All Wheel Drive experts chime in please.

[Irving]

Here is the situation.

2001 Subaru Outback with an automatic transmission. This vehicle comes with a space saver spare from factory. This spare tire is smaller in width, and smaller in diameter than the rest of the tires. If you get a flat tire, the procedure outlined in the Owner's Manual is as follows:

Change flat tire with space saver spare tire. Take an extra fuse from the fuse box (amperage does not matter), and install it IN to the fuse box under the hood, near the driver side strut tower. The fuse is marked FWD, and will eliminate a light on the dash board. All the standard warnings about not driving over 50 mph, and not driving over 50 miles before replacing the full sized tire.

Manual says that if the fuse is not installed, damage to the center differential and/or transmission may occur.

When this fuse is installed, it disables the AWD system in the car, by disabling a coupler before the transfer case, so no power can go to the rear axle.

On the automatic transmission cars, power is distributed 50/50 (front/rear) in the first two gears, and then 90/10 (front/rear) in 3rd and 4th gear.

-------

Here is what happened.

Front passenger side tire went flat. Space saver spare was installed, fuse was not put in place to disable the AWD system, and the car was driving from Durango,CO to Pagosa Springs, CO; a distance of approximately 60 miles. On the way there, the driver smelt something, pulled over, and "the differential was smoking," didn't want to wait for tow truck, so continued driving to Pagosa Springs, CO.

After arriving in town, and taking vehicle to three different places, the final place inspected the vehicle and found the following damage. The rear differential got so hot, that it melted all three of the bushings that hold the rear diff. Also, the fluid in the rear diff was burnt and needed replaced.

Recap:
Space saver spare on passenger front wheel. AWD still engaged. Driven 60 miles on the highway. Rear differential bushings melted and rear diff fluid burnt.

No problems with the front diff, transmission, center diff/transfer case/viscous coupler/whatever.

How did this happen? I've spoken with 7 different shops about this. 2 small town mechanics, Scandiatech, 4 Subaru dealerships, and even Subaru America. I've gotten answers across the whole spectrum from, "The front and rear diffs got into a fight and the rear diff lost. The rear will lose every time." on one end, to "That's weird, usually the diff on the axle with the odd sized tire will bind and come apart, or the center diff will get torn up, or the transmission, but not the diff on the opposite axle. Don't think the spare was related to this damage." on the other end.

I can't make sense of this issue. Given the circumstances, the transfer case should have had issues first, and the front diff a close second. I can not reason out how the rear diff got so hot that it burnt the fluid and melted the bushings. Even if the front and rear diffs were fighting each other by turning at different speeds, the rear diff is still a completely sealed unit, with two wheels of the same diameter on the rear axle. If turning a different speed than the front, the rear would still be behaving normally in relation to itself. The spider gears shouldn't be turning much at all, it's not like the car was doing lock to lock turns for 60 miles. The rear diff is the HoneyBadger of the equation.

How come it failed?

Thoughts?

[mcantar18c]

I have an Associates in Automotive Technology, ASE certs, a decade of experience working in the automotive field and a lifetime of wrenching at home, and extensive experience in offroading as a hobby where axles/diffs/t cases/transmissions/etc. are things you need to know a bit about to build them.

And I have no f*cking clue why your rear diff would kill itself from having different sized tires on the front end.
I would, however, be happy to take a look at it as someone who's not in a position to make a profit off fixing it and see if we can't figure out what happened.

[Irving]

So did I do an okay job explaining everything? I tried to get as much info as possible.

Thanks for the offer, but it's not my car and it has already been repaired. The repair was thousands of dollars cheaper than it could have been ($370 I think). Just trying to figure out why.

I've had all kinds of suggestions thrown out there. Perhaps the factory tires are 215s and the owner got a great deal on some 205s and now the spare doesn't match. Maybe, but it is a space saver spare to begin with, and the two tires on the rear would be the same size, so it wouldn't matter anyway.

I think the best way to look at this is by isolating the rear differential. Looking at nothing else but a rear diff, what could cause it to get so hot that it would melt all three bushings?


Oh yeah, I did a quick google search as well, and found a thread where some kid put a spare on his 2005 Forester in the front, and the front diff caught on fire (white smoke), and was binding. That thread looked promising, but it was the front axle with the odd sized tire and the front diff that failed, so no help. Very strange.

[spyder]

Yes stuving, it will hurt your car. Subaru has that little fuse box for a reason.

[mcantar18c]

Yup, I'd say you did a pretty good job explaining the cause and the symptom/effect.

Repairing a toasted axle for a few hundred seems a bit fishy. From the sounds of it I'd bet that the diff was beyond saving (hot enough to melt = heat distortion, pitting/warping of bearing surfaces, fusing of components to housing, etc.) and the entire axle would need to be replaced, which would be at least a few hundred for the part and even more for the labor.

I can kind of see the front axle being smoked from turning two different size tires, although the spider gears should save the diff (they're there so that the two tires can turn at different speeds while turning a corner or when one doesn't have traction) and transfer that stress to the t-case which would fail due to the front and rear shafts turning at different speeds.

Looking at nothing but the rear diff, what could cause it to get so hot that it melts the bushings? Already low fluid would be my first guess, Occam's Razor and all... could have had a leak and this just happened to be its time to go. Hard to say what killed it without seeing it.

[Irving]

But where will it hurt your car?

Oh yeah, I forgot to mention that Subaru America went as far as opening a claim file for me, then took their ball and went home. I was talking to this customer service girl, who was taking a lot of time to ask all her contacts my questions to get as clear of an answer as possible, until she was told to stop talking to me. She said, "They basically told me to stop talking to you and just give you that answer. I'm sorry I couldn't get a better answer for you."

The more I talked to Subaru America, the less info I got. They told me that they would just refer me to a dealer. I responded that I've already talked to 4 dealers, and wanted to go straight to the source. At first I got some feed back, but eventually they said, "Do what the manual says or stuff will break. If you want to bring it into a dealership, we'd be glad to check it out for you."

The mechanic that actually repaired the car was very adamant to not give me his opinion one way or the other. He clearly didn't want to get in the middle. The best answer I got from him was, "I have no idea how this happened. I really can't say one way or the other."


EDIT: I asked the mechanic who did the work about the fluid level. He said it was fine before he drained it. He said he checked the viscous fluid in the center diff and it was fine. He also said there was no damage to the rear diff, just burnt fluid and the bushings. I'm beginning to become concerned that he didn't check the car out all the way and this will turn into a nightmare.

[mcantar18c]

Where will what hurt your car? Having the smaller spare on the front? Third paragraph of my last post...

Talking to dealers generally gets you nowhere. They're all scum with IQ's of a goldfish. Now, if you physically go in to a dealership and ask to talk to one of their senior techs, and ask HIM about it, THAT might get you somewhere.

Did he check the transfer case itself, and not just the fluid? While the fluid of a component can usually give you an indication of what's going on inside of it, it doesn't always tell the whole story.

[Irving]

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.

[Outlaw1]

The shorter tire on the frontend effectively lowered (numerically raised) the gear ratio. When the VC is in 50-50 it puts the car is in a pretty good bind. Imagine putting 4.10's in the front and 3.73's in the back of a 4x4. Even without a bind, you'd be surprised to see how hot diffs get.

It's common practice to weld the VC and the front diff in a DSM. If you ever want to go for a ride in something that will shake, shutter, buck and generally freak out, just throw one odd size tire on it. Within a mile or two there will be some sort of broken drivetrain part.

[mcantar18c]

Irving:
Yeah, like I said dealerships suck. That's why I suggested actually going into a dealership and talking to them... if you put up enough of a stink the service manager will let you do it, and often times just asking will get you in.
What's wrong with your car?

Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.