79 Seville with 350 and TH400 won't shift into 3rd, any ideas?
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5.0 and 5.7 Discussion, 79 Seville with 350 and TH400 won't shift into 3rd, any ideas? in Cadillac Engine Technical Discussion; So I'm not sure if I should be posting this in this section, but here goes. I've got a turbo ...
  1. #1
    pacificnwrider is offline Cadillac Owners Member
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    79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    So I'm not sure if I should be posting this in this section, but here goes. I've got a turbo 400 transmission in my newly acquired 79 Seville that's not shifting into 3rd. It has a good feeling lockup in reverse, first, and second, but nothing in 3rd. Any ideas? Servo rebuild? shift solenoid? I'm stumped and would love to hear any ideas you guys might have.

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    carnut is offline Cadillac Owners Master
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    This is a 3 speed trans so you should feel 2 shifts as driven. First shift on light throttle at about 15 MPH to 2nd gear, then a second shift to 3rd at about 25 MPH. If these shifts are later at light throttle, I would suspect sticking governor weights. The governor as you know, is held in place with a clamp on the side of the transmission behind the speedo cable connection. Remove cable, remove clamp and pull the governor straight out. The metal weights should flop in and out easily. Also inspect the plastic speedo drive gear for severe wear. If the weights seem free, not binding and the plastic gear looks good, I would then suspect the kick down switch sticking depressed. I can't remember if the kick down switch is on the throttle body or beneath the gas pedal on a bracket. This trans used a vacuum modulator to control upshifts as well. Do you have vacuum to it? When you pull the vacuum hose off it, does trans fluid drip from it? If so, the diaphragm within the modulator is ruptured (torn) replace it. There are no "shift" solenoids as such on this transmission.

  4. #3
    pacificnwrider is offline Cadillac Owners Member
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    Carnut...THANK YOU! believe it or not, I've had a fairly difficult time finding good information about troubleshooting this transmission with this symptom. I really appreciate your great information. I'll keep you posted about what it might be. thanks!

  5. #4
    stoveguyy is offline Cadillac Owners Connoisseur
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    U could drop pan and inspect/change filter and fluid if u wish. Since car is new to you. Maybe it has been neglected?

  6. #5
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    Quote Originally Posted by stoveguyy View Post
    U could drop pan and inspect/change filter and fluid if u wish. Since car is new to you. Maybe it has been neglected?
    That's the first step. TH400 failures are very rare so I'm not surprised there is limited info about problems.
    What color is yours? Mine is black on black.

  7. #6
    brougham is offline Cadillac Owners Master
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    No 3rd seems to be pretty common with them, when you do a search anyway that is. If filter and fluid doesn't do it get a garage that knows transmissions to look at it.

  8. #7
    carnut is offline Cadillac Owners Master
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    20 days and no follow up?

  9. #8
    carnut is offline Cadillac Owners Master
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    One year and no follow up?

  10. #9
    carnut is offline Cadillac Owners Master
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    Re: 79 Seville with 350 and TH400 won't shift into 3rd, any ideas?

    Quote Originally Posted by pacificnwrider View Post
    carnut...thank you! Believe it or not, i've had a fairly difficult time finding good information about troubleshooting this transmission with this symptom. I really appreciate your great information. I'll keep you posted about what it might be. Thanks!
    rip!

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