The aux fans on our C220 seem to be activated by both AC use and engine temperature. At lower engine temps, they cycle on and off frequently with the AC on, but not with it off. At higher temps, they come on when the guage is just under the 100 mark, run for a while and shut off.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe