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) 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. See Figure 6B-3. 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 wiper located on the pump plate. A hole is located in front of each wiper. The speed differential between the clutch plate and the pump plated 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 of the 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 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. Thus, 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 in to satisfy the cooling requirements of each car model.