Fan clutch
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How does the fan clutch work?
  As you know, the cooling fan is located on the end of the water pump shaft on most vehicles equipped with mechanical ( non electric ) cooling fans, the cooling fan in this particular design is driven by the same belt that drives the water pump.  In some instances a fan shroud surrounds the fan. This shroud increases the cooling system efficiency by ensuring that all air pulled in by the fan passes through the radiator. Basically there are two types of fans: a fixed drive fan ( which rotates at water pump - engine rpm - speed ) and the automatic fan clutch, also known as thermal fan clutch. 

 Automatic fan clutches ( aka thermal fan clutches )are hydraulic devices used to vary the speed in relation to the engine temperature. Automatic fan clutches are used with many engines, especially those equipped with factory installed air conditioning units. Automatic fan clutches permit the use of a high delivery fan to insure adequate cooling at reduced engine speeds while eliminating over cooling, excessive noise, and power loss at high speeds.

 The automatic fan clutch has two modes of operation, the engaged mode and the disengaged mode. The disengaged mode ( engine cold or high speed driving conditions ) occurs when the silicone fluid is contained in the reservoir area of the fan clutch. As the temperature of the engine rises so does the temperature of the bimetallic coil. This bimetallic coil is connected to the arm shaft in such a way that as the temperature rises the shaft moves the arm exposing an opening in the pump plate. This opening allows the silicone fluid to flow from the reservoir into the working chamber of the automatic fan clutch.

 The silicone fluid is kept circulating through the fan clutch by wipers located on the pump plate. A hole is located in front of each wiper, the speed differential between the clutch plate and the pump plate develops high pressure areas in front of the wipers, thus the fluid is forced back into the reservoir. But as the temperature rises the arm uncovers more of the large opening and allows more silicone fluid to re-enter the working chamber.

 The automatic fan clutch becomes fully engaged when the silicone fluid, circulating between the working chamber and the reservoir, reaches a sufficient level in the working chamber to completely fill the grooves in the clutch body and the clutch plate.  The resistance of the silicone fluid to the shearing action caused by the speed differential between the grooves transmits torque to the clutch body. The reverse situation occurs when the temperature drops. The arm slowly closes off the return hole thus blocking the fluid flow from the reservoir into the working chamber. 

The continuous action of the wipers removes the silicone fluid from the grooves in the working chamber and reduces the shearing action. Less torque is transmitted to the clutch body and the speed of the fan decreases.  The temperature at which the automatic fan clutch engages and disengages is controlled by the setting of the bimetallic coil. This setting is tailored to satisfy the cooling requirements of each make and model. 

 - How do I know if my fan clutch is working properly?
   There are three ways to diagnose a bad fan clutch:
 1- NOISE: Fan noise is sometimes evident under the following normal conditions: a- when clutch is engaged for maximum cooling, and b- during the first few minutes after start-up until the clutch can re-distribute the silicone fluid back to its normal disengaged operating condition after overnight settling; however, fan noise or an excessive roar will generally occur continuously under all high engine speed conditions ( 2500 rpm and up ) if the fan clutch assembly is locked up due to an internal failure. If the fan cannot be rotated by hand or there is a rough grating feel as the fan is turned, the clutch should be replaced.

 2- LOOSENESS: Under various temperature conditions, there is a visible lateral movement that can be observed at the tip of the fan blade. This is a normal condition due to the type of bearing used. Approximately 1/4" maximum lateral movement measured at the fan tip is allowable depending on make and model. If the lateral movement exceeds the manufacturer's specifications, the fan clutch needs to be replaced.

 3- SILICONE FLUID LEAK: The operation of the unit is generally not affected by small fluid leaks which may occur in the area around the bearing assembly. However, if the degree of leakage appears excessive, the fan clutch needs to be replaced.

   Click on image to learn how to replace the fan clutch  in most GM cars and trucks from early 60's to early 90's