here's the factory manual section on OM603 fan clutch operation. Note the bimetal strip actuates at an air temperature of 71C, which should equal a coolant temp of "90-95C". There is no equivalent information for the new OM606 clutch but again, I suspect it triggers at a higher temperature; and/or is less solidly coupled when it does engage. (?)

PDF file, 115kb - save & open with Acrobat Reader:

http://www.w124performance.com/docs/mb/OM60X/OM603_fan_clutch.pdf