Our 300SDL gave us problems with the AC system. It just wasn't cold enough. The dealer wanted to replace the expansion valve. 12 hours of labor plus parts and suplies. I nixed that because they were not sure that was the real problem.
The expansion valve is behind a cage that was built around the steering column for the airbag.
Someone made a tool to get to the expansion valve without taking the steering support out. Do a search

I feel the problem we had was the ETR control. The AC would work great sometimes and poor to nil others. All this while the pressures were OK.

The Idea of a blocked eveporator I will agree. Heres the link to a pict of my 123 eveporator.

evaporator out of the 300TD

Lots of blockage.

As steve said, check your vacuum elements.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car