Before getting all carried away, grab hold of the return line up by the brake booster and see if it's cold and/or sweaty. If it is, you know you have a problem with the air doors, OR you have an issue with the monovalve not shutting off all the way. If it leaks through, the amount of heat the heater core throws is enough to cancel out any cooling that takes place from the A/C system.

Are you also CERTAIN that the compressor is coming on? You see the center section of the pulley pull in and start spinning? If you're low on refrigerant, it's likely that the compressor will cycle a lot.

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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]