Introduction- I wrote this because I replaced my clutch unnecessarily.
The fan should only rotate about 6 inches when the car has been sitting cold.( A thermal fan clutch is engaged on a cold startup because the fluid drains into the working area when the engine is shut off.) When the car has been driving and you shut off engine, the fan should rotate about a third of the way around. Mark one blade and observe how far it goes.
Another thing, the clutch has a bit of lateral play especially when warmed up. The fan should always be moving. Some fans on other cars seem to have entirely different specs which threw me. Not sure what happens if the silicone drains out. I assume it just free spins. the clutch fan is supposed to much superior to electric fan.

Thermal Fan Clutch

Varies the fan speed with temperature of the air behind the radiator.
Engaged (high speed) operation provides maximum cooling.
Disengaged (low speed) operation provides fuel savings and noise reduction.
Greater life expectancy than a non-thermal clutch.
Briefly engaged at cold start-up.
Engages at about 170° radiator air temperature, (about 30° lower than coolant temperature).
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.
Most models are designed to duplicate original equipment performance. Some Chevrolet/GMC truck models are specifically designed to engage at lower temperatures than the original equipment parts that they replace.

Thermostatic spring on the stock clutches:

The fan clutch operation is regulated by a valve that is opened and closed by a thermostatic spring. The valve controls the flow of a viscous silicone fluid between chambers in the clutch assembly. When the engine is cold, the clutch is essentially disengaged, which is why the fan runs at its slowest compared to the engine's speed. As the engine warms up, the air flowing to the fan assembly becomes hotter. The hotter air causes the thermostatic spring to unwind and open the valve. Silicone fluid from the reservoir chamber flows into the main chamber, engaging the clutch, and the fan spins faster (though it's still slightly slower than the engine).

Thermostatic spring is circled below:

Dismantling

4 10 mm bolts hold the plastic fan to the clutch. 4 10 mm studs hold the clutch to water pump.

Modified by vancouverbc at 11:46 PM 1/14/2009

Modified by vancouverbc at 11:50 PM 1/14/2009

Modified by vancouverbc at 11:51 PM 1/14/2009

Modified by vancouverbc at 11:53 PM 1/14/2009

Modified by vancouverbc at 11:27 PM 1/16/2009