300sd tranny jerks repeatedly on 1-2 shift

[flash!-ah-ah]

(I posted this in Tech Help but thought it might belong here instead.)

My W116 300SD automatic transmission jerks repeatedly in the 1-2 upshift when lightly accelerating -- like it can't make up its mind whether to shift into second or not.

It only does it when the car warms up.
It only does it on the 1-2 shift.
It only does it when lightly accelerating -- it shifts solidly when I accelerate hard.

There is no slipping occuring - it seems to be switching between second gear and neutral (or maybe first). The only way I've found to get around this is to hold the trans in first gear until revs are high enough to ensure a solid shift.

Crazy thing is that it started happening when I changed my fluid and filter. I'm considering changing the fluid/filter again just to see if that happens to fix it.

Thanks

[ericnguyen]

I think there may 2 reasons:

1) The ATF level is not correct yet.

2) Dirt may get into your transmission when you changed your transmission filter and ATF. You must be very careful when removing the transmission pan and replace the filter.

Eric

I've heard that changing the trans fluid in a car that
hasn't had it done in a while may lead to this. The
old fluid keeps the sludge (for lack of a better term)
in places like clutch packs which may have developed
a small leak. The new fluid flushes out the old and
the result of wear becomes more obvious. This is
just one possibility, especially on a high-mileage trans.
The easiest fix to try first, besides the correct level
of fluid, is a simple adjustment. On the driver's side
of the trans is a round piece protruding about 3/4"
with a vacuum line going into it. In the center of the
(~3" dia.) modulator, as it is called, is a rubber cap.
Pry it off, and use a small pliers or similar to carefully
pull out a little tee handle. Visualize its current position
before turning it, and just pull it out enough to be
able to turn it. By turning it clockwise, the pressure
is increased and the shifts become firmer. You may
give up other shift qualities but this is one way
that might take care of the 1-2 indecision. Try turning
it one full turn cw and see how the trans behaves.
You can turn it more if one turn doesn't quite do
it. There are many threads about getting the correct
vacuum signal, which would be the next step if
this doesn't work.
Adjusting the trans due to wear does not necessarily
mean it is worn out. If you do decide it is worn out
(hopefully not too soon) then you can use Lucas Trans
Treatment/fluid. It is like 90 weight trans fluid and the
extra viscosity slows the internal leaks enough to
make it shift close to normal. Good luck
Mark T 82' 300SD 280k

[flash!-ah-ah]

Thanks Mark. I'll try that.

Does anyone know anything about those trans fluids on the market that are supposedly designed specifically for high mileage vehicles? (Valvoline makes one I know.) Are they worth anything?