The AC switch is most important. It should turn on at around 290psi if I recall. If it comes on properly at the low speed, something is way wrong if you get to the high speed set point which varies by engine but 110 C is a good approx. AC systems low on refrigerant will often not set the low fan, which then causes the engine to reach the temp for high fan on some cars.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician