As far as I know the fan should slow down when the set(ted) temperature is reached and the climate control is switched in auto position. The dealer can plug in diagnostic tools to check the airco system (at least...that is a possibility on most of the models from that same year).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)