Viscous Fan Clutch
Viscous fan clutches are belt-driven and found on many older rear-wheel-drive passenger cars. They contain a drive and driven plate with annular grooves that fill with a thick viscous silicon fluid.
These fans are needed at slow speeds, typically below 35 mph and idle. They can cause a vehicle to overheat at idle and in heavy traffic. The fluid wears, rendering it incapable of grabbing the drive and driven plates and offering the gripping resistance needed to hold them together. If the clutch is constantly engaged, the vehicle will suffer from poor gas mileage and loss of horsepower. They make a loud whirling sound at highway speeds and revving the engine.
Because the crankshaft drives this fan, check the accessory drive belt for wear and glazing. A viscous fan clutch contains a thick fluid that can leak from the unit. This fluid's viscosity is measured in CST or centistokes and degrades over time. Check the assembly for leaks through the seams and around the shaft.
They also have a thermostatic spring. Check it by releasing it from its seat and measuring the distance between the spring and the retainer. Check the manufacturer's specifications for any special procedures.