All Wheel Drive experts chime in please.

[jmg8550]

Like mentioned before, it is an electronic AWD system and relies heavily on the wheel speed sensors. It is not a traditional 4WD system. So, since the front spare tire was spinning faster than the normal tire on the front, the ABS system is trying to compensate and telling the AWD to put power elsewhere ie. the rear diff. Thus causing more stress on the rear and damaging it. Should've put the 50 cent fuse in and saved lots of trouble. I'd say the driver is 100% at fault.

The auto manufacturers give you an owner's manual, I suggest reading it.

[Inspector Fowler]

It's also important to note that the automatic transmission equipped Subarus use a different AWD system than the manual transmission equipped Subarus. The auto Subarus have a computer controlling the power split based on what it senses from the ABS sensors. The manual Subarus use a purely mechanical system that uses no intervention from the computer or readings from the ABS.

[Irving]

How does sending the power to the rear cause issues with the rear differential? Even if 100% of the power was sent to the rear diff, the tires on that axle were the same size, so it should have been business as normal as far as the diff is concerned right?

I've had some of the dealers and other techs say something along the lines of every single one of these posts, but none of them have ever seen this before that they can remember. One shop said they have people call in with a similar question from time to time, but never saw the car.

[DFBrews]

How does sending the power to the rear cause issues with the rear differential? Even if 100% of the power was sent to the rear diff, the tires on that axle were the same size, so it should have been business as normal as far as the diff is concerned right?

I've had some of the dealers and other techs say something along the lines of every single one of these posts, but none of them have ever seen this before that they can remember. One shop said they have people call in with a similar question from time to time, but never saw the car.

with out locking in the diff power is not sent equally to both rear wheels so the spiders where working their ass off which they are not meant to handle.

the axle goes in the center

[jmg8550]

When everything is bound up from the transfer case out to the differentials, it causes friction, friction causes heat, which is the worst enemy of lubricants. Thus the rear diff failing and eventually (if driven longer) ripping itself apart.

[Irving]

Why wasn't the center diff damaged then?

How does binding in the center diff translate to binding in the rear diff?

[Not_A_Llama]

I can't contribute discussion on the mechanical side of things, but have you talked with Strictly Automotive, in Denver?

Probably the best and most honest Subaru shop I've ever dealt with.

[jmg8550]

The transfer case is allowed to slip to a point, and whatever the computer commands for pressure. The differentials will not slip because they are gears. I would imagine the transfer case operates sort of like an automatic transmission with clutches, they increase or decrease pressure.

This will give you a better understanding of how a differential works:
http://www.youtube.com/watch?v=K4JhruinbWc

It's an old video but is very informative.

[Inspector Fowler]

The transfer case is allowed to slip to a point, and whatever the computer commands for pressure. The differentials will not slip because they are gears. I would imagine the transfer case operates sort of like an automatic transmission with clutches, they increase or decrease pressure.

This is exactly how it was explained to me during my brief period as a (shudder) Subaru salesman. I love Subarus, I just hate selling cars.

The crude explanation I remember for the mechanical AWD system was that it uses the heat of slipping clutch-style plates against each other to warm up a fluid that in turn put more pressure on the plates, thus causing them to slip less and send more power to the shaft that was slipping less.

[spyder]

My question of where it would hurt the car was directed at Spyder, you just snuck a post in.

Every dealer I called, I told them I wanted to talk to whichever tech they thought knew the most about drivelines. Every single service manager said, "Well what's the situation/question?" Listened to me, then put me on hold for two minutes while they went to ask the techs, then returned with various answers. I specifically asked Subaru America if I could just speak directly with their techs (call center people not technically trained, and would just ask someone else), but they said they were internal only and would only talk to dealerships.

Basically, anyone who realized that this damage didn't seem directly related to the tire change, didn't want to get involved one way or another. Thanks everyone for the help so far.

Sorry stuving, I tried to make another post, but my wireless was acting up and after the thrid time trying to slightly explain and the damn thing telling me it couldn't connect and erasing what I was trying to write, I gave up. So, on my normal computer here goes.
The problem with the any tires being different sizes, is that the axles are always turning at different speeds even when the car is going straight. So the center differential is always working and that wears it out. There is a clutch assembly or viscous coupling that is really complicated to explain. The tires need to be within a quater of an inch in outer diameter size. The other part is that when the subaru's sensors read the different rotation speed of the tires because of the size, it activates the traction control system thinking that one tire is slipping because of the different rotation speeds. This then activates all the other little things that turn on to try to correct this normally on slippery road surfaces. Basically, you are keeping this system on and wearing it out. There is a timing thing also in the whole TC system that keeps it from turning on during normal turning and everything else, the system is a pain in the ass to explain. So, driving with different size tires messes with all the rotation speeds of everything under the car and the traction control system. Basically.