Start by testing the A/C performance. See if the return lines to the compressor are cold and sweaty with the temp wheel set to full cold. If not, you know you likely have A/C problems.

Next suspect is the monovalve (or duo valve depending on your model). They can fail where they stay open. My SDL did this and it was miserable... New cartridge solved my issue. The cartridge looked fine, but the shaft had gotten rough. Just enough that it wouldn't pull closed...

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]