Tranny Problem...

Discussion in 'Trucks and Trailers' started by Razzlo, Feb 12, 2001.

  1. Razzlo

    Razzlo LawnSite Member
    Posts: 40

    '84 C10, 350V8 w/TH350 Trans. Rebuilt w/ 30K on it. Does not shift properly from 2nd to 3rd. Can foot shift sometimes, other times have to slip into neuteral then Drive. This seems to work. Could this be a modulater or what? Thanks.
     
  2. 75

    75 LawnSite Senior Member
    Posts: 992

    Okay, I'm no trans expert and the book on my shelf is for TH-400's (Turbo Hydra-matic 400 Transmission by Ron Sessions) but in the troubleshooting chart for "No 2-3 shift, four entries appear: two of them deal with detent switch problems which I believe don't apply to a TH-350, the other two are "Valve body malfunction", it says to remove pan, check for loose valve body bolts. If bolts are tight, remove valve body & check for blown, incorrect or mispositioned spacer gasket. Also, shake valve body and check for stuck 2-3 shift valvetrain. Valves should move of their own weight. This can be done with the trans in place. Number two is more drastic, it says "Direct clutch circuit leaks", and starts with remove and disassemble transmission. Again, I'm no expert on the subject and the book I have is for the TH-400, but there are a couple of ideas. Hope this can of some help to you.


    1975 GMC C-35
     
  3. Razzlo

    Razzlo LawnSite Member
    Posts: 40

    Thanks. Forive my ignorance, but What exactly does the detent switch do? After looking in the book, it is behind the gas pedal. Also I saw in the book that there is a cable from the carb, i.e. detent cable????? Could this have anything to do with it? NOTE: The wife drove the truck yesterday and said that she just wiggled the shifter and the trans went into third. Could it be a worn linkage or is this just a coincedence? Thanks.
     
  4. mike reeh

    mike reeh LawnSite Member
    Posts: 229

    detent cable is sometimes referred to as the kickdown cable.. think of it as a throttle position sensor for the tranny.. th400 has electric detent switch, mounted on the gas pedal, th350 uses a cable hooked up to carb linkage


    mike
     
  5. 75

    75 LawnSite Senior Member
    Posts: 992

    Right on the money with your description of the detent cable Mike, it causes a forced downshift to 2'nd ("passing gear") when you're at or near wide open throttle. As for the linkage being worn as a possible cause of the problem, I would think it is possible, it could also be just out of adjustment. Again, I'm not expert on transmissions so I'm largely guessing here!
     
  6. 75

    75 LawnSite Senior Member
    Posts: 992

    Sorry I can't come up with anything specific, but here is another thought: Is it possible that the detent cable (kickdown circuit) is out of adjustment? I am wondering if this is causing the unit to hold in 2'nd, since the kickdown circuit causes a downshift to 2'nd under a heavy throttle application.


    1975 GMC C-35
     

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