HOW THE FAN CLUTCH WORKS
There are two basic types of fan clutches: thermal and non-thermal (also called "torque limiting"). Thermal fan clutches have a temperature-sensitive bimetal coil spring on the front that reacts to temperature changes. When the air coming through the radiator is hot, the spring expands and opens an internal valve that reduces clutch slippage. This causes the fan to spin faster for increased cooling. As the air cools, the spring contracts and closes the valve. This increases the amount of clutch slippage, allowing the fan to slow down and decrease cooling.
The clutch consists of a fluid coupling filled with a silicone based oil. In the cutaway view at the left, the area between the teeth on the clutch plates is filled with silicone fluid. An internal valve opens and closes a passage between the main fluid cavity and a fluid reservoir. When the passage is open, fluid enters the clutch and makes the fan to turn faster. When the valve is shut, fluid flows back to the reservoir but doesn't return, causing the clutch to slip and the fan to turn more slowly.


The non-thermal (torque limiting) fan clutch doesn't have a temperature sensing capability. It reacts only to speed, slipping to limit maximum fan speed to about 1200 to 2200 rpm depending on the application.

CHECKING THE FAN CLUTCH

A good clutch should offer a certain amount of resistance when spun by hand (engine off, of course!). But if the fan spins with little resistance (more than 1 to 1-1/2 turns), the fan clutch is slipping too much and needs to be replaced.

If the fan binds, does not turn or offers a lot of resistance, it has seized and also needs to be replaced.

Fan speed can also be checked with an optical tachometer, by marking one of the fan blades with chalk and using a timing light to observe speed changes, and/or listening for changes in fan noise as engine speed changes.

You should also try to wiggle the fan blades by hand. If there is any wobble in the fan, there is a bad bearing in the fan clutch, or a worn bearing on the water pump shaft. A bad water pump bearing will usually cause the water pump to leak and/or make noise, but not always. Remove the fan clutch and see if the play is in the water pump shaft. If it feels tight (no play or wobble), replace the fan clutch.