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transmission shiftiness : FIXED

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by scotto0609, Jul 30, 2004.

  1. scotto0609

    scotto0609 1/2 ton status Premium Member

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    Recently while driving truck the shifts are taking a long time to upshift,could it be vacuum modulator on tranny?A little backround is in order,its a 400 with a tired small block in front of it,also just put 3500# camper on it but it was fine and only has @ 50 camper miles on it. Do you think i killed it?.... Broken vacuum line,and it runs better!thanx guys.
     
  2. scotto0609

    scotto0609 1/2 ton status Premium Member

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    Re: transmission shiftiness

    No opinions? I feel like a red headed step child. /forums/images/graemlins/confused.gif
     
  3. JeremyK5

    JeremyK5 1/2 ton status Premium Member

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    Re: transmission shiftiness

    If it's just winding out in each gear, without a slipping sensation, it could be the modulator, which you can get at autozone for about $12. Another possibility might be the governor, it has plastic gears and sometimes they strip out. I'm not 100% sure the th400 has a governor though, it's been a long time since i've seen one in person.
     
  4. bigredk5blazer

    bigredk5blazer Registered Member

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    Re: transmission shiftiness

    How tired is the motor? If it's got a lot of blowby then it could be a matter of insufficient vaccum to properly operate the modulator. I'd do a compression test just to be sure. If it passes that then the motor is probably okay. The reading isn't as important as the difference between cylinders. More than 10% isn't a good sign. If you get one that's a little low try squirting some gear oil into the cylinder and re-testing. If it gets better the problem is the rings. If it doesn't then it's the valves. If the rings are good you can just send the heads out for a valve job and new seals. Then your motor will be back up to snuff.

    Typically if the governor gear fails it shreds apart and the trans just won't shift at all. Though don't overrule this either. It's simple to pop the cover off and take a look. Have a new cover handy though,because if it's a pressed on cover the factory uses locktite to hold the cover into the case, and it is usually damaged during removal. I can't remember if TH400s use this style cover. All 700R4s and TH350s do.

    If it slips then the porblem is inside the trans, and a rebuild is the only way to correct it. I find this unlikely though, the TH400 is a really tough trans, even stock. I've seen fleet vehicles go through two or three engines in front of a stock, regularly serviced TH400, even when the trucks were used to plow snow and tow trailers over the limit of the truck.
     

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