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  1. #1

    Default HELP, Swapped 4l80 to TH400 but now wont run, sort of

    I have a 1993 K2500 6.5 Turbo Diesel. I lost reverse in my 4l80 and replaced it with a TH400. When I start the truck it runs for about 30-45 seconds and then dies. My 4wd fuse also blows the second I turn the key (not sure if they are related). In that few seconds it runs I have forward and reverse and can drive. Any ideas? I am not sure if there is something I should be doing with all the leftover wiring from the 4l80 that I am not using that could be causing this as well.

    My wife is about done with me messing with this truck, I may be living in it if I can't figure this out soon.

  2. #2


    Find a schematic of the engine/transmission harnesses. There are some circuits that share grounds, or work in conjunction with others. It is possible this is causing an issue somewhere. If the fuse is blowing, there is a short somewhere; do some digging and see if you can locate it.

    What happens all when it dies? Does it restart right away or not?
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  3. #3


    Like Asphalt Animal posted you need to find a schematic, some of the wiring can be bypassed and some will need to be connected.
    Your new trans isnt computer controlled and the engine is looking for certain signals to stay running.

  4. #4
    Sr. Mechanic Ethan's Avatar
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    Sep 2009
    Cedar Park, TX


    The 4L80E and TH400 are vastly different transmission, one is electronically shifted, while the other is not. You need to put the exact tranny in that truck that it was designed for. The reason it keeps blowing fuses and dies after a few seconds is because the PCM realizes that there is no transmission, and shuts the truck off. PCM means Powertrain Control Module, meaning, it controls the engine and transmission, it has to have both to work properly.

    I believe the TH400 is mechanically shifted(pressures and the whatnot dictate when it shifts), which is why alot of guys don't like em, and the 4L series are all electronically shifter.
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  5. #5


    Ethan, On a '93 with manual injection pump there is very little to no engine control function from the ECM. In fact, it is actually only a TCM in this application.

    There is still a TPS (throttle position sensor) used for transmission control, and it could be possible that since there is no feedback from the transmission, as most of it is not connected, it can be creating some sort of electrical havoc here. What may need to be grounded that isn't anymore?
    Schematics I have looked at do not fully illustrate what, if any, malfunction in the TCM can have on engine operation. Most of this is fully mechanical, with the exception of the fuel shutoff solenoid and lift pump and neither of these should be affected by the TCM.
    If nothing else, I'd still say you should try to verify that nothing is compromising fuel delivery; severe blockage, faulty shutoff solenoid in the IP, bad OPS, and no massive air leak or failing lift pump.

    In the long run, you'd probably have been better off installing a manual.

  6. #6
    Sr. Mechanic Ethan's Avatar
    Join Date
    Sep 2009
    Cedar Park, TX


    The 92-95(me thinks) chevy C/K series used PCM(Powertrain Control Module) controlled engines, they don't have a ECM. The PCM controls both the engine and transmission, mainly the shift points, and the TCC.

    The 93's are very very similar to my 94, I know them like the back of my hand, and the transmission has a huge effect on the way the truck runs, if there's nothing sending signal back to the PCM, even though it's OBD1, it'll recognize it and stop operating properly. You have to have the connections hooked up, all of them.

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