Daryl

Can some one help me out and explain how the anti knock sensors work on a 88 S-4? Can they be disabled if they are not working correctly? How can I test them to be sure they are working properly? If they go bad do they default to ignition retard mode? Thanks for any info on this subject.

Daryl

Lizard928

the anti knock sensors detect detonation and then retard the ign slightly to prevent this from happening

Daryl

Hello Lizard931. Thankyou for the reply. I know what they do. I need to know how they do it, how to know if they are working properly and how to stop them from retarding the ignition if they are not working correctly.

Steve J.

Presumeably, they are accelerometers. You might compare a known good one with a questionable one by whacking it and looking at the output on an oscilloscope. I think the computer can sense if they are not working and default to retarded ignition mode.

Jfrahm

I believe they are piezoelectric transducers. They do sometimes go bad (quite often on 5 cyl Audis for some reason.) One failure mode is that they become hypersensitive and jabber away, killing power and fuel economy.
I had this problem last year on my 90 Quattro. As a temporary measure I cut a circle of cardboard to insulate the sensor from the block somewhat. In the Audi, The ECU tests the knock sensor so for proper performance you cannot just disconnect it. It has to be hooked up and able to hear some engine noise. Actually you can disconnect it, but then the ECU reverts to a conservative spark map and lights the 'Check Engine' light. In my case it was the dead of winter so intake temps were very low, I never heard any knock with the sensor insulated somewhat. I intended to get a new knock sensor, but rather than going to the trouble I wrecked the car instead.

-Joel.

martin D

Daryl

You can get a Bosch diagnostics 'Hammer' test done, this will show whether they are working (along with a lot of other sensors).You can also test them with an electrical multimeter at the connectors (but I don't know the exact procedure for this ). If they are faulty you will have to replace them or put up with the lost performance

Randy V

As Joel states, they are a transducer that actually 'listens' for the knocking sound associated with detonation.

They can be confused by other sources of noise, such as a loose exhaust manifold, and will retard timing as if the noise they are hearing is from detonation, resulting in performance degradation.

jpitman2

They are piezo things that convert vibration to an electric signal, and are tuned for a certain range of frequency, which is related to the knock sound of detonation. When you get a knock, the ecu gets a signal, retards timing till it goes away. You can buy aftermarket units that are tunable to suit your installation. The book should tell what happens when one fails - it seems quite common that disturbing an old one will break it on an S4.
jp 83 Euro S AT

ViribusUnits

When an engine knocks, there is a sudden explosion. This sudden explosion is like hitting the pistion with a hammer. Of course, it rings like a bell. This is what you hear.

A knock sensor is typicaly a peso electric sensor that is "tuned" to the frequency of the ringing in the block. This is basicly a microphone that turnes noise at the frequency of the ringing, and just that frequency, into electric signals.

Normaly, there is a big of background noise that it also picks up. When the strength of the electric sensors gets to a certain threashold, the comptuer retards the timeing. This signal processing can be quick enough that is it capible of narrowing it down to a diffrent cylinder by the timeing of the engine, and then retarding ONLY that cylinder. I don't know if it is on the S4.

Now that sensor can go dead, in which case it no longer transmitts the background noise to the computer. The comptuer realizes that the sensor is silent, and go to the most conservitive spark map. As as it would if it was experenceing sever detonation. This saves your engine.

The sensor can also go to hypersensitive, resulting in it transmitting the background noise too loud, resulting in the computer constaly thinking there's detonation, resulting in the most conservitive spark map. This also saves your engine.

The problem is that when the sensor go bad, you can't get the most advance that you would other wise get. The causes the power and fuel economey to go down on the engine. I also belive it causes the exaust temps to go up, but thats only really a worry for raceing engines that already have high exaust temps.

A methoid to test the system is to get a metalic object, and a timeing light. Let the engine warm up, and idle. From there, rap the metalic object near the sensor, and then use the timeing light to see if it retards. If it does, the sensor is working, if not, then it's not.

The sensor will pick up any ringing at the specificed frequency, even if it's not knocking. If the background noise is too loud, the comptuer doesn't care, it can't tell the diffrence between knocking and a loud valve train. It just assumes that the noise is knocking, and reacts accordingly.

Good luck.

John Speake

Yes, they are piezo devices. A common problem is bad connectors where they join into the main cableform. Have a careful look at these (they are 3 pin Bosch connectors). Sometimes insulation melts down into the contacts and causes problems.

One other problem is that sometimes the insulation on their wires breaks off due to the heat, then the screening braid of the wire touches surrounding metal, and causes fault "noise".

The signal level from the knock sensor is very low, so good connections and a low picked up unwanted noise are important.