Check the electric fans. My AC (300SE) had the same symptoms, crappy or no AC in the city and decent on the highway. The fans were not coming on, the wires under the fuse box were fried. Fixed it and cold AC now.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).