Let’s talk about those Gen 1 AWD Systems
Tech Talk about Gen 1 Volvo AWD System
On the viscous system Volvo used: It was a good system and if the tires were rotated and no odd tires ended up in the mix it gave good service. Age on the VC has introduced some problems and baked grease in the front CV joint and middle support bearing failures are the service issues with age. And the splined connector sleeve wearing.
Gen 2 VC’s on the Volvo P2 models seemed to have been very low trouble and so far I’m not aware of those getting vibrations and other VC issues. I also do not see the VC on those sling out oil from midsection of the VC.
jimmy57 » There are a few different systems.
Gen 1 that is the 850 platform is the delicate one.
Gen 2 2001-2002 XC70 is less so but still needs care.
All others since then are very low in problems other than a pressure/temp sensor that has been updated.
The front drive connection to rear drive is via wavy discs splined alternately to input and output with a silicone gel, viscous coupling, VC.
There is also a one way clutch in the VC that is overridden up to approx 30 mph so you can have power to rear wheels in reverse.
First point: Tires that move the car have slip that is minimal but does increase with speed or load. This is why the rear tires on highway driven RWD vehicle with perfect alignment wear out faster than frts and FWD those tires wear out way faster than rears. This slip means the VC will have some minor speed difference churning the silicone gel between the wavy discs when you are driving on pavement. The VC will tolerate this well when the tires are the same circumference or even better if the worn tires are on rear. The one way clutch lets rears overspeed fronts but is purposely designed to transmit power when front is faster than rear as that is what happens when you lose traction.
Second point: The VC’s get stiff with age. My hypothesis soon to be tested is that the silicone oil that is the liquid carrier of the fine particle silica sand that makes silicone gel slings out and lets the silicone gel get stiffer. The gel is a material with a material characteristic called anti thixotropism, i.e., it gets stiff when stirred. The concept is like having a bucket full of marbles. If you move your spread fingered hand through the marbles in bucket they would pass through your fingers but with effort needed. If you tried to move your hand fast through the marbles they would pack against each other and “lock” up.
If the silicone oil making the gel a viscous liquid gets lost then the gel is little more than sand packed in between the wavy discs. Sand would not want to flow. When the VC is too rigid due to tire issues or age then it no longer supports the tolerance of the speed differential and it strains the angle gear (AKA bevel gear) that turns the sideways output of transaxle 90 degrees and has the rear driveshaft connected to it. When the VC becomes intolerant of the speed difference then vibrations are felt and when you exit car after a drive you will usually smell gear oil as it is vent through the angle gear vent.
On a perfectly working system a problem with mixed tire sizes where rears are larger circumference the angle gear and VC will suffer and possibly fail.
Why Were Gen 1 AWD Systems so Delicate?
Last Updated on March 26, 2022
