slipping transmission on mercedes 420sel (1990) on 1990 Mercedes-Benz 420SEL

Please help. My transmission is slipping. When I put it into 3rd it works for a while (up to about 35-40 mph). Also seems to be some empty space where it looks like there may have been a belt for something?...near the base of the motor right near the front of the care. Doesn't look like there has been one there for some time.(The guy at the gas station couldn't figure it out at all.) Really need the car to be fixed as inexpensively as posssible. I'm broke and have a sick relative that needs attending to.If somewone could give me a basic idea of what might be causing this, I could get a better idea of what kind of cost it would take to fix it before i drop it at some mechanic.

Check your transmission fluid level, while the car is idling and in PARK, after it has warmed up. Then if the fluid is ok, go by an ATRA certified transmission shop and go for a test drive and get another opinion. Since I am not there, I can't really say what exactly is wrong with your transmission, there are hundreds of things it could be.
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I had the same problem on my 87 420sel, I was looking for possible problem when I noticed a small hose that goes from the tranny to the rear of the engine that was disconnected (if you stand in front of the engine, reach under the rear of the engine, between the wall and the engine actually, the hose is about 3/8 diameter), put it back on, drove the car and started shifting correctly.
Let me know
There is also a modulator on the transmission which is not to expensive to replace. It is a box shape and screws on the transmission