All Wheel Drive Viscous Clutch
(AWD) All-wheel drive vehicles are controlled by an electronic control module, vacuum, or viscous fluid. They use viscous couplings to provide additional stability. Viscous couplings have a thick fluid contained between a series of plates. One set of plates is attached to one axle, and the other set is attached to the other axle.
When the AWD vehicle is operating under normal conditions, the plates and the thick fluid are turning together at the same speed. It's not until the two axles are spinning at different speeds, such as when a tire breaks loose and begins to spin, that the unit transfers more torque to the slower axle. The faster disks are dragging the thick fluid that drags the other disks along with it. This action transfers torque to the other axle.
Most viscous couplings can be tested with a bench vise. Place the unit in the soft jaw vise being careful not to damage the case. Install the driveshaft on the companion flange and use a foot-pound torque wrench to apply torque to the other end. Measure the turning torque required to spin the axle. These torque measurements are typically around 25-35 foot-pounds. Check with the vehicle manufacturer's specifications before proceeding.
The viscous fluid is added by weight, so when a unit is found faulty, it is typically replaced with a new one. The fluid becomes thicker when it's heated to provide more gripping power between the disks when it's needed most.