Hi,
A few days ago my wife got her `03 XC stuck in some snow here on the ranch while backing out of the driveway. The snow was close to a foot deep & as we`d had a Chinook overnite the snow was "wet". Anyway the tires just kept spinning anytime I put it in drive. Yes, as soon I detected just "spin" I took my foot off the gas.
This car has Nokian winter tires so I expected better results with AWD. Does this sound like the AWD may not be functioning, period? She had the bevel gear replaced just as the warranty was close to expiry due to previous owner`s possible abuse.
My GMC 3/4 ton 4x4 does way better in snow even with tires that soon need replacement.
Is there something about AWD I`m not understanding?
Thoughts on this issue much appreciated.
Rich

Bone up a bit on how Viscous Couplers work. Here's a start:
http://en.wikipedia.org/wiki/Viscous_coupling_unit

http://auto.howstuffworks.com/differential5.htm

http://www.wisegeek.com/what-is-viscous-coupling.htm

http://www.awdwiki.com/viscous.html


TRACS and AWD are only remotely connected concepts.

The TRACS is just a fancy add-on to the ABS system and helps you get going from a stop when wheels have little traction. I believe the 1998 has a '3 channel' system, so one channel for each front wheel and one channel for the rear wheels. If I'm wrong and it's '4 channel' then there is one for each rear wheel separately. The TRACS system only applies to the front wheels (though ABS does apply to rears too). It monitors and compares the wheel speeds between front left and right wheels. If it determines that one wheel is spinning faster than the other, it can very lightly apply the brakes on the spinning wheel which will allow the differential to transfer power to the other side of the car. This is a pretty weak effect but it is very useful. You will hear the TRACS system working right behind the instrument cluster.

The Viscous Coupler transfers power front-to-back. Please read the links I posted, but essentially, the VC transfers more power in direct relation to the difference of input prop-shaft speed to output prop-shaft speed. So if the input shaft is spinning faster than the output shaft (ie: front wheels spinning) then it transfers torque to the rear prop-shaft. There is no computer control involved whatsoever. It is completely up to the properties of the fluid in the VC.

The VC can work VERY fast. Actually I think faster than the newfangled electronic systems (ie: Haldex), though I could be wrong there. I have read the specs on it but it's something like 1/8 of a turn on a wheel is enough to start transferring power to the rear. They /can/ currently transfer more torque than the new electronic systems.

The side-to-side torque slit is handled by the differential itself in the rear. In 1998 the rear differential is a locking limited slip differential. Sometimes known as positive traction of positrac. This is actually a much more robust system than TRACS and Volvo had good experience using these units from their RWD days.

Combined they pretty much give the car the ability to split to torque to any single wheel though some mechanisms used are better than others. TRACS is the 'first line' of defense against wheelspin but only has the power to split power from side-to-side at the front, and it can't transfer that much torque (maybe something like 20% but I'm guessing - I've never read any specs on that). The VC has the ability to transfer nearly ALL the torque from front-to-back given it's working properly and the front wheels have 0 traction. The rear LSD can transfer nearly all the torque from side-to-side in the back.

Consequently, if the two front tires and one rear have zero traction, you should still have a decent time putting torque to the ground.

With all that said, though I have never had a set of Blizzaks, I hear they're pretty fantastic on ice, and they would have served you well in this situation too. It's pretty hard to overestimate the importance of proper tires. Good snow tires and AWD together is a real killer combination.

Word of caution : I understand the temptation to really bang on the AWD when the bad weather comes but be forewarned - these AWD systems are not nearly as robust as the real 4WD/4x4 systems in trucks with beefy center differentials and heavy transfer cases. The stress of immediate traction after low traction situation (like, you're drifting away on some ice and then hit a patch of bare pavement for example) is often enough to break something in the front transfer case (Bevel Gear or Angle Gear). And supplying too much torque through the VC can be enough to cook the fluid and effectively 'lock up' or worse 'decouple' the whole thing. You'd be surprised how much power your 19x horsepower turbo 5 can put down, and unfortunately not every part in your drivetrain was built to take it.

Have fun though!



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1998 V70 AWD->FWD Turbo 200k+

Yes, and yes. Snow tires really do work on ice if the temp is close to freezing. The AWD responds faster than you think, in milleseconds. BUT, do not rely on it to save you from a panic situation, by then it will be too late.

I made my 1998 AWD spin all 4 tires on ice last week, but I also have 250bhp and the temp was around 15F and garbage all season tires.

What hurts is the ABS. It will not make your car stop any faster eventhough you can drive faster with AWD. What you need to learn is how to use the gas pedal instead of the brakes to drive around obsticles. Example: I was driving down hill to an intersection where I wanted to turn left. ABS kicked in and I realized if I continued to hold the brakes, I would just slide right past the intersection. So, I got off the brakes (only doing 5mph) and gently gave it gas. The AWD let me turn hard left without any slipping.

When the temp get to 20F or less, the compound in the tread will no longer be sticky on ice, so be careful.

Klaus
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Always willing to listen, just not able to take direction.