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want to swap my TH400 to TH700, anybody done the same?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by fireinthehole, Jul 14, 2004.

    I need overdrive because I do not tow enough to justify keeping my TH400 in. This is on a 89 3/4 Suburban with a 241 transfer case. I know the output shaft is different between the 400-700 transfer cases. I know I need the tv cable, and other stuff. Is this a real difficult swap to do? My only alternative is a gear vendors type unit, and they are $2600 + driveshaft mods. ANy tips or ideas?

    Thanks- Mat
     
  1. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    It would be a simple bolt on if you get the case and adapter from another truck . Or as you said you could swap shafts in the case. I don not know overall length , but assume driveshaft length will change . As for the TV cable , get a bracket off another truck with a th700 . The shifter will work , but th400 was 3 speed , so the shifter won't grab first gear in stock form . To correct that you can go floor shift , or get a detent plate and gear indicator off a th700 truck as well . /forums/images/graemlins/k5.gif
     
  2. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    Your reasons are justified, but the th400 to th700 is a big step down in the reliability and strength dept. Don't know what you use your truck for, but sounds like mostly street driving so you are probably ok. If you wheel a lot, I would look into something else as a DD and keep your wheelin rig as tough as possible.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Going to have to do something about lockup on the 700 as well...your ECM isn't setup to control it, thus that would have to be dealt with.

    Probably not an expensive obstacle, but an obstacle nonetheless.

    Sit down and add up what this swap would cost, then average out how much you will save per fillup if you got 1-2MPG better. (2MPG would probably be a good figure)

    $1500 for a 700R4 will still buy a LOT of gas. If my calculations are right, at 10MPG as a starting point, an increase of 2MPG will pay for the $1500 700R4 in 33,750 miles.
     
  4. Dorian,

    Your right. I was talking this over with a friend of mine and he said the same thing. I guess I should just slow down and drive no more than 65 mph on the highway. Really I just want to drop the rpm's down a little at highway speeds, that little 350 feels like its working hard, although I have no tach so I can only guess what the rpm's are.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Mines cruising at about 2500 @ 65, and I too don't like it running that high. Even where it's 70 I didn't feel comfortable running that fast, the RPM's were just too high. Engine wasn't complaining, just not necessary...the engine makes more than enough power at lower RPM's to push the truck that fast.

    Only options really are an NV4500 or numerically lower gears, but either are fairly expensive. Then again, if only doing it for engine speed reduction, and not just MPG, it could be worthwhile.

    But I doubt it hurts these engines to run 2500RPM all the time. If stock trucks came with 4.10's and the non-overdrive transmissions, I figure GM didn't think it was a problem either.
     
  6. I was thinking the same thing about GM. The truck came stock with 4.10's and I think 235/75r16 tires, which are tiny. So I think at 65-70 mph it was running at a higher rpm than around 2500. I now have 35's on mine now,so I know its running a little slower.
     

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