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TH400 swap in to replace 700R4

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Brian 89KBlazer, Jul 19, 2001.

  1. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    I know it's been discussed here & there in pieces but does anyone have a step by step & part by part description/write-up on this swap?? Either that or if there were details given in a previous CK5 post, what's the link?
    My tranny supplier is sending all I need in hard parts but is sending me a 32 spline input gear for my t-case instead of a 27-spline output shaft for the TH400 along with the correct adapter plate. He didn't seem to think I needed the B&M kickdown kit. Just vacuum for the modulator. Wes, anyone??

    Thanks
    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  2. HarryH3

    HarryH3 1 ton status Author

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    The Turbo 400 uses vacuum to the modulator to help control shifting. You'll need to wire up a switch to get the kickdown for passing gear to work. It mounts on the gas pedal bracket. I grabbed the switch for mine from an old Jeep Wagoneer. Lots of those are laying around in junkyards and most came with Turbo 400's. The switch from the Jeep bolted directly to the bracket on my K5. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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  3. CK5

    CK5 In my underwear Administrator Premium Member GMOTM Winner Author

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    I plan on swaping my 700 for a 400 but I am going with a manual valve body. would I still need the modulator?

    Steve

    Bandwidth FREE ZONE

    Mall-Running poseur 4x4 with curb-feelers
     
  4. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Thanks Harry

    I should have remembered that from a long ago post! [​IMG][​IMG]. Now I remember; both ARE needed. So that bracket kit from B&M racing basically just relocates the kickdown switch from the gas pedal to the end of the the 700R4's throttle cable in the engine bay, correct??

    Thanks again
    Brian
    89KBlazer

    BTW, when are you swapping Steve? I need a write up with pics by mid-late August. please!![​IMG][​IMG]

    Build it Right or Don't build it at all!!
     
  5. ken

    ken 1/2 ton status

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    I did this very swap a few months back in the wife's '90 Suburban. It had the dreaded 700r4 in it. The 32 spline input shaft must be installed into your 241 transfer case, and it has to be split open to install it. No biggie, just stand it on its end (the end that mates to the trans) and pull all the bolts and slowly seperate the case halves and look inside when you have it open only about an inch to see where everything is. About the only thing that needs to be lined up is the shift fork. Also, one tip, loosen the front output flange nut before you remove the transfer case from the truck, this will make it alot easier...also tighten it back again once its installed in the truck and you have some resistance to hold it from turning. If I remember correctly, the torque is 150 ft/lbs. Be sure to insure that you received all the correct parts, namely that the ourput shaft from the 400 has enough engagement into the input of the transfer. There were some 4x4 versioned 400s that had longer outputs than others.One nice thing is that I did not have to modify my driveshaft lengths or the crossmember mounting in any way...it was pretty much a bolt in! Good luck!

    ken
     
  6. 4x4blazer

    4x4blazer 1/2 ton status

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    You may also want to replace your gear indicator on your column. I got mine from a '77 Chevy pickup and it fits perfect.
     
  7. HarryH3

    HarryH3 1 ton status Author

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    Steve, I'm not sure if you need a modulator when running a manual valve body. I think the modulator also controls hydraulic pressure in the tranny, so it's probably required, even if you will do all the shifting yourself. No big deal since it's part of the tranny anyway. Just run a vac line from the intake manifold to the modulator and you're all set. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
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  8. 84_Chevy_K10

    84_Chevy_K10 Banned

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    No, you don't need a modulator with a manual valve body. There wouldn't be any hydraulic control going on at all--a full manual valve body is just that--completely manual. Vacuum or nothing else--except input from the driver--is going to affect the shifts.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  9. HarryH3

    HarryH3 1 ton status Author

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    Does the manual valve body also override the line pressure? Some trannies increase the line pressure with load, to increase the clamping power on the clutches.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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  10. ChadH82

    ChadH82 1/2 ton status

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    If I'm starting all over with a new 400 tranny from B &amp; M will I have to worry about the modulator? I had a T-350 before.

    Chad
     
  11. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Chad

    I think you're OK from what I recall. Both the TH400 &amp; TH350 hook up the same way. Harry?? Ken?? Anyone else care to confirm?

    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  12. HarryH3

    HarryH3 1 ton status Author

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    Yep, the modulator will already be installed in the tranny (just like on a Turbo 350) so just hook the vacuum line to it. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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  13. californiak5

    californiak5 1/2 ton status

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    Is there a differance in length between the TH350 and the TH400?

    Thanks

    See Ya,
    Keith

    [​IMG] <font color=green>Born to Drive My K5</font color=green> [​IMG]
     
  14. Executioner

    Executioner 1/2 ton status

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  15. californiak5

    californiak5 1/2 ton status

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    Hey there Ex,

    I don't no what I would do without your help.[​IMG] I asked a straight forward question and got a straight forward answer.[​IMG]

    Now since I know there is a diferance in length between the two does anybody know what the doferance is? Thanks

    See Ya,
    Keith

    [​IMG] <font color=green>Born to Drive My K5</font color=green> [​IMG]
     
  16. thebigdaddyof2

    thebigdaddyof2 1/2 ton status

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    According to info I have from ORD:
    SM465 - 18 3/8"
    TH350 - 21 1/2"
    TH400 - 23"
    TH700R4 - 23 3/8"
    Hope this helps.

    thebigdaddyof2
     

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