Full Time Awd

[desertdave]

We have a 2007 RX350 which we like for many reasons. However, we had occasion to be in the mountains last summer, and experienced the following. When climbing a rather steep driveway, and turning left into a parking pad, the LR wheel encountered some dirt and leaves and did nothing but free wheel. The remaining 3 wheels did nothing indicating that all power was going to the wheel with no traction. This experience went against everything I've ever known or experienced with full time AWD.

The dealer claims that this is the way it's supposed to work which, to me at least, makes no sense at all.

I then consulted an independent Toyota certified mechanic who told me that the way it's supposed to work is that the ABS system, upon slippage, will apply a braking action to the wheel without the traction with power remaining on the wheels with traction. . So, first of all, am I correct that the AWD wasn't working? Secondly, is that the way, (the braking action) is supposed to work. I should add that the independent mechanic, at no cost, did an error code check with no results.

[jgr7]

I'm no expert on this but with a primarily front wheel drive vehicle like these I would have said it wouldn't have even slipped when a rear wheel lost traction. These RXs are based on front wheel drive with the rear helping out not the other way around. Did you get the bells and flashing lights on the dash when this happened? Did the RX work through the spinning rear tire and finish the climb? Your independent mechanic seems to have the correct grasp on the way it works. What you described is like a rear wheel drive car with out a posi or limited slip rear end not a front biased all wheel drive vehicle. Keep after the dealer for a more detailed explanation.

Jeff

[The G Man]

I agree with jgr7, there should always be some power to the front wheels. The system cannot send 100% power to the rear wheels.

[SmilingBoognish]

Your left rear tire was spinning while none of the other three pulled the vehicle up the grade? This makes no sense to me, given that the RX is based on the front wheel drive Camry...

[pauljcl]

Your left rear tire was spinning while none of the other three pulled the vehicle up the grade? This makes no sense to me, given that the RX is based on the front wheel drive Camry...

My understanding is that, except for the 2010RX, the (older) RXs have a viscous center (differential Torsen-type). I take this as meaning that the center differential will allow some slip; however, as slip increases the viscosity of the liquid increases and the differential become less accepting of slip, thus 'forcing' the rear wheels to drive (push) the car in addition to the fronts. -- The 'axle' differentials (front and rear) are both 'regular' differentials, i.e. they allow slip and the wheels with more grip gets less power. -- It would thus appear that if both a front and a rear wheel have no grip, the RX would be unable to drive forward.

I would be really interested to read a post from someone who knows for sure ....???

[lemon]

I think the first gen had a different system than the second gen with most ppl agreeing that the first generation was a better AWD system. The third gen is different again from the second gen. The hybrid is a different beast entirely, with no center diff - the rear differential being electrically driven.

[pauljcl]

I think the first gen had a different system than the second gen with most ppl agreeing that the first generation was a better AWD system. The third gen is different again from the second gen. The hybrid is a different beast entirely, with no center diff - the rear differential being electrically driven.

First and second gen. are supposed to have had viscous center diff.; third generation replaced that with clutch diff. - yes, hybrid seems another dimension...

[SmilingBoognish]

Your left rear tire was spinning while none of the other three pulled the vehicle up the grade? This makes no sense to me, given that the RX is based on the front wheel drive Camry...

My understanding is that, except for the 2010RX, the (older) RXs have a viscous center (differential Torsen-type). I take this as meaning that the center differential will allow some slip; however, as slip increases the viscosity of the liquid increases and the differential become less accepting of slip, thus 'forcing' the rear wheels to drive (push) the car in addition to the fronts. -- The 'axle' differentials (front and rear) are both 'regular' differentials, i.e. they allow slip and the wheels with more grip gets less power. -- It would thus appear that if both a front and a rear wheel have no grip, the RX would be unable to drive forward.

I would be really interested to read a post from someone who knows for sure ....???

first - a viscous coupling is what you described, but is definitely not the same thing as a TorSen center differential. A VC has clutch packs floating in a liquid that responds to heat. When there are speed differences between the front and rear axles, the clutch packs slip, making friction and the liquid heats up causing the clutch packs to grab one another, essentially trying to match the axle speed between front and rear. This is why on vehicles equipped with a VC it is crucial to keep your tires rotated and replace all 4 at once, otherwise the VC is constantly getting hot and will burn up in a few thousand miles. A TorSen unit is a gear driven mechanical unit as used in most Audi Quattros, and does not rely on a VCD. You are correct that the front and rear differentials are open, so either the left or right side could spin freely; however if the rear was spinning then the VC should have already sensed that the front was spinning to send that much torque to the rear in a front biased setup like the RX has. I suppose it's possible that the hill was steep enough and that the rear tire spinning had so little traction that the VC could not transfer enough torque to the front. VC's do not "lock" like a traditional 4X4, and depending on the design there will always be some degree of slip.

edit: After reading my own post, I'm wondering if the original poster ran different diameter tires on his vehicle at some point, which caused premature wear on the VC?

[pauljcl]

Your left rear tire was spinning while none of the other three pulled the vehicle up the grade? This makes no sense to me, given that the RX is based on the front wheel drive Camry...

My understanding is that, except for the 2010RX, the (older) RXs have a viscous center (differential Torsen-type). I take this as meaning that the center differential will allow some slip; however, as slip increases the viscosity of the liquid increases and the differential become less accepting of slip, thus 'forcing' the rear wheels to drive (push) the car in addition to the fronts. -- The 'axle' differentials (front and rear) are both 'regular' differentials, i.e. they allow slip and the wheels with more grip gets less power. -- It would thus appear that if both a front and a rear wheel have no grip, the RX would be unable to drive forward.

I would be really interested to read a post from someone who knows for sure ....???

first - a viscous coupling is what you described, but is definitely not the same thing as a TorSen center differential. A VC has clutch packs floating in a liquid that responds to heat. When there are speed differences between the front and rear axles, the clutch packs slip, making friction and the liquid heats up causing the clutch packs to grab one another, essentially trying to match the axle speed between front and rear. This is why on vehicles equipped with a VC it is crucial to keep your tires rotated and replace all 4 at once, otherwise the VC is constantly getting hot and will burn up in a few thousand miles. A TorSen unit is a gear driven mechanical unit as used in most Audi Quattros, and does not rely on a VCD. You are correct that the front and rear differentials are open, so either the left or right side could spin freely; however if the rear was spinning then the VC should have already sensed that the front was spinning to send that much torque to the rear in a front biased setup like the RX has. I suppose it's possible that the hill was steep enough and that the rear tire spinning had so little traction that the VC could not transfer enough torque to the front. VC's do not "lock" like a traditional 4X4, and depending on the design there will always be some degree of slip.

edit: After reading my own post, I'm wondering if the original poster ran different diameter tires on his vehicle at some point, which caused premature wear on the VC?

Thank you for the explanation(s), and also for explaining incidentally something that I wondered about, i.e. the requirement that tires rotations be accomplished every 5K miles - obviously it responds to the need to maintain relatively little work of the VC on an on-going basis. Very interesting. Though I would have thought that the issue of tire pressures equalizing circumference would be more important, i.e. slightly higher front psi than rear on the AWD RXs, since the preponderance is in the front (and this visibly affects the set of the tires).

[The G Man]

SmilingBoognish, thanks for that informative post. :)

[SmilingBoognish]

...Though I would have thought that the issue of tire pressures equalizing circumference would be more important, i.e. slightly higher front psi than rear on the AWD RXs, since the preponderance is in the front (and this visibly affects the set of the tires).

Tire pressure is important, too. On my 2002 Subaru WRX (which had a VC), if the front tires weren't 3-4 psi higher than the rears, you would hear a very pronounced and unpleasant noise when decelerating in 2nd or 3rd gear. It was probably making the racket in other gears, too, but there was enough road noise to drown it out. I always hypothesized that it was the VC. I keep our Lexus pumped up slightly more in the front than the rear out of habit for this reason. The engine weight does have a pronounced effect on the roundness of the tires.