It will either come on due to refrigerent pressure reaching a redetermined temp or for engine temp.

For engine temp, they will come on low around 105C and be on high around 115C I believe. I forget what the refrigerant pressure cut in points are. On my 97, they are infinitely variable so it setermines speed based on its logic.

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Ali Al-Chalabi

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