Centre Differential Viscous Coupling Failure.CorgiWerx have rebuilt over 500 Subaru Centre Differentials and have built up a wealth of product knowledge on the viscous couplings. Apart from the occasional mechanical failure of the diferential gearset, the common failure is deterioration of the silicone viscous fluid. The viscous fluid can expand, the fluid can thicken, the fluid can separate.
Here is one of the typical fluid failure mechanisms.
Attached is a photo of the clutch plates inside the viscous coupling.There are outer clutch plates with external teeth that engage in the body of the centre diff and inner plates with internal teeth that engage with the differential gears. There are interlaced A-B-A-B=A=B etc. The plates are not designed for friction contact, the plates float between each other dragging themselves through the viscous fluid.
In the picture the outer plates are on the left and the inner plated on the right.
Here, the upper two plates are "normal" and the lower two plates show the signs of the viscous fluid failure coating the plates with a black tar like wax. This can be best seen on the plate on the lower right on the teeth from the 8 O'Clock to the 2 O'Clock position.
When the diff gets hot, the waxy coating black coating on the suface of the plates acts like a brake and casues the centre differential to bind up.
CorgiWerx - The Subaru Centre Differential Subject Matter Experts