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700/900 Series General Forum for the Volvo 740, 760, 780, 940, 960 & S/V90 cars |
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Faulty Knock sensorViews : 828 Replies : 7Users Viewing This Thread : |
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May 9th, 2022, 09:22 | #1 |
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Faulty Knock sensor
So my car has thrown its first "code" after 206K and 27 years (excluding minor problems caused by weak mixture and dump vaccum pipe failure).
It was 1-4-3 knock sensor signal missing and just came up randomly mid journey with no other symptoms or noticable loss of power, using normal unleaded. The lambda light came on permanently. I reset it after figuring out you have to "wait" for the led to come on after the 5s press. The warning light hasn't come on again yet. Anyone experienced this? Is it likely to come back, should I get a spare sensor in the mean time? Does it test the sensor for continuity to decide the light or does it expect a knock oaccassionally? |
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May 9th, 2022, 10:48 | #2 | |
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May 9th, 2022, 12:57 | #3 | |
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Also check your wiring to the knock sensor as they tend to be tough little animals and don't fail often. You can check continuity with your DMM, can't remember the figures and they're usually pizeoelectric devices so the continuity will be different to the usual electromagnetic sensors such as ABS, wheel speed, timing and so on.
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May 9th, 2022, 13:39 | #4 | |
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May 10th, 2022, 10:58 | #5 | |
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We don't know how it decides to generate the code. As you say a piezo crystal is hard to test with resistance. Maybe you could put a pulse in and look for a slighly delayed one back. Maybe you can feel or hear it in test mode, but its normally an input, it would need a strong output to do that so I doubt it. Maybe there is a low level noise signal from the engine running. I have experienced it working with 95 and 12psi boost, you get a big flat spot. In any case it hasn't come on again. |
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May 10th, 2022, 11:39 | #6 | |
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Running it on 95 octane and 12psi boost is a good recipe to generate detonation so definitely worth upgrading your fuel to a decent super unleaded (97/98 octane) and particularly losing E10 and using E5 instead.
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May 11th, 2022, 04:59 | #7 |
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The ECU expects a signal of some kind constantly from it. A guy on turbobricks unbolted and wrapped it in a cloth and it threw the knock sensor missing code.
Different fuels aren't going to a fix a wiring or sensor fault.
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May 11th, 2022, 11:25 | #8 |
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Perfect, thats the information I was looking for. Its getting a low level noise signal. I'm also thinking with fuel too good, maybe it might get very quiet.
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