Just thought I'd toss this in from my experience on other all wheel
drive vehicles:

In general, Viscous Couplings work on the principle of thermal expansion
and friction.  The VC is a multi-plate "clutch" with the plates closely
spaced and immersed in a specially formulated silicon fluid.  This fluid
lubricates the plates to the degree that they will slip somewhat to
allow differential wheel rotation between the front wheels and rear
wheels.  At low rpm it also allows slippage, and that is the basis of
the VW test mentioned.  If the VC has lost it's fluid, it will be
"locked up" and unsafe at worst (skidding on high speed turns), or will
rapidly destroy your tires at the least.

As slippage between the plates speeds up with lost traction at one or
more wheels, the fluid heats up and expands, forcing the plates tighter
together, effectively "locking up" the VC so you can off-road
effectively.  Without a VC full time all wheel drive would not be safe
due to wheel skipping during cornering.  That is why "four wheel drive"
without a VC can only be used on mud and snow, which allows the wheels
to slip.

This component is usually very durable, but if you note any sign of
leakage have it serviced.  This will be expensive but not as expensive
as replacement.

Have fun!

--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext 208

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