Your aux fan should come on well before 375! You are going to damage your compressor running it like that. Jumper the high pressure switch on the receiver drier and see if it comes on. If not, unplug the aux fan and apply 12 volts to it, see if it engages. If it does, then the problem lies somewhere in the circuitry.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280