Testing the viscous coupling
There is a dearth of information as to how to test the function of a viscous coupling installed in the Vanagon Syncro. Below are some links that might help. Starting with the best, Derek Drew’s comprehensive analysis and test procedures.
Another approach is to measure the torque required to turn one front wheel as described in this link to a Volvo document. Syncro owners are encouraged to try this test and report findings here. Perhaps a range of torque values can be gathered and related to viscous coupling behaviour.
A discussion on the North American Subaru Impreza Owners Club, and extracts from a SAE paper on how the VC works (not bad, still does not fully explain how the plates are forced together in the “humping” stage). This Subaru site has a few discussions about vc function and testing, its worth a look around.