEL stayed on... the only way I could read the code digitally was with our high tec scan tool at work(apprentice auto sparky has it's benefits )

Bought a cheapie off ebay and my car still ran like crap, then bought a bosch one and that did the same thing(wierd as bosch stuff is usually pretty good).. IN the end just went genuine which cost me $180 2 months ago..

yeah i was punting on whether to say it or not haha. The poor VW has had afew problems since ive owned it which is a shame because its such a well built car and a pleasure to drive!

Your right about them being a nice car! Driven one at work and was amazed what it was like

I washed my EJ22 engine in the vortex yesterday and it threw a knock sensor code (22) when I started it. The knock sensor is second hand and I think it only works part time.

I have done this and after letting it dry the CEL went away

I have a question (gannon) about the OBD2 system...

the car was getting the CEL come on along with the EPC light and not letting it rev past about 2000rpm.

I was disconnecting the battery to get rid of the light and putting everything back to normal, until the warning lights came back on again (i did this 3 times)

Does the OBD2 system remember old codes? Say if i was to pick up some bad fuel and the car went into limp mode because of it, but then i disconnect the battery and reset the system (lets say that all the bad fuel is gone) will the car still log the bad fuel in its system and throw up the code at a later date until the ECU is cleared of codes properly by a scanning tool?

Depends on the car.... Your liberty for example, I know for a fact that if you disconnect the battery then let all the residual power drain(20mins or so) it will permanently clear the codes and all the old codes will be gone, unless there is a problem with a sensor. In which case it'll appear again..

With more modern cars, especially the more advanced ones the codes will be kept in the memory until cleared with a scan tool.. Other system are usually disabled too depending on what problem is wrong in the system.

Heaps of BMW's and Merc's pop codes because a brake light globe is blown... wtf

[Gannon]

I have a question (gannon) about the OBD2 system...

the car was getting the CEL come on along with the EPC light and not letting it rev past about 2000rpm.

I was disconnecting the battery to get rid of the light and putting everything back to normal, until the warning lights came back on again (i did this 3 times)

Does the OBD2 system remember old codes? Say if i was to pick up some bad fuel and the car went into limp mode because of it, but then i disconnect the battery and reset the system (lets say that all the bad fuel is gone) will the car still log the bad fuel in its system and throw up the code at a later date until the ECU is cleared of codes properly by a scanning tool?

Depends on the car.... Your liberty for example, I know for a fact that if you disconnect the battery then let all the residual power drain(20mins or so) it will permanently clear the codes and all the old codes will be gone, unless there is a problem with a sensor. In which case it'll appear again..

With more modern cars, especially the more advanced ones the codes will be kept in the memory until cleared with a scan tool.. Other system are usually disabled too depending on what problem is wrong in the system.

Heaps of BMW's and Merc's pop codes because a brake light globe is blown... wtf

What he ^ said.
Newer CANBUS vehicle management computers (controls everything including the brake lights) may use a small lithium battery or a memory storage capacitor to preserve the memory much like a home PC or even your smartphone does, even when all power is removed.

In this case, the only way to remove codes is with an OBDII scanner.

This one is a standalone for under $20 and although a cheap chinese unit, should show and erase codes.
ebay.com/U281 OBDII OBD 2 ENGINE SCANNER FOR VW AUDI SEAT SKODA

or search ebay as there are hundreds of different types

[Alex]

thanks guys youve been most helpful! They erased the codes yesterday hoping for a fix (which i was doubting) and sure enough i was able to get the CEL on again last night when i was test driving it. Its going back to them today, but at least theyll have a clean slate to work with now.

the VW has a screen in the middle of the cluster which tells me everything about the car, it came up there when one of the parkers blew a little while ago

its abit different from the days plugging the connectors together and counting the flashes!

alex

[Alex]

okay, it was only throwing one code this morning. Which came out as 'warm air flap motor' this is aircon related. They recommended me to take it to a VW specific service place.

I since did some googling and found you need a VW specific scan tool to get the correct reading. I then found out soemone used the regular OBD2 tool and came up with the warm air flap fault. He then had it checked by a VW specific tool, and the fault turned out to be cam position sensor.

I know the cam position sensor is more related to dodgy starting, but it has been trouble to start afew times before, which i was assuming is all related to this same problem.

The car has two cam position sensors which are extremely easy to replace, so ive ordered a new genuine pair ($80ea) and i'll give that a go. If it doesnt fix it, i'll get it scanned properly.

The problem is that every VW specific service place in perth has a 2-4week wait.

alex

[El_Freddo]

The problem is that every VW specific service place in perth has a 2-4week wait.

Make a booking, when the cam sensors arrive fit them, if the problem is still there hang on to the booking - if it's gone cancel the booking

Cheers

Bennie

[Alex]

Make a booking, when the cam sensors arrive fit them, if the problem is still there hang on to the booking - if it's gone cancel the booking

Cheers

Bennie

That's the plan

Sent from my Motorola RAZR HD

[NachaLuva]

Good luck Alex, I hope the genuine CPSs do the trick.

This one is a standalone for under $20 and although a cheap chinese unit, should show and erase codes.
ebay.com/U281 OBDII OBD 2 ENGINE SCANNER FOR VW AUDI SEAT SKODA

or search ebay as there are hundreds of different types

Gannon, will this work for a '97 Foz as I don't think its OBD11...isnt the '97 OBD1?

[Gannon]

Im unsure if it will work with your Foz but I know that the professional data readers that most auto electricians use can read Subaru's SSM protocol

For $20 its not a particularly expensive experiment. Id search for one labeled for Japanese models though, not European ones.

[Gannon]

I was gonna make my own thread, and I still might...

I got my $8.99 chinese knock sensor today and installed it


Restarted the car and the check engine light isnt on, so I guess so far its a sucess.

Looking at the old sensor, it had a few cracks in it, so I busted all the plastic off it and took some photos




Im gonna make a guess and say that the grey plastic looking section between the 2 connection lugs is the piezo element, a weight above it and then the nut on top used to compress, and thus change the tuned frequency of the unit.

[Alex]

how careful were you when torque-ing it up? I once read that you need to be fairly specific with them.

However when i changed my liberty sensor i just did it up to a 'tight enough' level and havent had any problems.

alex

[Alex]

if anyones interested, the VW ended up having a stretched timing chain. Parts had to be ordered from overseas so i'm still waiting, then i'm getting a new timing chain and all tensioners/guides installed.

just as well the chain didnt break, kiss $14ks worth of motor down the drain!

alex