The guessing game could go on and on. To begin, you should hook up a manifold gauge to verify charge and pressures. The system could simply be low on refrigerant. This may trip the low pressure switch, which is there to protect the compressor from being damaged by running undercharged. Without any empirical data, it's anyone's guess...

My 91 300E has such a slow leak that I just top it off with a can or two of R12 each year (depending on the length of the heat season here in TX). If I let the charge go low enough, the compressor will eventually fail to engage, at which point the system will blow hot air.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280