Aux fan is activated by a pressure switch on the low pressure side if the AC system. My fan began running slow when the system needed to be topped off. Once the low pressure reading was over 40psi on a 90 deg F day the fan sounded like an F4 Fantom at takeoff.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!