K 20 P/U 4WD V8-454 7.4L (1985)
Fan Clutch: Description and Operation
Fig. 1 Fan drive clutch assembly, 1984---87. 1980---83 similar
Fig. 2 Variable speed fan with flat bimetallic thermostatic spring
Variable Speed Fan W/Bi-Metallic Coiled Thermostatic Spring
WARNING: Do not operate engine until fan has first been inspected for cracks and/or separations. If a fan blade is found to be bent or
damaged in any way, do not attempt to repair or reuse damaged part. Proper balance is essential in fan assembly operation. Balance cannot be
assured once a fan assembly has been found to be bent or damaged and failure may occur during operation, creating an extremely dangerous
condition. Always replace damaged fan assembly.
The fan drive clutch, Fig. 1, is a fluid coupling containing silicone oil. Fan speed is regulated by the torque-carrying capacity of the silicone oil. The
more silicone oil in the coupling, the greater the fan speed, and the less silicone oil, the slower the fan speed.
There are two types of fan drive clutches in use, one with a flat bimetallic thermostatic spring, Fig. 2, and the second with a coiled bimetallic
thermostatic spring, Fig. 3.
The fan drive clutch with the flat bimetallic spring works with a control piston on front of the fluid coupling to regulate the amount of silicone oil
entering the coupling. The bimetallic strip bows outward with an increase in ambient temperature and allows a piston to move outward. This piston
opens a valve regulating the flow of silicone oil into the coupling from a reserve chamber. The silicone oil is returned to the reserve chamber through a
