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th400 shifting ????

Discussion in 'The Garage' started by bigbadchev84, Mar 25, 2006.

  1. bigbadchev84

    bigbadchev84 1/2 ton status

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    well last night we pulled the sm465 that kept popping out of gear out of my brothers truck and installed a th400 that he picked up out of an 81 diesel, we put in a new convertor and swapped out the governor, it ran great previously in the other truck but now it dosent seem like it is going into 3rd gear behind his 350. maybe i am just not used to it being an auto, we dont have a tach on it but hes running 35s and 4.10s there is a noticible diference in rpms at about 50-60mph, is there something we did wrong? what should i look at?
     
  2. tomseviltwin

    tomseviltwin 1/2 ton status

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    Did you hook up the vacume modulator? Shifting linkage adjustment? Electric kickdown swith from gas pedal?
     
  3. bigbadchev84

    bigbadchev84 1/2 ton status

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    yeah its all hooked up...........anyone i need help!!!
     
  4. hack500

    hack500 1/2 ton status

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    what're you using for a shifter? didn't the diesel's come with a cable modulator... so was there anything internal that needed to be changed when putting on a vacuum modulator?

    you'll probably need to wire up a tach to see where it's at at 50-60 mph. you say there's a difference at 50-60mph but different from what?:thinking:

    edit: is the kickdown switch sticking at all?
     
  5. bigbadchev84

    bigbadchev84 1/2 ton status

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    its a standard shifter stock run of the mill one, i had forgot to add that it sat on the shop floor for 3-4 months before installing it, the kickdown kicks it down when you stomp on it and lets off, just the rpms are up a noticible difference as far as the engine loudness goes, i would say a good 3-500 rpms more.
     
  6. sweetk30

    sweetk30 professional hooker Premium Member

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    dont forget the stick is 1:1 ratio with clutch . and the auto has a convertor. this will give a bit more rpms . but around the same range.
     
  7. HarryH3

    HarryH3 1 ton status Author

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    Yep, there is a 200-300 RPM "penalty" with an automatic that doesn't have a locking torque converter. If you installed a converter with a higer stall speed than stock, then the difference can be even more pronounced. It should be pretty easy to tell if the trans is actually shifting twice...
     
  8. bigbadchev84

    bigbadchev84 1/2 ton status

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    it's only going up to 2nd gear and tops out at 60 MPH. When you let off the gas- you can feel it going into 3rd. Slight throttle after that will put it back into 2nd. Whats goin on?
     
  9. diesel4me

    diesel4me 1 ton status Premium Member

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    modulator...

    The diesel TH400's used a vacuum pump on the motor to give vacuum to the modulator,a switch that controls the amount of vacuum,it was spliced in the vacuum hose,it made the constant vacuum from the pump act more like a gas engine does during normal driving( more vacuum at low speed,less or none at higher speeds and wider throttle opening)...

    I'm not sure if a diesel's modulator is set up any different than a gas motor is or not..mine has a "rebuilt" TH400 that appears to be a car tranny (not a "K" case),and has no provision for the cast cover or truss rods..the switch I mentioned before is FUBARED on my truck,and was causing it to not upshift until it was screaming,it would finally bang second gear at 50 mph,then be very reluctant to shift into third,or stay in third once it did shift..

    I tried by-passing the switch and hooked the modulator directly to the pump,and screwed the adjustment screw inside the nipple on the modulator for the hose all the way in, until it gently bottomed out,at the advice of a local tranny shop guy I know who is considered one of THE best around here..he says thats what he does in cases like mine on a diesel truck,since GM has discontinued the switch,as far as he can tell..(I haven't found anyone else who makes it either)..

    I'd make sure you have a good source of MANIFOLD vacuum,a good modulator with no diaphram rupture or ATF leaking out of it,and if all that checks out maybe unscrew the adjustment screw a turn and see if that improves the shift into 3rd..mine now shifts a bit early and smoother than I'd like,but its in 3'rd at 20 mph like its supposed to be..if your detent solenoid is sticking or being activated it wont upshift until its wide open..there is a slight chance the valve body gummed up a bit while sitting,but its a long shot...

    Dont forget the GOVENOR on a diesel used different weights,made the tranny shift at different speeds!---and diesels often had highway gears in the axles,that might be fooling you ,..the speedometer gear might be causing it to read differently too!
    :crazy:
     
  10. bigbadchev84

    bigbadchev84 1/2 ton status

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    thanks for that reply, i just got ahold of a tranny guy at home and he said that it was probably a bad modulator, or the fact that i dont have a metal line going down to the tranny, its just a rubber piece of vacuum line the whole way he said it might be getting too much vacuum and pulling the daiphragm flat, he is the same guy that i went too about the governor a couple months ago since i heard that those were different and he gave me a gas one to stick in it, that was changed out on the shop floor a few months ago too. so tomorrow i will pick up a small vacuum tube and a new modulator and start there, does anyone know how to adjust the modulators?
     
  11. bigbadchev84

    bigbadchev84 1/2 ton status

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    ok well took it to the tranny shop today and found that it lost the direct drive clutch, so we took the one out of my blazer i got for sale, look in the for sale section its getting parted
     

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