Page 1 of 2 12 LastLast
Results 1 to 15 of 19

Thread: very crude rev limiter on 7747 ecm.....

  1. #1
    Electronic Ignition!
    Join Date
    Jan 2014
    Posts
    15

    very crude rev limiter on 7747 ecm.....

    I was thinking of using the shift light pin to activate a solid state relay that would control the power to the coil. Can you guys think of a reason this wouldn't work.

  2. #2
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    54
    Posts
    3,847
    It would work but might not be smooth or nice. Potential problems include point corrosion causing poor/no-start, wider than desired RPM swings, rpm limit lower than desired, unexpected / unknown variables causing inadvertent light operation.

  3. #3
    Fuel Injected!
    Join Date
    Feb 2013
    Age
    46
    Posts
    23
    Reminds me of when we were kids we'd run an engine up to speed shut off the switch let it spin down and load up fuel and turn the switch on causing a huge back fire POP... this is essentially what you'd be doing.. I never really hurt an engine doing that as a kid but the older much more wiser guys at the time told me that it's not exactly a good idea.... Most soft touch rev limiters hit individual cylinders at intervals to kinda avoid that. So you'd have yourself a hard touch rev limiter. Does the 7747 not already have rev limiting?

  4. #4
    Electronic Ignition!
    Join Date
    Jan 2014
    Posts
    15
    no it does not. and I know its not going to be pretty when I hit it but its better than blowing up my motor if I pop a u joint try to climb a hill

  5. #5
    Electronic Ignition!
    Join Date
    Jan 2014
    Posts
    15
    rather than letting it load up I could cut the power to the injectors

  6. #6
    Fuel Injected!
    Join Date
    Feb 2013
    Location
    Euless, TX
    Posts
    2,308
    Quote Originally Posted by diver14_98 View Post
    I was thinking of using the shift light pin to activate a solid state relay that would control the power to the coil. Can you guys think of a reason this wouldn't work.
    Repin to a 7427, fix the lack of a rev limiter and gain a better PCM in one move.

  7. #7
    Fuel Injected! JeepsAndGuns's Avatar
    Join Date
    Sep 2011
    Location
    alabama
    Age
    41
    Posts
    1,702
    I think personally I would rather cut power to the injectors. But instead of doing that, I agree with fast355. The 7427 swap is not that hard and you gain so much more tuneability. You also will get a rev limiter, and faster data logging. When I swapped to the 7427 I never looked back.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  8. #8
    Super Moderator
    Join Date
    Mar 2011
    Location
    Camden, MI
    Age
    35
    Posts
    3,026
    Quote Originally Posted by JeepsAndGuns View Post
    I think personally I would rather cut power to the injectors.
    I'm not sure I would recommend it based on if you cut the injector pulse while an injector is spraying, you're going to get an engine cycle that is going to do some really funny things, could either be not enough fuel to light off or enough fuel to light, but could be a super lean mixture.

    cutting the coil power could possibly be worse.... if you cut power while it is in dwell, it's going to spark.... either on the previous cylinder very late into the combustion process or super early into the next one.

    if you could monitor if the circuits are active before cutting power, that would solve both of those problems. depending on where the code lays, you might be able to write in a very small patch to quit delivering fuel during engine cycles where engine speed is above x RPM.

    but honestly..... i'll never recommend a C3 where a P4/P6/P66 can be used.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  9. #9
    Electronic Ignition!
    Join Date
    Jan 2014
    Posts
    15
    looks like its time to upgrade ecms

  10. #10
    Fuel Injected! JeepsAndGuns's Avatar
    Join Date
    Sep 2011
    Location
    alabama
    Age
    41
    Posts
    1,702
    I bet you will not regret it. I dont think I have seen where anyone swapped to a 7427 from a 7747 and wanted to go back.
    I will warn you, there is a slightly steeper learning curve as there is probably twice as many parameters. So get a bin file, XDF and just spend some time looking through it in tuner pro and get yourself famillar with it before the actual swap takes place. If you do that, it will help make the swap go easyer.
    With the change to the 7427, you will open yourself up to being able to run a E-trans if you want. The ability to have wide band o2 sensor data in the datastream, not to mention hacks/patches to be able to control a electric cooling fan, or PFI.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  11. #11
    Fuel Injected!
    Join Date
    Jul 2013
    Location
    Newzealand
    Posts
    483
    we used to use tachos with adjustable shift lights for rpm limiting on carby dirt speedway cars just cutting the coil power with a relay activated by the tacho works mint.dont do it off the earth or you can do engine and exhaust damage LOL.if you isolate the coil power only the ignition module should still run the ecu fine without spitting the dummy.if it doesnt cut and return fast enough for your liking a solid state relay might switch faster if you can find one that will handle the amps required

  12. #12
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    54
    Posts
    3,847
    Quote Originally Posted by diver14_98 View Post
    no it does not. and I know its not going to be pretty when I hit it but its better than blowing up my motor if I pop a u joint try to climb a hill
    I have some code written to limit rpm in a 7747 based on speed. I believe it could be rewritten for an RPM variable.

  13. #13
    Electronic Ignition!
    Join Date
    Jan 2014
    Posts
    15
    im clueless when it comes to moding the code

  14. #14
    Fuel Injected!
    Join Date
    Feb 2013
    Age
    46
    Posts
    23
    I think I'd rather kill fuel than fire, but as pointed out depending on circumstances leaning out could be an issue.

    Just a theory, but I wonder if you can't power the coil with two alternative sources, a 12 volt and a much weaker voltage say 4-6 volt. Have a switching relay setup that when your shift light is triggered it kills the 12 volt source but activates the lower volt source. My thinking is that it should still fire to prevent loading, but with much less voltage maybe even enough to keep the engine from raising RPMS. Another theory..lol, I have lots.. is maybe using an flasher relay (turn signals) or similar that quickly switches on/off coil voltage during times when the shift light is active.

  15. #15
    Super Moderator Six_Shooter's Avatar
    Join Date
    Feb 2011
    Posts
    2,968
    Sounds like it's time to add a 6AL, and have a proven limiter.
    The man who says something is impossible, is usually interrupted by the man doing it.

Similar Threads

  1. Removing or Bypassing Speed Limiter
    By Dolsey056 in forum Introductions
    Replies: 4
    Last Post: 08-12-2014, 04:39 PM
  2. Creating a "rev limiter"
    By trippyjoey in forum GM EFI Systems
    Replies: 12
    Last Post: 07-26-2014, 06:13 AM
  3. No Rev Limiter, Add to XDF?
    By UglyBoost91 in forum TunerPro Tuning Talk
    Replies: 5
    Last Post: 03-13-2014, 10:52 PM
  4. 7427 Rev limiter / 4L80E question
    By dubwise in forum GM EFI Systems
    Replies: 42
    Last Post: 02-20-2014, 07:29 PM
  5. rev limiter launch control
    By denziotech in forum TunerPro Tuning Talk
    Replies: 2
    Last Post: 02-29-2012, 05:42 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •