Can anyone recommend some links to look at for troubleshooting? Seems like I'm not getting power to the rear wheels either at all or it's spotty! 92 Passat Syncro. I assume I would start at checking the oil in the viscous coupler. Is there anything electronic?
in theory it is serviceable (their life is only rated around 60 000miles).
the problem is that the VC fluid is so thick that its almost impossible to get out, you have to re-fill it to a very accurate level (84-86%), and the main supplier of the fluid doesnt produce it anymore.
finding a real-world solution is on my to-do list. I just havent had time.