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TBI high idle mod

Discussion in 'The Garage' started by mouse, Mar 14, 2006.

  1. mouse

    mouse 1/2 ton status

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    While researching my swap to TBI, I ran across a couple of people who suggested applying a switched power to the A/C sensing circuit to cause the engine to idle higher when the switch is on. While there were references to doing this, I could not find anyone who actually did it. It sounds great for winch operation, or extra warm-up time on cold mornings.

    Well, I did, and here is what I found:

    First, this probably means you are not using the A/C circuit for A/C. I do not have the A/C hooked up, and did away with the compressor by installing an A/C delete pulley on my newly installed serpentine belt system.

    To start this project, find the dark green wire going to pin B8 on the ECM. This wire provides 12v sensing signal to the ECM, making the computer go to the higher idle rpm to compensate for a higher engine load from the A/C compressor.

    I made a small switch panel in the space previously occupied by a dash vent above my stereo. I added a lighted switch that has a red LED to show me when the circuit is on. I wired it to an "ignition hot" circuit that was available on my new painless fuse block that I used when converting to TBI.

    Easy install, just a switched 12v circuit.

    Results:
    Oddly, I expected the switch to cause the engine to idle up a couple hundred extra rpm whenever switched on. This proved NOT to be the case. When cold, the engine would increase rpm by around 200 rpm. Once warm, most of the time the increase was only 50-100 rpm:confused: . Sometimes it would go higher, somtimes not.
    This should work well for additional power from the alternator during winch use. This is the primary reason I decided to try it. 50-200 rpm will increase the alternators output in those times of winch use (but I'm hoping to get stuck less with the new detroit installed;) ).
    I think the ECM adjusts rpm depending on temperature. If that is true, in order to get a full 200 rpm when switching the circuit on, a double-pole, double-throw switch could be used to disconnect the temperature sensor and cause the ECM to run in open-loop when you want the high-idle mode. I have yet to test this theory.

    Minor project, but looks to be a useful design and purpose.

    Please feel free to add anything I missed.:D
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    In the MAF TPI programming, there is a "fan anticipate" setting, which is how far the IAC moves to anticipate the load from the fans.

    I've never gone into it farther to see *exactly* how that works, because it may still be going towards a target RPM, but I've seen it in there. Didn't mess with it, whatever GM setup works perfect for my fans. It is small enough that you don't know the fans are going to come on.

    That's a nice idea you came up with though.

    Another option might be to wire up your switch idea with a resistor of the rigth value, to trick the ECM into thinking the engine is cold (via the ECM temp sender) and needs more idle RPM. Again, on the TPI at least there is a table that increases or decreases idle RPM based on coolant temp. Thats some real easy stuff to do in the PROM, and easy to test out after changing too. Of course, then you MIGHT run into problems with the mixture, but that be something you could watch and most likely correct.
     
  3. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    I was thinking of doing this too for when my OBA is used. I'm thinking of burning a chip to increase the value of the idle rpm adder for a/c demand. I should, in theory, be able to set it at what ever I like. Only down side is that I do still have a/c so I'd have to keep it fairly low otherwise driving in traffic with the a/c on could get a bit scary!!!
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    There might be some "unused" pins on the ECM that could be used, like electric fans or AIR in truck applications, you'd have to look at a pinout for a truck ECM and see what is "not used" compared to say the cars, or different years of the same ECM.

    I know this is done with other ECM's to switch accesories, etc.
     
  5. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    As opposed to using the a/c command? interesting idea, I'll look into it. They may be 'tunable' too.
     
  6. mouse

    mouse 1/2 ton status

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    Those are some great ideas. I plan on getting into re-programming this summer. I will keep this in mind and see if I can get a little better high idle. The 550 rpm takes some getting used to, and 750 rpm isn't exactly "high."
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Yep, I know I've seen mention of it over at thirdgen.org DIY-PROM (of course) just never really paid attention to what they were switching.

    But the '747 ECM is immensely popular, so I'm sure there is plenty of info about this, if not the general idea how to go about it.
     

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