| The air temperature coming through the radiator is sensed by the bi-metal thermal spring on the front of the thermal fan clutch. It expands and contracts with the change in air temperature operating a valve inside of the clutch. When cold, the silicone drive fluid is pumped from the working area to the reservoir. When hot, the valve opens allowing fluid from the reservoir to be transferred to the working area thereby increasing the fan speed. The clutch disengages as the air temperature decreases, closing the valve and allowing the silicone fluid to be pumped back into the reservoir.
A thermal fan clutch is engaged on a cold startup because the fluid drains into the working area when the engine is shut off. The fan clutch will slow down shortly after startup as a result of a pumping action produced by a difference in speed between the shaft and the body of the clutch.
Our Special Clutches are specifically designed to engage at lower temperatures than the original equipment parts that they replace. | 
Brand new massive 21 inch GM Duramax HD cooling fan with special fan clutch which will adapt the Duramax high volume fan to any serpentine belt driven 6.5 Turbo Diesel thru 1999 with flange-mount type clutch (as pictured) in C/K and G/P vans and buses. (excluding motorhomes & H1's) Also available for all 1999 1/2 - 2002 screw-on mount type clutch (additional fee) see our products page. From SSDiesel SupplyTM, the 6.5TD specialists.
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