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Faulty Knock sensor

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Old May 9th, 2022, 10:22   #1
TonyS9
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Default 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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Old May 9th, 2022, 11:48   #2
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Originally Posted by TonyS9 View Post
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?
Very unusual and difficult to test . you might as well get a spare knock sensor to try and make sure it is torqued to the correct torque AND aligned properly . I have never come across a faulty one before though . It might be worth checking the wiring to the ECU . Has the knock sensor ever been removed ?
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Old May 9th, 2022, 13:57   #3
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Originally Posted by TonyS9 View Post
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?
Try running it on decent petrol Tony - super unleaded, preferably BP Ultimate, if not Shell V-Power as a (sort of) passable second.
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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Old May 9th, 2022, 14:39   #4
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Try running it on decent petrol Tony - super unleaded, preferably BP Ultimate, if not Shell V-Power as a (sort of) passable second.
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.
You read my mind Dave, I was going to say that multiple people including myself have found 940's hate running on 95 with the new E10.
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Old May 10th, 2022, 11:58   #5
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Try running it on decent petrol Tony - super unleaded, preferably BP Ultimate, if not Shell V-Power as a (sort of) passable second.
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.
Would that not be less likely to generate a knock signal?

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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Old May 10th, 2022, 12:39   #6
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Would that not be less likely to generate a knock signal?

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.
Higher octane fuel is much less likely to generate a knock signal, correct. The piezo in the sensor is tuned to resonate at the frequency produced by detonation/knocking (it's like a piezo-electric buzzer in reverse) and when detonation occurs, it will therefore produce a voltage spike. The ECU reads this as detonation and retards the ignition timing, reducing the power output.

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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Old May 11th, 2022, 05:59   #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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Old May 11th, 2022, 12:25   #8
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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.
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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