Also check your belt tension or the quality of the belt. If the belt slips at all, the compressor will be disengaged until the engine is restarted. Mine used to do that, especially if/when I drove through a puddle or if it was raining out. I never heard a belt squeal or anything. After tightening properly, the issue disappeared.

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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