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  1. #1

    Default Replacing 4l80 with TH400 on 6.5l - Need help

    Howdy, I have a 1993 k2500, 6.5l Turbo Diesel. Until recently it had a 4l80 transmission in it, but I lost reverse and overdrive was getting to be useless. It is an old farm rig so it is not worth putting a ton of money into. So i got a TH400 from an old military rig and put it in.

    After cutting and welding the shift arm at the transmission (just turned the dog leg about 180 to get it to reach right), adding flexible lines to match up the oil cooler lines, shortening the front drive line (th400 is about an inch shorter than the 4l80) and moving the support for the transfer case forward about an inch, I am finally to the point of dealing with the vacuum modulator.

    I do not have a vacuum pump since my turbo has a spring driven waste gate, so I added a line form the air intake ahead of the turbo and ran it to the modulator. I assume this would work for the shift vacuum but would love some input. Is there anything I am missing? Are there any other modifications I should make before hitting hiway speeds? So far I have only driven in 1st and reverse a few feet just to make sure it was working.

  2. #2


    you'd get a little vacuum in front of the turbo, but not much, and it's gonna be delayed. there's air going through there, but not much real vacuum, because there's no resistance on the other (filter) end. However if you're only running around locally and on the farm and such, I don't think it'd really hurt to just leave it unhooked (not a trans expert though).
    1986 Chevy G20 Tow Rig - 5.7 TBI conversion
    1978 Mustang II King Cobra Road Racer - Holley 650, MSD 6AL, 5 speed, 10 point Roll Cage, 9" posi, fuel cell, custom interior.

  3. #3


    I am also curious about all the electrical stuff that is now unplugged since this is not an electric transmission. Should I be doing anything with it?

    Thanks dualdj1, I am not a tranny guy either but didn't figure it would hurt much for it to just be unplugged as well.

  4. #4


    yeah, some of it has to do with the lockup torque converter and such. Just make sure you run it in Drive and not OD and it should be ok. Running in OD will probably hurt your converter.

  5. #5
    Sr. Mechanic Ethan's Avatar
    Join Date
    Sep 2009
    Cedar Park, TX


    It's not going to run for more than a minute, another guy on this forum did the same thing, and the truck would turn off after about a minute. The PCM needs the transmission connected to complete the system, otherwise, the PCM will recognize that there is no transmission, and shut the truck off as a fail safe.

    You need to go get the exact replacement transmission, and put that in, or get a ECM that will run only the engine for that truck. But if it does run for more than a minute, then oh well, it works it works, but I don't think it'll work.
    1994 Chevy Silverado 1500 Ext Cab 350 cuin TBI in dark Chevy blue
    160,000+ miles
    Original paint
    Original interior
    Original engine and tranny
    wrecked twice stolen once
    Diamond Plate bed rails
    Power windows
    Original Alloy wheels
    Original fancy smancy cup holder
    Tow package
    Original plastic bed liner
    K&N Cone Air Intake
    Dual MagnaFlow exhaust(not true dual) with chrome tips
    MSD Street Fire Performance Ignition coil

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