ezrider,

I forgot to address the question of why a fan clutch directly. I believe it is related to need and economy.

The fan is loaded on what is essentially a cubic curve, meaning at twice the speed it uses 8 times the power. If the fan were locked on all the time and ran linearly with engine speed, it would use more fuel, and at higher speeds this would be significant. For example, as a fraction of a horsepower at idle or low rpm city driving might be several horsepower at 80 mph. Additionally, a fan designed to run at 2 or so hp would have to be redesigned to run at 6 or 8 hp, meaning the fan would have to be quite a bit more robust and cost more initially, and then use more fuel all the time.

The other factor is that you should not need such a fan. The machine at higher loads should be running fast enough that the air being forced through the radiator is more than adequate to remove the heat from the radiator without a fan at all. An exception might be towing a heavy load up a hill where you are in a lower gear and not moving that fast. So, considering the permanently on concept gives you the most output when you need it least or not at all, and cost extra money to buy and operate, it is not used by MB or any other automobile manufacturers these days. Good luck, hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)