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Two TH400 questions:

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Brian 89KBlazer, May 7, 2001.

  1. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Just talked to a local tranny guy & he said that he'd supply & install a TH400 rebuilt with B&M shift kit for $450!!!

    Question 1

    What model years did the TH400 come bolted to an NP208 t-case (1980-??)? I need an adapter plate to mate the two & need to know what year to ask for?

    Question 2:

    Where do I get a vacuum lead for the modulator on the TH400 on my '89 TBI 350? The tranny guy I was just talking to said that the 400 hooks up with a vacuum modulator & an electric switch. Where/How do these tie in to my engine??

    Thanks
    Brian
    89KBlazer
     
  2. thatK30guy

    thatK30guy 1 ton status Premium Member

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    You can use ANY year TH400 to mate to the 208 as long as its the 4wd version, with the short shaft. Even the older 400's that mated the 203's will work fine. Just dont use a 400 that was bolted to a 205. Those have a different shaft length.

    The vacuum modulator will require a simple hard line bent to run from the modulator to a simple vacuum tree up on top of the intake manifold. Somewhere by the back of the carb or by the passenger side valve cover should be a fitting for those such trees. Simply hook some short rubber hoses from the modulator to the hard line and then on top, from the hardline to the tree.
    The electric switch you need is for the kickdown. TH400's use electric kickdowns instead of manual ones. You would need to check out Summit Racing or Jegs for the conversion kit. B&M makes these. Price is around $45.00. It includes everything you need to make the TH400 kickdown work.
     
  3. Sparky87k5

    Sparky87k5 1/2 ton status

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    The TH400 needs to have 32 spline output for 208's, at least mine was on 88 Suburban I had. Didn't use electric kick down and manually shifted down when needed. Ran that trans 142,000 miles with no ill effects behind a 513 CI cadillac engine.

    Gone fishing
     
  4. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Thanks Westy!!

    I'll check with Summit Racing on that electric kick-down kit. On the year & makes though; I've actually got an NP241 t-case in my Blazer which is basically the same as the NP208 it replaced except for an internal pump system & a re-inforced tailshaft. Will the spline count be the same? Advanced Adapters site says it has a different adapter application for TH400's that came with NP208 vs NP208's that came behind the 700R4 which is what mine is behind.

    Thanks again
    Brian
    89KBlazer
     
  5. thatK30guy

    thatK30guy 1 ton status Premium Member

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    No problem on the Summit deal!! Glad to help! [​IMG]

    The 400/208 combo shaft has 32 splines and its a little bigger around than the others.

    The 700/208 combo shaft has 27 splines. These are also the same as the TH350.

    The adaptors for the 400 is different than those of the 700/350. The 400 uses 6 bolts to hold the adaptor to the tranny housing. The 700/350 uses 4 bolts. If you have the 700/241 combo, you would need a conversion kit in the 400 in order to run the 241 t-case. That conversion kit is simply a 400 out put shaft machined down to 700/350 size and spline count. You can get these from B&M also. TCI also has them. In order to install these, the tranny would need to be torn down.
     
  6. HarryH3

    HarryH3 1 ton status Author

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    I got my Turbo 400 kickdown switch from a Jeep Grand Wagoneer. Cost about $2 at the JY. Most of them came with 400's underneath. The switch attaches to the accelerator pedal bracket. It screwed right onto the stock bracket on my '75 K5.

    And as stated above, the modulator is looking at intake vacuum.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
    It's a great day to be alive...
     
  7. ken

    ken 1/2 ton status

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    Hey "thatK30guy", I beg to differ, but the TH400s used with 203s had a short output compared to the Th400s that used the 208s or 241s. I have a 1978 TH400/203 combo that I had the tranny rebuilt for sitting on the floor for my '77 Suburban. Then the wife's 1990 Sub's 700r4 took a crap for the 7th time, so I decided to put in a TH400. Had a 1985 TH400 (that had a 208 transfer) and the shaft stuck out alot further (like 2 or more inches) than the 1978 TH400 (203 version). ALL were 32 spline, but the stickout prevented me from using the 1978 version TH400 in my 1990 Sub with 241. AND, I had to change the input gear in my 241 to the 32 spline version (it had the 27 spliner from the 700-junk-4). The gear was $140 from Parts Mike and was easy to install...just had to split the case halves apart and unbolt a few things and plop in the new gear and re-assemble...total time with cleaning was 1 hour. The adapter i used was from the 1985 TH400-208 adapter. It bolted directly to the TH400 and the 241 transfer. Then, when I bolted it all in, the crossmember mount was such that it had enough forward to aft adjustment in it so that NO modifications were necessary to the mount, crossmember, or driveshafts. Truly a BOLT-IN operation!! And BOO-HOO, I don't have overdrive!!! And guess what else I don't have...a tranny thats going to fail AGAIN!!!

    ken
     
  8. thatK30guy

    thatK30guy 1 ton status Premium Member

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    Well, thats weird! I pulled a 400/208 combo out of an 83 K20 once and compared them to an older 79 K20 with the 400/203 set up and both trannies had the same length shaft. Thats why I went with that quote. Just from what I have seen and dealt with. Who knows? Maybe GM has more than one set up for these combos?
     
  9. ken

    ken 1/2 ton status

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    Maybe the TH400 shaft length went to the longer shafts in the 1984 time-frame...that would explain why your 1983 shaft was the same as a 1979, and why my 1978 and 1985 were different. Who knows for sure??!!

    ken
     

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