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Thread: Desensitize the knock sensor 94/95 LT1

  1. #1
    Electronic Ignition!
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    Desensitize the knock sensor 94/95 LT1

    Iíve been chasing some false knock for a couple of months now.
    Ever since I installed some headers itís pulled timing 4000+ at WOT.
    Iíve knocked it down to only pull 4 degrees so itís helped out a lot.
    But what table(s) does one adjust to desensitize the knock sensor?
    I thought it was the fast attack knock rate table but it doesnít seem to be helping.
    Im running 91+ octane. Timing tables are stock. Iíve actually taken 1 degree out at 90kpa and 4000rpm.
    AFR is 12.7 @ 4000 and creeps to 13.0 @ 5400. All at WOT.
    Iím thinking about just zeroing the table out at WOT 4000+
    I know itís not real knock. Iíve actually lowered the timing live with EEHack and had the same result.
    If someone needs a log Iíve got a couple.
    Thanks.

  2. #2
    Fuel Injected!
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    There is no table for it. Knock Detection is handled by a separate PCB entirely, in what's known as the "Knock Module." it's attached into a slot on the underside of the PCM, under a cover.

    This module's job is to take the raw output from the knock sensors, filter them based on its internal values as to which are "real knock" and which are "noise," then if I recall correctly will pull a pin low to tell the PCM that knock is occuring. When and how long this pin is pulled low is then used by the PCM to retard timing.

    At present there is no real good way to fix it. Some will say that the LT4's knock module is less sensitive to false knock because of the switch to roller rockers in the LT4 requiring a new knock module program to compensate, but I have not seen any empirical data comparing the two modules against one another. In addition, these knock modules are now going for over $300 on eBay as they have been out of production for many years.

    If you are handy with an oscilloscope, you could hook up to the knock sensors themselves and try to figure out which frequencies represent real knock on your car versus which don't, then tear apart another knock module (one less expensive than the LT4, haha), reverse-engineer its functionality, and build your own. I'm actually looking into doing that personally.

    Or you could just give up on the electronic spark control system altogether and set maximum knock retard to 0 across the board. If you know your car isn't knocking, then the system is only hurting you.

    Best of luck no matter which path you choose.

  3. #3
    Fuel Injected! spfautsch's Avatar
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    Have you thoroughly checked that the exhaust system isn't making contact with the body somewhere? Possibly due to driveline torque loading?

    I've dealt with all sorts of false knock. The roller rockers and pushrod guides seem to have been my primary source, but since going to shaft mount rockers it seems to be completely cured.

    Long story short, the knock sensors on these cars are extremely sensitive and LT4 knock modules are very scarce so I'd eliminate all possible sources before investing in one. I once had a cracked exhaust tip causing resonance that was causing severe false knock.

  4. #4
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    The passenger side header likes to tap the floor a little bit lol.

  5. #5
    Fuel Injected! vilefly's Avatar
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    Well, I am not sure will work, but I know what won't work. An inline resistor to the knock sensor won't work. There is a 5v reference voltage sent to the 4K impedence of the sensor. The ecm expects to see this voltage drop to 2.5v when connnected. This means there is a 4k resistor in the ecm series with a 5v supply, which has a tap that heads to an input to the processor. (voltage divider) Any extra resistance will set a code, so it gets a little tricky messing with it.

    Perhaps a mechanical extension on the knock sensor might dampen things (or let it resonate more, dunno). Not sure how close it is to the headers, though. I can't remember if the knock sensor uses pipe thread or not.

    2008-08-30_182514_41845650.gif
    Last edited by vilefly; 10-27-2019 at 04:35 PM.

  6. #6
    Fuel Injected! steveo's Avatar
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    you don't really have control over when knock counts occur but you definitely have a control over how they're interpreted

    if it occurs in a narrow-ish RPM band i'd reduce ignition advance in that region to safe-ish levels and zero that area in the max knock retard vs RPM table.

    if it occurs in a wide area, then disabling the entire system is usually what i'd do.

    as far as a hardware solution, i'd suggest passive filtering on the sensor itself rather than hacking the knock module. if you can identify the frequency, a notch filter isn't that hard to build.

    headers setting off the knock sensor isn't really a problem i've had unless they're smacking the frame, usually valvetrain changes do it to me.

  7. #7
    Fuel Injected! vilefly's Avatar
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    Passive filtering. I suspect this would be how to wire it up.

    knock sensor hack.png

  8. #8
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    Quote Originally Posted by steveo View Post
    you don't really have control over when knock counts occur but you definitely have a control over how they're interpreted

    if it occurs in a narrow-ish RPM band i'd reduce ignition advance in that region to safe-ish levels and zero that area in the max knock retard vs RPM table.

    if it occurs in a wide area, then disabling the entire system is usually what i'd do.

    as far as a hardware solution, i'd suggest passive filtering on the sensor itself rather than hacking the knock module. if you can identify the frequency, a notch filter isn't that hard to build.

    headers setting off the knock sensor isn't really a problem i've had unless they're smacking the frame, usually valvetrain changes do it to me.
    around my area we usually only have 91 octane. i found a station with 93 so i filled it up. im running the stock timing tables too.
    the car has 209k on it but doesnt make any noises. ill see if i can post up a log.

  9. #9
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  10. #10
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    Well now EEHack wonít open after clicking on a log and windows wanted to open EEHack. No clue.

  11. #11
    Electronic Ignition!
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    anyone run across this issue? ive tried uninstalling and reinstalling
    Attached Images Attached Images

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  13. #13
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    Quote Originally Posted by spfautsch View Post
    Try a system restore to a snapshot before the problem.
    lol shit

  14. #14
    Fuel Injected! vilefly's Avatar
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    Hmmm. same thing happened to me months ago. I will have to try that. I had switched computers earlier.

  15. #15
    Fuel Injected! spfautsch's Avatar
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    That'll be $49, each.

    vilefly you seem tech savvy enough to reload winblows. That fixes all software problems, always.

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