I believe that on certain settings that the compressor is supposed to cycle to remove moisture from the vehicle but this setting probably varies depending upon the year. My 80 says on bilevel that it cycles.
I'm with Greg and just turn it on every now and then while driving. Let it run for 5 minutes or so and then turn it off but then I have the manual switch on my dash for the compressor.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique