Why is Volvo's AWD system so delicate?

Post by **Nevada1906** » 27 Apr 2012, 16:05

I've been hearing all kinds of horror stories about the AWD system in our cars. What gives?

Volvos are generally pretty tough. I'm not necessarily saying I'd like to approximate an EVO with my 850 using OEM parts, but just for laughs, why couldn't I?

Post by **jimmy57** » 27 Apr 2012, 17:09

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 characterisitic called antithixotropism, 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 differntial 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.

Post by **Nevada1906** » 27 Apr 2012, 17:42

Wow, thanks for all that.

Sounds like the durability problem comes from using a viscous coupler instead of a mechanical differential. I'm going to guess this was done with the intended yuppie demographic in mind; they want smooth and quiet, not bank-vault tough.

Post by **jimmy57** » 27 Apr 2012, 23:47

VC's were used on a lot of AWD cars in 90's but many didn't make it to US. There were several duds.
There were many less than stellar AWD systems devised to try to make the power flow as invisible as possible.

Mad Dog Tannen · Post by **Mad Dog Tannen** » 28 Apr 2012, 01:14

Nevada1906 wrote:Wow, thanks for all that.

Sounds like the durability problem comes from using a viscous coupler instead of a mechanical differential. I'm going to guess this was done with the intended yuppie demographic in mind; they want smooth and quiet, not bank-vault tough.

Viscous couplings provide a difference in power distribution (generally around 65/35). So there can be more power to the rear wheels than there is to the front. If it were just gears then it would be equal 50/50 power to the front and rear. Hence the only debatable difference between 4 wheel drive and all wheel drive.

Post by **jimmy57** » 28 Apr 2012, 10:39

The torque distribution numbers have to be taken with a dose of physics.
Volvo says up to 95% can go to one wheel. That means if you park on 3 patches of ice then the one tire with traction will theoretically have 95% of available torque. This does not mean the other three do not spin. If you take a torque wrench with the intent and capability of tightening a fastener to 500 lb-ft but the fastener is stripped and rotates freely then you have zero net torque on fastener. The torque output of engine through transmission and sent through shafts to wheels must be matched be matched by traction to be effective torque to move the car.
IF you have traction on all tires, a locked AWD/4WD system and 500 lb-ft of torque then effectively 125 lb-ft to each wheel.
The likelihood of breaking a tire loose on take off is diminished greatly as the torque through any one tire is too low to exceed the frictional coefficient of the tire on the pavement that gives the torque limit of that tire to slip.
Those who have taken rear driveshaft out of AWD cars making them FWD probably have discovered torque steer that never was an issue until then. The seemingly dormant AWD system was sharing (not equally) the torque and the reduction in torque to front wheels dropped the loading on front tires enough to make this not be felt.

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 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.

Post by **rmmagow** » 28 Apr 2012, 21:58

So, what's a good check to see if it is worth putting a propshaft back? Mine started making a lot of noise and vibration, I took it out and the ends were more or less shot as was the center bearing. Now I drive FWD with the plan to put a reman propshaft in at the end of summer. Is there any way to determine the health of the VC? I don't see a lot of crud there or anything like a leak. I will be changing the bevel gear fluid later this summer.
So, put in the shaft or just leave it FWD? Car has 169000 and runs very good, 600 miles a week mostly highway.

Mad Dog Tannen · Post by **Mad Dog Tannen** » 29 Apr 2012, 00:35

I am not sure about the Volvo VC's per say, but in other AWD systems that I've owned leaving a drive shaft out for extended periods of time can hurt your VC. I do not recommend it.

Post by **jimmy57** » 29 Apr 2012, 11:31

The location of the VC in volvo's system is not the same as others. It is fine to run w/o rear sahft in.
The VC's in center diff used as a "soft" limited slip will be destroyed as without front and rear drive working. Tthe VC is the prime torque path in that case.

Post by **dcarlson12** » 29 Apr 2012, 17:57

To jimmy57:
Are the AWD drive after Gen 2 2001-2002 XC70 a new/different design? i.e. with a system that doesn't use the viscous coupling or ???