Truck Repair: transmission slip, vacuum modulator, vaccum pump

Question
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Followup To
Question -
Van,
I have a 1985 Suburban, 6.2 Ltr diesel, turbo 400 trans.  The trans tends to slip under acceleration in first gear in the mornings.  The shift through 2nd and 3rd is very strong with no slip.  Once it warms up, no problem.  Could this be a sticking valve body?
Thanks,
Karl
Answer -
Hi Karl,
Possible, or a dirty filter, or leaking filter o ring allowing a little air to be drawn into the pump.
I think a filter and fluid change might fix you up.
That trans probably has a lot of miles on it, so some easy takeoffs would probably help also.
85 also probably has a vacuum modulator on the right side, above the pan flange. Check the vacuum hose, and the modulator for leaking vacuum.
Good luck,
Van

Van,
Thanks for the quick response.  Had a trans service about two months ago, the filter was pretty much plugged up.  I also replaced the vaccum modulator, the vaccum pump, and the vaccum line at the modulator.  
A scenario describing the slip: just started it and the engine is cold, put in gear and accelerate - this is where I get the slip in first gear.  The trans does not slip through shifting.  The shifts are always strong.  Only in gear.  Once warmed up, there is no slipping at all.  This is why i'm thinking a sticking valve body.  Could there be other components causing the slip?
Your thoughts?
Thanks,
Karl

Answer
Well Karl, the part of the valve body that selects the gear is mechanically controlled by the linkage.
But the shift governor could stick, I suppose, even though it should have shifted to the low position the last time the truck was parked.
It is easily removable, without even draining the fluid.
On the right side of the trans, near the rear of the housing, there is a little square cover with four bolts. Remove the cover, and the governor assembly will come right out.
There are some balls in the valve body area that could be slow to move and cover their respective holes if there is sludge in there. May want to change that fluid again.
If you let it sit a few minutes running before trying to take off, and it doesn't slip then, it may be that the fluid is just draining back out of the pump, and needs to draw up again.
You don't want to allow it to slip if possible, cause they can't take much, before an overhaul is needed.
Van