I have a viscous coupling on my S80 T6 and have always been curious as to its function. Looking under the car, I believe it is inserted in the axle shaft going to the left side, between the tranny and the bearing block, and before the inner CV joint.
I have heard this called a viscous and/or visco differential. It is a $310 feature on a TDS S80. My concept of a viscous differential would be a viscous link between the left and right axles which would resist differential action and would be, at least logically, inside the differential. The visible coupling is external to the tranny, and I sure would like to know how it could affect the other axle. Any techs out there? A word on this would be appreciated.
Thanx,
Larry P.

I believe that it's only function is to attempt to control excessive wheel slippage from the right side wheel as the turbos add alot more HP/torque. I say "attempt" because even with it, the RF wheel spins too easily on full or near full excelleration as is. I can only imagine that it must be far worse w/o it.

This has a rudimentry explaination:

http://www.howstuffworks.com/differential7.htm

Are you in Europe? I think the viscous coupling is only
available in Europe (that was the case on the S70Rs which were
only in Europe). Just curious.

Thanks for the URL Ray. I am in the US and ordered a TDS car. Volvo has quietly included this feature on some T-6's over the years. I ordered it when it was offered as a feature for my custom built TDS car.

The reference describes viscous couplings joining two halves of a drive shaft, and looks like the Volvo device.
When showing the differential application, the website shows the device internal to the differential.
If the external Volvo coupling is simply joining the two pieces of the right side front axle, how does it interact with the differential? What benefit does one derive from the device? I'm still puzzled.

The coupling is to reduce wheel spin. A problem on high tourqe
front wheel drive cars. I'm not exactly sure how it works in the
applications external to the transmission.

I do recall reading British Car magazine and they liked the S70R's
suspension (which has the viscous coupling) but found the S70T5 had
problems with wheel hop (no viscous coupling). So, I suspect it
works rather well based on this description. I wish my T5 had
something like this.