TH400 slushy shifting.

Discussion in 'Automatic Transmission' started by 84 M1008, May 15, 2018 at 11:41 PM.

  1. 84 M1008

    84 M1008 Member

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    Gentlemen, my newly acquired 84 K30 doesn't seem to shift right. It's hard to tell when it shifts. It's more or less sliding when it upshifts. Could it need a new modulator or could the 37 inch tires be the cause?
     
  2. HotRodPC

    HotRodPC Administrator Staff Member

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    All of the above could be the cause. Probably doesn't need a new Vac modulator. If it's an adjustable one, try turning the screw in half to a full turn and see if that makes a difference. My 400 in my C20 was pretty soft and soggy shifting too. I prefer a more positive shift myself so I did adjust the modulator to delay the shift a bit, and when it did shift it was a bit more positive. If the trans is working properly, a Stage 1 Shift Improver Kit to gain a more positive shift.
     
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  3. 84 M1008

    84 M1008 Member

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    Thanks HotRod, this trans was suposedly rebuild 10K miles ago but who knows. I have to find out the line pressure parameters and see if this thing is sick before I start throwing parts at it like I usually do. I'm thinking it may need a different stall converter because of the tall tires, but I don't know too much about how that effects performance. It's a 6.2 so it is what it is. Thanks for the info.
     
  4. HotRodPC

    HotRodPC Administrator Staff Member

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    Oh yeah, if it was built 10K miles ago and shifting soft, I'd for sure tighten the screw up on the Vac Modulator and see what that does.
     
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  5. 84 M1008

    84 M1008 Member

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    It should have been serviced three or four times since also which it probably wasn't.
     
  6. Blue Ox

    Blue Ox Full Access Member

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    Converter stall only has to do with the engine's power curve. @$$uming you have the correct transmission components you should have a lower RPM stall converter for the diesel to take advantage of the low RPM torque and the governor should upshift sooner because of the diesel's lower maximum RPM.

    My experience with incorrect components was the truck accelerated poorly and if you kept your foot in it you would hit the engine governor which did nothing for acceleration, to say the least.

    I don't think either of these items would affect how firm the shifts are.
     
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  7. 84 M1008

    84 M1008 Member

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    It accelerates as I expect a na 6.2 to, but I'd like a crisper shift. I know TH400s were never known for that.
     
  8. da_raabi

    da_raabi Trailer Park Supervisor

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    Shift kit will make it crisp. I rebuilt my 400 in January and put a TransGo TH400 1&2 kit in it with towing settings. The shifts are progressively firmer the more throttle you give it, but always crisp. Reverse is a bit harsh but otherwise the shifts are EXACTLY what I like. Plus the dual-feeding the kit provides will make your direct clutches last longer.

    If your trans is otherwise good this kit would be very helpful, and can be done in the truck. Take a look. Its a $40 well spent!
     
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  9. 84 M1008

    84 M1008 Member

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    Thanks da_raabi! I'll look into it.
     

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