These “Viscous Couplings” are located in the Model 247 and Model 249 Transfer Case Assemblies. They are filled with a special fluid, along with sets of internal plates. The plates are connected to either the front or rear output shafts. They are used to transfer torque from the rear wheel to the front, or visa versa, in the event that either set of wheels begin to slip. When one set of wheels spins freely, the viscous fluid between the plates drags the slower set of plates along. This transfers torque to the wheels where it is needed! The viscous coupling does not interfere with turns. The advantages of these units are not felt until wheels actually lose traction. Because these units are filled with a viscous fluid, the viscosity of this fluid will break down over time. This occurs especially in hot climates, or severe use. When this happens, the effectiveness of 4WD is severely impaired.
Images courtesy of Andy Rusho & Crown Automotive