Had my 89 300SE serviced. A/C man told me the Aux. Fan on and off limit
pressure switch is set much higher than normal systems. I know nothing about this system. I believe he said pressure, but he could have said temperature.

Anybody know the actual on/off numbers?

The fans cut in at low speed based on refrigerant pressure in the air conditioning system. High speed comes on based on coolant temp. I don't know for sure, but I believe the trip point is somewhere around 110C.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

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