Fluid level is fine since I've had it, and it still red/pink in color and doesn't smell.

I did some manual shifts to hold my speed on hills the other day, now the car will slip out of gear while accelerating in first gear and shudder/chatter when it goes to shift from first to second, and from 2nd to third. Seems to hold gear until I give it gas and then it slips free. So this isn't just WHEN it shifts, its when its just engaging regularly.

The transmission modulator holds perfect vacuum. I disconnected it to intentionally let the transmission shift hard and get the modulator out of the picture, but it still slipped. It will go into reverse and drive just fine every time, it seems as though when I ease into the gas it slips out of first gear and revs up on me like it's in neutral. I just did the head job on this guy for a couple grand and don't want to rebuild a transmission too!

I would think vacuum leak would make hard shifts, not slipping shifts or slipping while just in a gear.

B2 piston? The modulator pressure is okay as I've had the car 5 months and the pressure was set for all gears just fine. Gear shifter itself? Gear shift switch? Cable?

Any help appreciated!

How many miles on the car?

Just to rule out simple things....when was the last time the filter was replaced? If you don't know I would at least change the fluid and the filter so that you have a baseline for maintenance and can rule out any filter/fluid related issues.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

K1 and k2 springs are cheap to replace if you open the pan.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

These transmissions seem to need very clean fluid and filters, or you'll get the weirdest issues.

I had a very similar issue to yours about half a year ago:
Lost 4th gear....

In my case, new fluid + filter + a bottle of Trans-X (to remove varnish from the transmission) worked like a charm.
Might be worth a shot.

P.S. My old fluid was clear red and did not smell either. However, after I dropped the pan I realized the filter was in pretty bad shape. It appeared the previous owner(s) had drained and refilled the transmission (hence clean fluid), but never replaced the filter.

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

You can change the filter and fluid, and a few hundred miles later you'll be sucking enough friction material against the filter that it can't breathe. BTDT.

Drop the pan, if it has a layer of silt on the bottom it's probably toast.

__________________

Gone to the dark side

- Jeff

@Babymog
Well right, but to replace the filter one would have to drop the pan anyway...

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

Yep, I cleaned the pan in my '95 like I was going to bake in it, a few hundred miles later 4th gear came and went. The key symptom was that it would clear up after sitting a while, I guess the silt would fall away from part of the filter and allow some flow.

There are probably 20 different things that can cause the AT to not shift properly though, hopefully yours is a cheap one. Unfortunately without the ability to check pressures you're working a little blind.

Good luck.

__________________

Gone to the dark side

- Jeff