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