Spot on Mark, centre diff failure put me out of the RAC in 1996..something ive composed to try and explain the centre VC

The viscous coupling is a small sealed drum filled with a measured quantity of silicon fluid. Inside the drum are two sets of interleaved plates. One set of plates is splined to the outer drum and one set is splined to the inner shaft.

The viscous coupling is installed in the transfer-case to act as a limited-slip. The outer drum is attached to the front output shaft, for example, and the inner shaft is attached to the rear output shaft. As long as both front and rear output shafts turn at the same speed, both sets of plates in the viscous coupling turn at the same speed and there is no relative motion between the plates.

If the front shaft were to begin turning faster than the rear output shaft, indicating front wheel slip, the two sets of plates would develop relative motion and begin slicing through the silicon fluid. The silicon fluid would heat up and solidify, momentarily locking the sets of plates together and counteracting an open differentials tendency to transfer only limited amounts of power. With the plates locked together, relative motion stops; the silicon fluid cools down and returns to its liquid state, freeing the plates, but if the plates were continually slipping, or the locked state was overridden by continual abuse they would eventually overheat in a catastrophic fashion and lose drive completely.