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TH400 shift points

Discussion in '1982-Present GM Diesel' started by u2slow, Oct 29, 2002.

  1. u2slow

    u2slow 1/2 ton status

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    I'm finding my Burb shifts religiously. 2nd hits at 10mph and 3rd at 30mph. Doesn't matter if its uphill, flat, downhill, lazy on the skinny pedal - although it will hold the gear a while when I'm into it pretty good.

    If I was going to be picky, I'd say it holds onto 2nd a bit too long for my day-to-day driving. No biggy - I can dig it.

    When my 14SF finally packs it in (and it will /forums/images/graemlins/grin.gif) I've got a 14FF to stick in with 3.73's. It has 4.10 now.

    I don't like the idea of doing most of my around town driving without leaving 2nd gear. Is there an easy way to tweak the shift points? Or at least a way to make it "coast" up into 3rd when I let off the pedal?
     
  2. BARRAZA

    BARRAZA 1/2 ton status

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    Sounds like you may have a vacuum regulator valve problem. Try this test:
    Hook a line directly from the vacuum pump to the vacuum modulator at the tranny and take a test drive to see if there is any difference. A turbo 400 under no or low load should shift from 2nd to 3rd way before 30mph. Also make sure your vacuum pump is providing vacuum and that there are no leaks in the line to the modulator.
     
  3. u2slow

    u2slow 1/2 ton status

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    I didn't think the vacuum lines looked too good... I'll try your test on the weekend, with some fresh vacuum hose.

    What does a direct hookup do? Does full vacuum simulate a no-load situation?
     
  4. BARRAZA

    BARRAZA 1/2 ton status

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    Yeah, a direct line will simulate no load. It should shift very early and softly. Will tell you if the modulator in the trans is working. Also check the vacuum pump output and for leaks in the line.
     
  5. AzzKicker

    AzzKicker 1/2 ton status

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    I tried everything and my tranny still shifts late. I just gave up.


    At 25mph it goes into 2nd and around 35-40 it goes into 3rd.

    The shift from second to thirds is smooth. But from 1st to second better have your seatbelt on /forums/images/graemlins/smile.gif.

    I replaced modulator and get vacuum.
     
  6. Txstruck

    Txstruck Registered Member

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    Mine does the same thing, but from second to third (34mph) is always the same unles im in it. /forums/images/graemlins/truck.gif
     
  7. AzzKicker

    AzzKicker 1/2 ton status

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    ya same here. MY SECOND TO THIRD is Excellent no matter what.


    Its just first to second that really screws it up.
     
  8. Orange Crush

    Orange Crush 1/2 ton status Premium Member

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    I'm having the same kind of trouble with my T-400 except mine shifts into 2nd at 30 MPH and 3rd at 55MPH /forums/images/graemlins/k5.gif
     
  9. AzzKicker

    AzzKicker 1/2 ton status

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    I found the problem to be the Vacum Regulator Valve. Located on the side of the Inject Pump.


    BY PASS it and go for a drive /forums/images/graemlins/smile.gif You wont even KNOW when teh truck shifts its soooo smooth!!!
     
  10. u2slow

    u2slow 1/2 ton status

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    Since I gutted the AC heater box, it shifts nicer. Probably had a vacuum leak in there somewhere.

    Its shifts a weee bit sooner, but not jarring anymore. /forums/images/graemlins/wink.gif
     
  11. DieselFreak

    DieselFreak Registered Member

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    My gas blazer with the TH350 (or was it 400?) shifts like this

    1-2 at 18 MPH
    2-3 at 30 MPH

    <font color="purple">Drive a diesel - be happy</font>


    /forums/images/graemlins/k5.gif <font color="red">DieselFreak</font> /forums/images/graemlins/k5.gif
     

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