Rebuilt TH400 shifting way too soon

Discussion in 'The "Juice Box"' started by BrianTrick, Sep 8, 2017.

  1. BrianTrick

    BrianTrick Brian Trick

    I'm working on a customer's car that had a new drivetrain in it. It went from a 350/TH350,with a 2:73 rear,to 455,TH400,with 3:08 rear. When I drive it,the transmission shifts way too soon. 1st gear is pretty much nonexistent,and it shifts into 3rd before you get to 25mph. Normally,you don't have to start messing with governor weights,unless you have steeper rear gears installed,but these are only 3:08's. I bought a governor kit,and I installed the weights & springs that were right in the middle of the selection. That made no difference. I then adjusted the vacuum modulator by turning the screw all the way in,then backing out about a 1/2 turn. That made no difference either.
    The kickdown does work nice,but the normal shifting needs to spread out. As I drive it more,the shifting is still real close together,but it is starting to shift very hard.
    You could barely feel it shifting before,but now it's too hard.
    Any ideas?
     
  2. BRUCE ROE

    BRUCE ROE Well-Known Member

    Adjustable vacuum modulators may have an adjustment range around a dozen
    and a half turns. Half a turn will do nothing. The weight kits will surely have
    an effect over their range, if the trans isn't broken.

    I found it takes a lot of adjustment and driving trials to get these things right. I
    get a spare governor, start making changes, and KEEP A RECORD of what has
    been done/results, or lose track of cause/effect over the amount of driving
    involved. The theme here is set the vac modulator for harshness, adjust the
    weights for heavy throttle shift point, adjust the springs for light throttle
    shift points. Your results may vary. good luck, Bruce Roe
     
    matt68gs400 likes this.
  3. BrianTrick

    BrianTrick Brian Trick

    The screw on the modulator was wound way out when I started. I expected to notice something when I turned it all the way in like that,but I didn't.
    The lighter weights are supposed to lengthen or delay the shift points. I tried those first,drove it,then did the adjustment on the vac modulator. After each test,there was no change. The shifts were still real early,and you could barely feel it shift. After putting some more miles on it,the shifting became harsh,but still real early. I need to figure out why,all of a sudden,they got harsh,when no additional adjustments were made. Then I still need to lengthen the shift points.
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    What kind of torque converter does it have? What kind of vacuum does the engine make? When working with the governor, make one change at a time. Work with the weights first, one weight at a time. Once you have the WOT shift points close, change the springs, one at a time. It took me quite a while to get my shift points where I wanted them.
     
    matt68gs400 likes this.
  5. Bens99gtp

    Bens99gtp Well-Known Member

    Are you sure the valve in the governor is not stuck? When the weights move you should see the valve inside the stem moving too. If that valve was stuck in the potion of the weights swinging out, it would be wanting to give the trans too much governor pressure all the time causing early upshifts.

    To get these style trans to upshift governor pressure has to over cone throttle pressure. When you crank the modulator in you are raising throttle pressure making it harder for governor pressure to over come it making a later shift.

    Another thing to check is make sure the valve behind the modulator is not stuck also. If it is stuck it could also not be generating enough throttle pressure causing early upshifts
     
  6. BrianTrick

    BrianTrick Brian Trick

    I did check the valve in the governor,when I swapped the weights,and that seems to be working correctly.
    Yes,when I turned the screw in,in the modulator,I expected the shifts to shift later,but it made no difference.
    Another suggestion was to remove the modulator,and see if the trans builder installed a gold spring behind that. If so,try removing it.
    The engine is making great vacuum. The trans line is connected to the 3-port tree,like what you see on a 70 455 and similar engines.
    The converter is a Hughes 2500 stall.
    I'm just a little baffled as I have never had to goof with an automatic transmission like this. Mine have always worked as the should,when I installed them. My own transmission builder is somewhat baffled. He did not build this one.
     
  7. telriv

    telriv Founders Club Member

    Just a FYI. On my '64 Riv. which I bought new with a 3.07 rear at very light throttle pressure it shifts 1-2@ 6MPH & 2-3@12MPH. Been this way since 1964. So have MANY of the earlier TH400's I've worked on. The governor springs have more to do with partial throttle upshifts as well as the modulator adjustments.
     
  8. BrianTrick

    BrianTrick Brian Trick

    The transmission core is a 68,and if I remember correctly,it was from a big car,so you might be onto something. I know the original governor was retained,nothing done with it when it was rebuilt. Other than being converted to a shortshaft,it was a fairly basic rebuild.
     
  9. Bens99gtp

    Bens99gtp Well-Known Member

    Is the valve behind the modulator stuck....if it can move freely it could be stuck in the spot causing low throttle pressure. I think if I recall if the valve doesn't shuttle in towards the center of the trans that would cause early upshifts.

    The spring behind the modulator would add more pressure pushing the valve in the same way as cranking the modulator in, both of these would add pressure cause later shifts.......if it has the spring removing it will only let shift sooner.

    I've had those valves get hung up before
     
  10. BrianTrick

    BrianTrick Brian Trick

    That valve has not been checked yet.
     
  11. BRUCE ROE

    BRUCE ROE Well-Known Member

    Something is wrong. See if the valve behind the vac mod is missing or highly
    modified. Its several inches long, but will slide out of the vac mod hole. Bruce Roe
     
  12. Matt Knutson

    Matt Knutson Well-Known Member

    "The trans line is connected to the 3-port tree,like what you see on a 70 455 and similar engines."

    My vac line has always been connected to the manifold all by itself and never shared the port with any other function
     
  13. BrianTrick

    BrianTrick Brian Trick

    Yes,I have had some that were by themselves,directly to the manifold. This one is connected to the tree,per diagram. It's a bit of a sleeper,that is supposed to look factory stock(iron intake,Q-jet,ported iron heads,light rotating assembly).
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Curious, what diagram?
     
  15. GS44667

    GS44667 Worlds First Stage1 Conv

    All the trans lines hook to this fitting. Never a T on 400-430-455. thumbnail_20170911_120706[1].jpg
     
  16. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    does the metal line have the tip pinched down ware it goes into the manifold vacume tee
     
  17. Matt Knutson

    Matt Knutson Well-Known Member

    I sure hope that engine pictured - above - has PCV valve in play while running
     
  18. Bens99gtp

    Bens99gtp Well-Known Member

    Your thinking the wrong way. Low vac would cause a later upshift. The vac helps pull on the valve to allow gov pressure to over come throttle pressure sooner making a sooner shift. When you floor the car you have low to no manifold vac which causes higher upshift cause it's harder for the gov to over come throttle pressure without the vac assistance. When you turn the modulator in you are adding extra pressure causing later upshift. Just like if you leave the vac line off.
     
  19. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    ^ I like to start with the basics, basics need to be right or were just puffin smoke up the dudes tailpipe.
     

Share This Page