On engines with belt-driven cooling fans, a fan clutch is often used to save energy and reduce noise. The fan clutch disengages slows or disengages the engine's cooling fan when extra cooling isn't needed. The fan pulls air through the radiator and air conditioning condenser when the vehicle isn't moving fast enough to provide adequate airflow for cooling. A fan can eat up anywhere from a couple of horsepower up to 12 or 15 hp on a big V8, so by reducing the parasitic horsepower loss on the engine the fan clutch makes a noticeable difference in fuel economy.
The fan clutch also reduces noise by slowing down or disengaging the fan at high way speeds, and some even help speed engine warm up during cold weather.
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.
A slipping fan clutch is often overlooked as the cause of an engine overheating problem.
As a fan clutch ages, fluid deterioration gradually causes an increase in slippage (about 200 rpm per year). After a number of years of service, the clutch may slip so badly that the fan can't keep up with the cooling needs of the engine and the engine overheats. At this point, replacement is often necessary.
Other signs of fan cluch failure would include any looseness in the clutch (check for fan wobble), or oil streaks radiating outward from the clutch hub.
If the clutch is binding, the fan may not release causing excessive cooling and noise, especially at highway speeds
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.
Many experts say it is a good idea to replace the fan clutch at the same time as the water pump if the water pump has failed. The reason is because both age at about the same rate, so if the water pump has failed, the fan clutch may also fail soon. As as we mentioned earlier, a high mileage fan clutch may be slipping excessively increasing the risk of overheating.
When you buy a replacement fan clutch, make sure you get the same type (thermal or nonthermal) as the original. You can always upgrade from a nonthermal to a more efficient thermal fan clutch, but never the reverse. Or, you can get rid of the fan and clutch altogether and install an aftermarket electric fan kit to cool the radiator.