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More transmission issues

Discussion in 'GM Powertrain' started by rustyk2, Mar 26, 2010.

  1. rustyk2

    rustyk2 Rockstar 4 Years 500 Posts

    Hey guys, got the tranny to stop dumping fluid but now it developed a real problem. When it first starts up and I engage the tranny, it runs through the gears fine. After a few minutes it will drop from third to second. Can manually shift down, then slid the shifter to D and it shifts up then drops again. Think the bands toasted but not sure. Replaced the modulator valve and fluid and filter. Hope it can be saved, but do not hold out much hope.
    Thanks in advanced

    Dana Pelletier
  2. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    A restricted vacuum line can cause this problem. Make sure the top hose at the intake manifold is not cracked ot pinched.
  3. rustyk2

    rustyk2 Rockstar 4 Years 500 Posts

    Hi Jim, Tried cleaning out the steel line had some restriction, cleaned it out. replaced the rubber hose connections on the line but still same problem. Question, can you tighten or adjust the band? if so how and where. I heard the adjuster is under the pan by the valve body.
    I really appreciate the help.
    Thank You,
  4. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    Dana, Unfortunately your transmission's band has nothing to do with your problem. The THM 350 and THM 400 both have one band. The band is only used for engine braking in 2nd gear. You didn't say what trans you have but your problem is not the band. If your transmission is full of fluid it should engage immediately when shifted into gear. If it delays you either have a problem with the filter, restricted, missing o ring, etc, or the forward clutches are burned up. How did the pan look when you serviced it?
  5. rustyk2

    rustyk2 Rockstar 4 Years 500 Posts

    Sorry about the lack of info. Have the th400. The fluid was medium dark, but not burnt smelling, the filter was really bad the magnet had the usual crap on it and some grit in the pan. no peices or chunks of gears or the like. I'm afraid the only cure is a rebuild. but any help is greatly appreciated.
    Thank You,

    Edit...The tranny engages immediately when shifted to D, runs through the gears then drops into 2nd and stays there. When I stop, it shifts through the gears goes to third then drops again
    Last edited: Mar 28, 2010
  6. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    Try unplugging the kickdown wire at the gas pedal or at the trans. Either end. If the switch is stuck it will do what yours is doing.
  7. rustyk2

    rustyk2 Rockstar 4 Years 500 Posts

    Thanks, will try that tomorrow, getting late and raining in SC. thanks again for all the help. Could the detent switch be bad?

    Last edited: Mar 29, 2010
  8. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Please keep us informed im kinda learnign as this thread progresses. Since alot of my buddies have these trannies...
  9. rustyk2

    rustyk2 Rockstar 4 Years 500 Posts

    tried to unplug the wire, got it off, no change still dropping third. Any more options?

    Thank you,
  10. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    Are you are sure you have good manifold vacuum at the modulator? If you pull the line off the engine should speed up. If the vacuum is good and there was no oil in the vacuum line then what I would do next is look at the governor. Take the 4 bolts out and remove the cover. It's on the left rear of the transmission. These governors have a plastic gear which can wear out. Once it wears out the trans wont shift but if it's almost worn out or the gear is loose it can fall out of high gear.

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