Knock sensor question--93 850 GLT

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Dave M - 16 Aug 2007 02:35 GMT

My friend just got this car and the Check Engine light is on. The guy he
got it from said he'd had the computer scanned and the message was to
replace the knock sensor. However, the car isn't knocking, no matter what
kind of gas is used. If the knock sensor in NG, shouldn't it knock?

Does the computer actually detect a bad sensor? Or is it merely indicating
that it's time to change the sensor after a certain number of miles? If the
sensor is actually bad, why isn't the car knocking? Is it possible that
when the computer detects a bad sensor it retards the timing so the engine
can't knock, in an effort to protect the engine?

Also, how does one clear the computer of such messages, to make the light go
out?

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Vitus Jensen - 16 Aug 2007 22:41 GMT

Hi Dave!

> My friend just got this car and the Check Engine light is on. The guy he
> got it from said he'd had the computer scanned and the message was to
> replace the knock sensor. However, the car isn't knocking, no matter what
> kind of gas is used. If the knock sensor in NG, shouldn't it knock?

When the knock sensor is bad the ECU is runung the engine in a "limp home"
mode. In this mode the engine can't knock, you get a terrible mpg ratio
and the car won't run very fast.

> Does the computer actually detect a bad sensor? Or is it merely indicating
> that it's time to change the sensor after a certain number of miles?

It's detected. There is no "bad after so and so many miles" in a 93
Volvo.

> If the
> sensor is actually bad, why isn't the car knocking? Is it possible that
> when the computer detects a bad sensor it retards the timing so the engine
> can't knock, in an effort to protect the engine?

Yep! see above.

> Also, how does one clear the computer of such messages, to make the light go
> out?

There is a black box in the engine compartment, inside there is a button
and a LED. You have to press the button a certain time and the error
codes are a told via a blinking LED. To clear the error another timing of
press/blink is needed. Google for swedish bricks or something like that
to get a description how to do it.

You may remove the battery for half an hour to clear the codes but read
*all* the codes (there may be several!) before clearing and write them
down. This way you can accertain that it's the knock sensor and compare
codes if the Check Engine Light ever comes back.

Bye,
Vitus (964, 1996)

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Vitus Jensen, Hannover, Germany, Earth, Universe (current)

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Tim.. - 17 Aug 2007 09:02 GMT

> My friend just got this car and the Check Engine light is on. The guy he
> got it from said he'd had the computer scanned and the message was to
[quoted text clipped - 9 lines]
> Also, how does one clear the computer of such messages, to make the light
> go out?

There are 2 knock sensors on the 5 and 6 cylinders, and there was a TSB to
update the wiring loom between them on the early 850's.

If you replace the sensors, beaware they must be torqued to a specific
figure in order to work properly.

This could be your problem?

Tim..

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JDG - 21 Aug 2007 03:34 GMT

Could be bad spark plug wires.Don't ask me how that kicks out bad knock
sensor codes, but it did on my 94 850.

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Dave M - 22 Aug 2007 04:13 GMT

> Could be bad spark plug wires.Don't ask me how that kicks out bad knock
> sensor codes, but it did on my 94 850.

Bad wire causing rough-running engine which might mimic gas knock?

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