Can anyone tell me what a knock sensor is please? I have one on my Subaru Forester. I am given to understand that it may be the cause of some engine hesitation in accelerating away from 1st to 2nd. My ECU was upgraded by Subaru to try to solve the hesitation problem. It worked fine for a couple of days, but now the hesitation is back. Could the ECU be learning from the Knock Sensor and take me back to square one?
By the way, I noticed that none of the Subaru mechanics had dirty fingers. Does this mean that they are now technicians who rely on what the computer tells them?
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Mechanics usually wear those disposable gloves now. Old engine oil is thought to be a bit bad on the skin.
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The "knock sensor" picks up when the engine is starting to "pink", in simple terms the ignition is too far advanced for the load on the engine. The ecu receives a signal from this sensor and retards the ignition to suit.
It is an "active" signal since the ecu is nearly always trying to run the timing as advanced as possible.
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Can anyone tell me what a knock sensor is please?
"A knock sensor is fitted to some management systems, the sensor being a small piezo-electrical device that, when coupled with the Electronic Control Modules (ECM), can identify when knock occurs and retard the ignition timing accordingly."
www.picotech.com/auto/waveforms/knock_sensor.html
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Although the knock sensor is tuned* to pick up the vibrations caused by knock, typically around 4KHz, it is possible for a knock sensor to 'hear' other signals which the ECU interprets as knock.
Things that are loose, rattling or impacting together can confuse a knock sensor.
As the ignition advance map is adaptive, it is possible for the ECU to learn an incorrect mapping if the knock sensor is hearing things.
*The sensor may be tuned by torquing the attachment bolt is the bolt is mechanically in parallel with the piezo crystal. If the bolt is too loose, or too tight, the centre frequency of the sensor can be incorrect, so the sensor might pickup the valve train noise rather than knocking, for example.
Also, as the knock sensor signals are very small, they are easily swamped by electrical interference and noise. The noise may be interpreted by the ECU as knock. I have ended up fitting metal braiding over the knock sensor wires, effectively extending the ECU metal casing all the way to the sensor, in order to shield out electrical noise in one particularly difficult case.
Number_Cruncher
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Thanks for the quick replies.
So, it ain't necessarily that the sensor is faulty, just a bit too sensitive? Car is under warranty and it has been back 4 times. No one at Subaru has mentioned the sensor, I found out about it on the net with a lot of Subaru owners complaining about it. Not sure how many times more I can stand going back to have it fixed.
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tack, just seen your post. If your Forester is a 4 cylinder motor this might be helpful, www.subaru.org there is a thread in there in which members report symptoms identical to yours and it appears a tweak of the software does the trick. Hope it is some use.
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Apologies for my carelessness. The website is www.subaruoutback.org but it contains threads on other subies. Good job my head is properly secured. :-((
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Apologies for my carelessness. The website is www.subaruoutback.org but it contains threads on other subies. Good job my head is properly secured. :-((
Thanks for that, will have a dekko.
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