I’m happy to explain a little about the knock sensor if you like. First of all the actual sensor is really a piezoceramic transducer, or to you and I, a specially tuned microphone to listen for knock. The sensor works all of the time and knock response is a normal part of the sensors operation. Knock creates a very specific sound frequency when it happens, but this varies from cylinder bore size. Imagine the bore as a tuned bell being struck by a hammer, the larger the bell the deeper the note. On our engines with an 86mm bore the frequency of knock is around 6.3 – 6.8Khz and this is where the knock sensor it tuned to listen too.
As I have just mentioned, knock response is a normal part of the ECU’s cycle. To obtain maximum spark advance the ECU will increase the timing until the point where the knock sensor starts to detect (hear) detonation, at this point the ECU backs off the timing slightly until knock stops, then slowly starts the advance cycle over again. This happens very fast, many times per second in closed loop mode to ensure optimum advance is always achieved.
Another function of the knock sensor is to protect the engine from uncontrollable combustion. By this I mean serious detonation caused by bad fuel, excessive base advance, too much boost, excessive intake temps, etc (i.e. not caused by the normal closed loop cycle). If during the normal timing advance cycle the knock sensor starts to register excessive detonation that isn’t cured by a few degrees retard, the ECU will pull out as much advance as is possible (taking you back to the base advance settings), at this time it will also close of the turbo VSV to limit the boost. When this happens the engine is put in what we all call limp home mode and the performance is terrible. If you have eliminated the ECU’s means of boost control you might find that because of the massive spark retard put in place your turbo will over boost, and if you have an EGT gauge you will see the exhaust temperatures go through the roof.
So now you can see why the knock sensor is very important to the engines operation, but why do they fail? Well there are two main causes for this, the first is simply excessive vibration if the sensors come lose in the block. The second is far more common and unfortunately a lot more serious, and that’s excessive detonation. The knock sensors aren’t that fragile but if you have killed one and it’s recording an error on the ECU, you have a serious problem. Before you replace the sensor you must find out what was causing the detonation or else if it keeps up it will be bad enough to destroy the engine.
One of the first signs of detonation is a blown head gasket. If you think that it’s boost pressure that caused the gasket to fail, you are very much mistaken. It’s very likely the excessive boost pressure was the cause of the detonation. If you have ever blown a head gasket and notice pitting on the piston crown, that is also caused by detonation.
