Could be....

Klima relay
pushbutton control unit
slipping clutch
bad compressor speed sensor

You really need the factory manual, there is a separate book on A/C, with a very detailed problem solving section.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles