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knock sensor failure? symptoms?

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Old 07-13-2005, 03:26 PM   #1
Oddjob
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Default knock sensor failure? symptoms?

Unanswered x-post from the 944 board:

Anyone ever have a knock sensor go bad, and if so, what were the symptoms?

Thanks,
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Old 07-13-2005, 03:30 PM   #2
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VERY underpowered and max boost would be 1.2bar (stock gauge), i tested the knock sensor by unplugging it, night and day difference compared when the sensor was connected. The car feels slow, very slow, not responding properly, heavy etc.
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Old 07-13-2005, 03:53 PM   #3
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Im pretty sure that the KLR performs a periodic diagnostic check on the knock sensor, and if it does detect a problem, it will run in safe mode, which is the retarded timing and reduced boost - which is what you found when you unplugged it.

My interest, which was triggered off a discussion from another thread on power loss, can the knock sensor fail in a way that makes the KLR believe the engine is knocking, so the KLR/DME just retard the timing w/o cutting the boost? Or a follow on question, could there be a fault in the KLR or DME that triggers retarded timing, either internally or in the wiring harness (ground/short)?
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Old 12-07-2006, 03:13 PM   #4
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Sorry to bring up an old thread, but can anyone answer this for Jim. I believe I'm having similar problems.
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Old 12-07-2006, 04:08 PM   #5
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No, the knock sensor can't fail such that it will "detect" knock and tell the DME that it's hearing knock. It'll be either good or "open" which will set a blink code in the KLR.

The knock sensor is just basically a microphone that picks up a very specific frequency. Very simple.

Dal.
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Old 12-07-2006, 06:05 PM   #6
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Kevin,

Do a search on this forum for samluke's post on knock counting. He rigged up a little event counter to count the knock signals sent from the KLR to the DME (which would be telling the DME to retard timing). Thought it was interesting and possibly a good way to check for potential problems (either actual knock conditions, or for a false detection - bad KLR?).
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Old 12-07-2006, 06:13 PM   #7
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Thanks Jim. I'll check that out. I need to do some testing. I've got about 50 rwhp that is hiding somewhere and we think it's a timing issue. I've replaced the klr, and knock harness. The klr isn't throwing any codes. I may try unplugging the knock sensor just to see if the car changes. I also still need to check the dme with my spare.
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Old 12-07-2006, 06:33 PM   #8
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Just fixed one with a bad knock sensor. The symptom was no power. The car was not building boost. The code it gave me was 2-2. I did not try to unplug it but, to check it: connect an ohm meter on term. 11 and 13 of disconnected klr control unit plug. You should get 270-330ohms. If 0 or inf. ohms check wires, if good replace sensor. torques to 11nm.
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Old 12-07-2006, 08:03 PM   #9
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Thanks MD. I'll try that. Ignition doesn't need to be on right to test?
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Old 12-07-2006, 08:38 PM   #10
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On a somewhat relate note, if you've pulled the CV does the KLR just retard the timeing?
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Old 12-07-2006, 09:41 PM   #11
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I had one fail on an Audi such that it detected *lots* of knock and pulled out tons of timing. Very bad fuel economy but no error codes. The crystal must have cracked or something. It's more common that they fail open due to wiring issues.

-Joel.
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Old 12-07-2006, 10:21 PM   #12
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I just probed pins 11 and 13 and I'm not getting any readings. The wiring is new, so it must be the sensor. Is there a way to pull the sensor and test it outside of the wiring?
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Old 12-07-2006, 10:27 PM   #13
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On my 86, I was able to take out the old knock sensor without pulling the intake manifold. I used a "woble" extenstion and a long standard screwdriver to push the ISV bracket out of the way.
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Old 12-07-2006, 10:39 PM   #14
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Blink code 2-2 on the diagnostic plug after 1.2-bar "limp home" mode.

Check the resistance across - should be 280-330 kOhm IIRC. If it's bad, a replacement is about 75 bucks but beware, I've been told they're going on backorder with Porsche N.A., which means it might take months to get a replacement.
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Old 12-08-2006, 12:47 AM   #15
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Quote:
Originally Posted by por951turbo
Thanks MD. I'll try that. Ignition doesn't need to be on right to test?
No the ignition should not be on when you test it.
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