Winter Mode Auto 5 speed

[revolver]

On a steep icy slope in Winter mode my front wheels spin (madly) and there is no drive to the rear. Push the button to normal mode and the rear wheels engage and spin and we go forwards. 2001 5 speed viscous coupling. How can this be happening?
If I really rev the engine (over 4000) the rear wheels start to turn but do not really push. Could this explain the famous Subaru video?

[lexefx]

Sorry I can't help anwser this question.

I have been advised never to use the Winter mode on our cars by a wise Volvo sage.

[revolver]

Well to put it another way, apart from engaging a higher gear earlier and a gentler throttle response, does anything else happen in winter mode. I am wondering whether the antilock brakes work to stop a wheel spinning like a Range Rover or is that too clever. Winter mode is certainly having an effect on the viscous coupling and I cannot see the throttle response or higher gear selection being the reason.

[skibo]

I believe the viscous coupling is a completely passive device ( ie, no means of changing its function from the outside - it does what it does, which is transmit torque as a function of the difference in speed between it's input and output ).

So, if 'W' mode is really stopping the rear wheels from turning, then I think your 'automatic braking' theory is a pretty good one. I do recall reading that the 'system' would apply brakes to a spinning wheel to shift power to the other wheel on that same axle. I think that's when the yellow light on the dash illuminates.

I've always wondered how it does that, though... How does it generate the pressure? Likewise, how does it relieve and reapply the pressure when the ABS keeps a wheel from sliding. I've replaced brake pads on my '04 3 times and don't recall seeing any mysterious devices on near the wheel - maybe these gizmos are nearer the master cylinder.

[revolver]

Thank you for your thoughts Skibo. Could you or anyone else with a viscous coupled car just give it a test on a steep icy hill, and see if it has the same effect. At least I will know whether or not it is just my car with a fault.
It is also interesting to hear the advice about never engaging winter mode. Why should that be?

[kain]

+1 on leaving the 'W' button alone. The only reason I can give is because JRL said so. I trust him.

Last time we had a big snow I took the XC up a hill in 6-8" of unplowed snow with relative ease leading me to believe the AWD was working.

Out of curiosity, did you have visual confirmation of rear wheels either spinning or not from someone outside of the car? I don't know how easy it is to tell from inside the car (engine RPM?)

[sjonnie]

So, if 'W' mode is really stopping the rear wheels from turning, then I think your 'automatic braking' theory is a pretty good one. I do recall reading that the 'system' would apply brakes to a spinning wheel to shift power to the other wheel on that same axle. I think that's when the yellow light on the dash illuminates.

I've always wondered how it does that, though... How does it generate the pressure? Likewise, how does it relieve and reapply the pressure when the ABS keeps a wheel from sliding. I've replaced brake pads on my '04 3 times and don't recall seeing any mysterious devices on near the wheel - maybe these gizmos are nearer the master cylinder.

If the brakes were applied to stop both rear wheels from turning while the drive shaft is turning, you'd toast your VC. Like you said, it's a passive device very sensitive to a difference in speed between the input and output shafts, a 6% difference is all it takes to make it engage. So maybe in this case the TCS stops one rear wheel while the other is spinning, I think the XC70 has an open diff.

The Traction Control System is controlled by the ABS hydraulic modulator, basically it has a pump and a bunch of valves to generate hydraulic pressure in whichever brake line it determines needs it. On the XC70 this pump is indeed next to the master cylinder under the fusebox.

[revolver]

I cannot really see how the traction control would affect the VC in the way I described. As you rightly say if it locked both rear wheels it would toast the VC. I ought to experiment with the handbrake I suppose to see if a gentle application has any effect in normal mode on a slippery hill. When I was on the hill in the snow, I opened the drivers door to observe the rear wheel on my side (right hand drive) In winter mode I could see the front wheel on my side spinning freely, quite fast, and by really revving the engine the rear wheel just started to turn but with little forward motion. I did not like doing it for very long for fear of damage to the VC. Push the winter mode button and the rear wheel on my side engaged (could not see the other side) with a soft clunk and began to spin as you would expect it to with full four wheel drive and we had forward motion on the very slippery surface. So it is acting like an on off switch for 4x4 and yes I am sure it is not a Haldex someone may have retro fitted!
You still have not given the reason for avoiding the winter mode button??
As I say I need someone to confirm if they have the same effect on a similar car

[revolver]

I should add to Kain's comment. It is very difficult to know if the 4x4 is working just by driving on snow. I have another little car a Fiat Panda 4x4 which is front wheel drive and you pull a handle to engage the rear wheels. It will go nearly anywhere in these same conditions in 2wd as long as you have forward momentum. It is only when you stop on a steep icy hill and try and restart that you realise the difference that the 4x4 makes.

[TrueBlue]

The VC was a bit of a disaster. Early models were too sensitive and there were issues with transmission windup.
The last year (MY2002) the sensitivity was turned down so much that it just didn't work.

I did some static tests, compared it with an XC90 which has the Haldex transmission and proved to myself (an others), that it just didn't work if the vehicle was at rest. It was not possible to test it if the car was moving - I was not prepared to hire a 4 wheel dynamometer at the time.

There were a series of videos posted on the web which prove my point.
It the car is rolling, then the VC makes some contribution, if not then it does nothing.
Haldex always works....

My conclusion - based on observation - is that the design of the internals allows the fluid to bypass the output shaft "blades" if it's not actually rotating. If it is, then the vanes pick up the goo and produce thrust.

A poor description, but I did try.

[wgriswold]

Mike,

I think you are right. I once watched an XC stuck in a driveway by a small 6" berm that any competent awd vehicle should have not even noticed let alone be stuck on. It was an early 2000's year model perhaps a 2002.

[kain]

It the car is rolling, then the VC makes some contribution, if not then it does nothing.

The VC (if it is working correctly) should bring power to the rear wheels regardless of whether or not the car is rolling but based on the delta in wheel speed between the front wheels and the rear wheels correct? If starting from a dead stop on a slippery surface and the front wheels begin spinning before the car moves then there should be some power that gets transferred to the rear wheels through the VC. Yes or no?

[Mike_v]

+1 on leaving "W" button alone. Just be gentle on the gas. If you then need or want power, it's there (as opposed to when "W" is on).

[Allen]

Trueblue, it sounds like you're talking about a torque converter.

[sjonnie]

The last year (MY2002) the sensitivity was turned down so much that it just didn't work.
That sounds like a faulty VC. If you remove the front drive axles your car will still run. I've driven my '02 w/o front drive axles and it will drive although with some problems associated with backlash during coasting.


If starting from a dead stop on a slippery surface and the front wheels begin spinning before the car moves then there should be some power that gets transferred to the rear wheels through the VC. Yes or no?
Yes. If the traction control is working properly you shouldn't get much spin anyway. The major failing of all AWD system is the ability to transfer torque to individual wheels. These are not serious off-road vehicles, they are just meant to get up up hills (when traction to the font of the vehicle is reduced) or prevent skidding when one wheel looses traction. If you loose traction in both one front and one rear wheel I doubt all but the most sophisticated AWD will get you very far.

[revolver]

1. I know that the 4x4 system on my car is working properly in normal mode
I am a farmer and have to drive in a variety of vehicles often at the limit of their abilities. I even have a sixwheel drive Land Rover. The XC70 is a very capable car for even quite serious off roading, the limiting factors usually being tyres rather than ground clearance or transmission' and the fact I do not want to chip the paint or make dirty!

2. The way I understand the VC to work is that the drive from the bevel gear goes into the VC on a shaft with a series of discs on it. These disc plates have holes drilled in them. The output shaft to the rear axle has a similar arrangement and the discs are very close to one another on both the shafts, alternating between bevel shaft and rear axle shaft but not touching. all this is immersed in a special silicon type fluid which is very temperature sensitive . It remains in liquid form if all the plates are turning at the same speed, as would be the case on a normal straight road. As soon as you turn a corner there will be a speed difference between the shafts and if everything has been designed properly this should allow the viscous coupling to slip without locking up. I feel that the problems with these cars comes from the fact that the VC is locking slightly when turning on the road as the fluid or disc design is too sensitive at these low differential speeds. However when off road, or very slippery conditions cause the front wheels to spin then there is a large differential speed between the discs, the fluid heats up and goes solid. The holes in the discs help lock them together. I believe this occurs over a very small temperature range maybe less than five degrees is all the change that is needed to lock the fluid and this happens in a fraction of a second. So if everything is as it should be and the car is at a standstill, you try to go forward but the front wheels spin then the VC should lock even if there is initially no forward motion. And indeed I have shown this to be the case as I have described before.

What I need to know is what else if anything does winter mode do apart from altering the gear change and throttle settings. If the traction control is made to work harder in winter mode then surely I would see a difference with the front wheels, they should surely spin less in winter mode but in my case they spin more! If the brakes are somehow applied to the rear by the TC in winter mode then I would expect to see the rear wheels locked and being dragged along in the snow by the front wheels.

Can somebody just see if they can make a similar car behave the same way as mine. All you need to do is a standing start on a slippery driveway which has a slope up to the road with an observer looknig at the wheels.