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Knock Knock sensor

Discussion in 'The Garage' started by fourbyfourforfun, Aug 8, 2006.

  1. fourbyfourforfun

    fourbyfourforfun Registered Member

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    What damage can be caused by driving with a non-functioning Knock sensor(causing detonation under acceleration)? How long would it take of regular driving to do damage?

    fourbyfourforfun
     
  2. readymix

    readymix 3/4 ton status

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    1st...
    Knock sensors do not cause knock.

    2nd....
    Fix the issue and you can drive without a knock sensor (just bypass it with a 3.9K Ohm resistor) should be fine.


    Double check the figure for the resistor though. I know I have posted it here before.
     
  3. neverendingproject

    neverendingproject 1/2 ton status

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    Well the knock sensors retard timing when detonation occurs. Ive nere run with out one, but it would keep advancing timing till it couldnt go anymore. I guess you would run a resistor to simulate a partial knock?
     
  4. dleroy43

    dleroy43 1/2 ton status GMOTM Winner

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    or go buy a sensor :dunno:
     
  5. midnitewarya

    midnitewarya Sounds like a problem for future me. Premium Member

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    :rolleyes:
     
  6. beater_k20

    beater_k20 Banned

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    or fix the damaged wiring if the sensor isnt bad.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Assuming TBI and TPI are the same, the diagnostic for the knock sensor via the ECM is to force knock, and "listen" for it, telling the ECM it's working. If it doesn't hear that knock from the test, it won't run the timing like it would if it were working right.

    ESC/EST on the trucks I don't know, especially the ones that were ALL electronic. I would assume they were advanced enough to design it to do that, but who knows.
     
  8. sled_dog

    sled_dog 1 ton status

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    I'm sorry I'm confused. How does a knock sensor on the Chevy trucks work? As I knew it, all Knock sensors were devices that created their own voltage, not variable reluctance sensors.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    You are right, it's a piezo-electric device. Knock creates voltage, ECM sees that, retards timing a set amount. (set by the PROM) The ECM knock test I am familiar with (could be different depending on ECM) is that the ECM initially advances timing past the point where GM has determined knock will occur under a set condition (making up numbers, feed say 30* advance at idle 90 seconds after startup) and if the ECM gets the feedback from the sensor saying knock occurred, then the sensor passes the test. If knock has theoretically been induced, and there is no feedback, the sensor has failed the test.

    Many people don't understand that "maximum spark advance" is not the goal in an engine, and the EFI systems do not attempt to run as much advance as possible. The advance is a carefully (that's up in the air) calculated curve in the PROM, and the knock sensor is there to make sure that IF something does go wrong (bad fuel, hot, etc) the engine doesn't detonate itself to destruction.

    As clarification, some systems use a standalone module (MAF TPI is one, I assume TBI is as well) that handles timing/knock (ESC) but still gets its info from the ECM. Later on the knock capabilities were integrated with the ECM, so there are no standalone modules. MAP TPI is an example of this, and I guess yet another "advantage" over MAF. The knock sensor is just that, a sensor. It does nothing other than sense knock. The ESC components and ECM are what do the actual timing/retarding work based on the sensors output.
     
  10. sled_dog

    sled_dog 1 ton status

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    ok thought they were piezo-electric like every other knock sensor I know of... so why is it you guys find it possible to rid yourselves of one with a resistor?
     
  11. fourbyfourforfun

    fourbyfourforfun Registered Member

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    I understand what the sensor does, my question was


    I already replaced the non-functioning sensor on the truck that had been being driven daily for months with detonation occuring. I wanted to know if damage was likely and where it would occur so I could verify that no damage was done. With detination happening and the timming being improperly advanced or retarded by the ESC wouldn't it put stress on bearing, rods, valves? There is nothing obviosly showing damage now. Is it worth being concerned about.


    fourbyfourforfun
     
  12. sled_dog

    sled_dog 1 ton status

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    What engine do you run? What year? Induction?

    You KNOW for a fact that detonation was occuring that whole time? You could have damaged any of the things you listed if thats the case. Why would someone knowingly drive for a long time with detonation occuring?
     
  13. fourbyfourforfun

    fourbyfourforfun Registered Member

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    It is a 1986 K5 carberated 350. We are a mutiple car family and all to often I find my self asking someone "HOW LONG HAS IT BEEN DOING THAT?" only to be told "DOING WHAT?"
     
  14. sled_dog

    sled_dog 1 ton status

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    ... I was completely unaware that 350s ever came with ESC. Thought only 305s did.

    I know what you mean, I am an auto tech. Amazing the things you see people drive in with, when you bring it their attention they had no idea the bald tires were a problem, or that it was misfiring really badly...
     
  15. dyeager535

    dyeager535 1 ton status Premium Member

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  16. sled_dog

    sled_dog 1 ton status

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    yeah I'm still confused by that... I think they are over complicating the sensor itself...
     
  17. dyeager535

    dyeager535 1 ton status Premium Member

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    I don't think the sensor is that complex, it's just the way the ECM "tests". If it advances timing to test for "knock", a resistor won't do anything.

    If it simply looks for resistance, then a resistor would work.

    In the example given, both tests are used, so one of the two would fail with a resistor in place, and throw a code.
     

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