Does the A/C compressor actually engage when switched on? Feel the suction line with your hand - it is the thicker of the two lines going to the compressor and runs toward the brake booster on the driver's side of the car. It should be cool or cold, and sweating. If it's not, you have a refrigeration problem. If refrigeration doesn't seem to be an issue, clamp the lines going to the heater monovalve and see if the heat goes away. The monovalve may have failed.

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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