Trans appeared to be working fine till a recent restart. No unusual noises, shifting ok. Moving into drive or reverse, gives no action or noises. I had checked the fluid level the day before. I attached a pressure gauge to the test port that's beside the Vacuum Modulator, resulting in zero pressure reading. Approx. 2 months prior I checked the pressure and had to increase to the 40 psi from ~25 psi level. I can see the Torque Converter turning, so not a disconnect there.
Prior to this new issue engine speed would always drop from 900 rpm to ~ 500 when moved into drive or reverse.
Engine is strong M110 with 170psi + cyl. pressure.
Can anyone give some possible causes or suggest any additional tests before pulling trans?
An auto trans is the only component on a car that I've never rebuilt. There's alot of good info available to follow on this site & others. I'd prefer to learn a new skill & rebuild myself. I'm sure there are good trans specialists close by, but don't have a good recommendation for anyone in Seattle/Portland area

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

For transmissions in general, sudden loss of drive / line pressure is usually caused by ears breaking off the torque converter hub / inner front pump gear. The pressure regulator spring could have broken, but I'd expect you to have at least some pressure.

I'm not sure what pressure port you are using, is this modulator pressure? ( 40 is way too low for line pressure )

Find the line pressure tap and check that ( I don't have a location or pressure specs )

You can also pull both cooler lines from the rad, place them in a bucket and start the engine looking for flow.

A 900 to 500 RPM drop is pretty large, that is a function of a miss adjusted idle ( fuel ) system. ( Probably lean )

Thanks for the info. I did disconnect both Radiator coolant lines & test for flow. I am getting flow.
The test port described in original listing is a 12mm Bolt just fwd of the Vacuum Modulator.
This procedure has been shown in previous posts such as

W123 Modulator Adjustment DIY - measure it the 'right' way

Hope this helps someone suggest new thoughts on source of problem

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

If you got flow at the radiator, then pump and torque converter are fine. Did you loose Drive and reverse at the same time?

Disconnect the vacuum modulator at the modulator, and see if oil dribbles out? Pull vacuum on it, or attach a hose, and suck on it, yet might hear the diaphram, but that is not a big deal if do not hear it, the driping oil out of the nipple is a big deal.

The mechanical modulator, the car would run, but not shift, yet you can disconnect it from the throttle linkage...

A loose valve body can do this

A worn B2 piston seal can do this for the forward gears, or a broken B2 piston, and it is on the outside of the transmission. I just wrote about this B2...

722.350 4 speed automatic slipping under load in 1, 2, 3 and reverse. Solid in 4th

Gearbox 722.368 repair manual / tips - Mercedes-Benz Forum

All the best,

Martin

I believe the Vacuum Modulator controls shift quality & the B2 Piston controls 1st gear.
In my case the transmission doesn't create drive in any gear. It doesn't even try to engage. When in park you can't turn the rear wheels.

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

mw:

Seems like a pan removal & inspection is in order.
As MAVA suggested, there is a possibility of a loose valve body.
Check that the shift linkage/arm is actually moving the selector valve in the valve body.

Just completed a Pan Removal & could see good movement of Shift Linkage & Selector Valve. Oil was clean & there wasn't any debris in the Pan from a part failure.

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

High mile cars wear / strip the splines on the trans output shaft flange causing no motion. This is where the driveshaft bolts to the trans. Does the car hold in park?

Yes holds in Park. I jacked up car & tried putting in Drive & Reverse (before draining fluid & removing Oil Pan). There was some motion if I rev'd engine up ~2000rpm

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83 280TE- 5 spd, 16.5mm rear sway
85 280SL- 15" Bundts, 5 spd

Internally the one-way clutch(Sprag as some may call it) is no longer locking, and it is common to first and reverse gear..

Look at the attachment on post 8

300TE 722.358 tranny 3rd,4th are gone

Martin