The A/C compressor runs when the climate control is in Automatic, Bi-Level, or Defrost mode. The compressor does not run in Economy mode. At least that's the way GM did it when they used the V5 compressor. I suppose it's possible that MB shuts the compressor down under certain circumstances, but the whole idea of variable-displacement was to do away with a layer of electronic control.

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1989 300 SEL that mostly works, but needs TLC