The first thing I would do is identify whether you are running on the low speed fan or the hi-speed fan. I would identify the two relays and pull them one at a time to see which one is working. the k10 is the low speed and the k9 is high speed.

The low speed fan is controlled matter of factly by the S32 high pressure switch. The idea being when the refrigerant pressure gets too high the fan comes on.

The high pressure fan relay is controlled by the A/C pushbutton control unit under some program. Look for the ground signal at pin 5 term 85 on the K9 relay. If this is grounded both switches S25/5 and B11/7 need to be evaluated as the pushbutton controller is telling it to come on for some reason. These relays should be the standard silver 5 pin relay. There should be a number of them in the box. Try different ones to evaluate the possibility of a stuck relay. Check the number on the side of the relay if its the same then it will be interchangable.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician