How is XC70 AWD supposed to work?

[Silvestris66]

I've accidentally chirped the front tires (Michelin Harmony) a few times on my '03 XC70 when pulling out into traffic from a stop, and that's made me wonder: If this car is really All Wheel Drive, what does that mean? What situation would make it noticeable that this is an AWD car and not just a front-wheel drive?

I've wondered if the AWD function is actually working.

[MacNoob]

It needs a bit of speed differential front/rear before the Haldex engages. You'd really notice it working in snow/ice/mud - anywhere that there is a pretty consistent loss of traction.

It's fantastic here in the frozen North.

[pbierre]

The Haldex AWD takes about 1/4 rev of front wheel slippage to engage the rears. That's about 20" of wheel surface travel.

I've never tried it, but you could place two layers of cardboard under the front wheels, with at least 20" of feed length. Then, you could measure
how much slippage occurred, and back-calculate it into front-wheel revs.

my source:
http://www.awdwiki.com/en/haldex/

[Silvestris66]

It needs a bit of speed differential front/rear before the Haldex engages. You'd really notice it working in snow/ice/mud - anywhere that there is a pretty consistent loss of traction.

It's fantastic here in the frozen North.

I'm in the American Southeast where we get maybe 2-3 snowstorms per year. I have noticed that it drives very well in snow -- on the few occasions where I've had the chance.

[gary g]

Does this mean that the rear wheels do not engage until the Haldex senses slippage?
I have an old CRV and that is how that system works. It is a FWD car until the front wheels slip.

My Volvo is an '01, so it is not a Haldex and I suspect that the rear wheels are always engaged since it is sensitive to tire tread evenness.

Just curious...

[NTXC]

I'm interested in this as well. I just got an 05 XC70 about a month ago and I'm wondering if I should change the fluid in the haldex or do the fluid & change the pump.

~108K miles, likely never done previously (I'm doing a transmission drain & fill this weekend to prepare for my first long trip with this car. Shifts beautifully but the fluid is definitely looking like it's time to replace).

[hoonk]

Does this mean that the rear wheels do not engage until the Haldex senses slippage?

Yes, the 03+ cars are FWD until the fronts slip and then the haldex clutch engages to drive the rear wheels. The 01 -02 (and 98-2000) cars use a viscous coupling in the driveshaft that "stiffens" and transmits more power to the rear wheels when the fronts spin faster than the rears. Perhaps the advantage is the electronic haldex system can transmit power directly, where the viscous coupling always slips

[sjonnie]

Does this mean that the rear wheels do not engage until the Haldex senses slippage?

It's a change in torque distribution, from 90/10 to 50/50

I have an old CRV and that is how that system works. It is a FWD car until the front wheels slip.

That describes 4WD, no AWD systems work that way. In 4WD the system is either off and 100% of the torque goes to the front wheels or the driveshafts are locked and the torque is always split 50/50. In AWD you install a mechanism for shifting torque between all wheel all the time. Your old CRV uses a viscous coupling like your Volvo.

My Volvo is an '01, so it is not a Haldex and I suspect that the rear wheels are always engaged since it is sensitive to tire tread evenness.

It is a purely mechanical systems that senses differences in rotation between input and output shafts and modifies the torque distribution as a result. When rotational speeds are the same torque distribution is about 90/10. When wheel slippage occurs speeding up the rotational speed of the input shaft, the coupling locks up thereby splitting the torque 50/50. You cannot have 4WD always engaged in a high traction environment because the front wheels need to rotate faster than the rear when cornering.

http://www.awdwiki.com/en/home/

[gary g]

Thanks and understood.
I think the CRV has a pump though that engages only with slippage -not a viscous coupling. Pretty sure anyway.

gg