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The aux fans are triggered to run at low speed via the temp and pressure switches on the AC drier. When the temp rises too high the switch on the head triggers the fans to run at high speed (loud).
When AC is off, the aux fans turn off when refrigerant temp and pressure lowers.
Check the drier switches first, then the switch/sensor at the head.
Replaced the head switch on my m103 (190E) because the fans always ran high speed (switch might have probably shorted out). Now the aux fans work on low with the AC on, then switch to high when engine temp rises.
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