View Full Version : All Wheel Drive experts chime in please.
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.
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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
09-13-2011, 00:08
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.
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.
Yes stuving, it will hurt your car. Subaru has that little fuse box for a reason. [Beer]
mcantar18c
09-13-2011, 00:27
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.
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
09-13-2011, 00:40
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.
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.
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
09-13-2011, 00:55
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.
Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.
This is the way that I understand it.
Outlaw:
Different gear ratios will grenade the t-case, but shouldn't effect the diffs themselves.
I have limited experience with Scubbys, but I know they don't have a traditional T-case. They have longitudinally mounted engine/trans and a center diff that transmits the power to the rear. Sorry if I'm stating the obvious.
Having the drivetrain in a bind with mismatched tires and a locked center differential will affect the whole drivetrain, from one end to the other. IMO, when someone says they 'smoked' the fluid, or 'melted' the bushings, it doesn't mean it got hot enough for the fluid to catch on fire, or actually completely melt away the soft rubber bushings. They might have just been deformed a little and the fluid just smelled. Regardless, you wouldn't know until you drained it. Also, the mounts for the rear diff are on the rear cover, so if the ring and pinion got hot it transferred it right into the mounts.
Heat + drivetrain in a bind = tweaked bushings.
I have limited experience with Scubbys, but I know they don't have a traditional T-case. They have longitudinally mounted engine/trans and a center diff that transmits the power to the rear. Sorry if I'm stating the obvious.
Having the drivetrain in a bind with mismatched tires and a locked center differential will affect the whole drivetrain, from one end to the other. IMO, when someone says they 'smoked' the fluid, or 'melted' the bushings, it doesn't mean it got hot enough for the fluid to catch on fire, or actually completely melt away the soft rubber bushings. They might have just been deformed a little and the fluid just smelled. Regardless, you wouldn't know until you drained it. Also, the mounts for the rear diff are on the rear cover, so if the ring and pinion got hot it transferred it right into the mounts.
Heat + drivetrain in a bind = tweaked bushings.
Here is my guess this is based on big trucks so i may be completely off. there are 4 driven wheel ends and normally power is provided to a front wheel end and than the opposite wheel end on the rear. Like a criss cross pattern. there are spiders that allow the axles on the same diff to turn at different speeds when going around corners etc. when you put the smaller tire on the front it made the spiders in the rear diff work really hard trying to provide equal power to the rear to compensate. which made it get hot and start melting stuff and burning fluid.
when i dyno a semi i have to lock in the power divider so both axles get equal power. if i do not BAD things happen
Here is my guess this is based on big trucks so i may be completely off. there are 4 driven wheel ends and normally power is provided to a front wheel end and than the opposite wheel end on the rear. Like a criss cross pattern. there are spiders that allow the axles on the same diff to turn at different speeds when going around corners etc. when you put the smaller tire on the front it made the spiders in the rear diff work really hard trying to provide equal power to the rear to compensate. which made it get hot and start melting stuff and burning fluid.
when i dyno a semi i have to lock in the power divider so both axles get equal power. if i do not BAD things happen
In this case the rear spiders would be take a beating, but they're not having to work the rear wheel tire/tire bias while going in a straight line. The front would have the spider gears working, but they are not in an LSD, so it's not THAT bad on the spiders.
Because of the short tire up front, the front tires are trying to spin slower than the rears with the 50/50 split. The drive side of the ring and pinion in the rear diff was in a serious bind (as well as the coast side of the front diff & the center diff).
blackford76
09-13-2011, 02:10
DFBrews gets a cookie!
You have to remember on a Subaru it isn't just a mechanical system, the computer lost it's mind with the odd sized tire and seriously over worked the rear diff trying to compensate. That is why they wanted to add the fuse to disable the awd system.
The whole time the small wheel was on the ground, the computer sensed that one wheel was losing grip, so it sent power to the rear, it uses the abs wheel speed sensors to tell which wheel is "in trouble".
DFBrews gets a cookie!
You have to remember on a Subaru it isn't just a mechanical system, the computer lost it's mind with the odd sized tire and seriously over worked the rear diff trying to compensate. That is why they wanted to add the fuse to disable the awd system.
The whole time the small wheel was on the ground, the computer sensed that one wheel was losing grip, so it sent power to the rear, it uses the abs wheel speed sensors to tell which wheel is "in trouble".
Thanks Rob when the hell are we going shooting again[Beer]
and outlaw no AWD is a true 50/50 split in power... ever
blackford76
09-13-2011, 02:16
Thanks Rob when the hell are we going shooting again[Beer]
I dunno, I have weekends off now, but work nights. We're gonna have to figure something out, I've got 2 new 1911's I haven't shot yet.
Subaru's computer is used to figure out which wheel needs to have power sent to it dynamically so that when you start to slide it will transfer power away from wheels that are slipping and to the ones that are gripping. Perhaps it kept sensing that all the wheel except the right front were slipping (turning slower) and as a consequence kept trying to send power to the back?
I didn't know that about the Subaru and it's undersized spare. I've owned one for almost 12 years now.
H.
rockhound
09-13-2011, 10:36
you can cause the same issues in a standard rig also.
the spider gears in a differential are not meant to be used for extended periods of time.
you turn the corner the work, you go in a reasonable straight line the see no use.
if you are running different size tires on even a normal axle you can overheat the system and fry it. one end of the differential will be turning at a slower pace than other causing the differential to work at normal highway speeds causing it to overheat.
if you are doing 2500 rpms going down the highway and the ratios of your tires is off by say 5% the axle is then differential is turning at 125 rpms for the next 60 miles. under normal circumstance the spider gears would never see those kinds of rpms and if the did it would be for seconds at a time.
Once you put a binding situation into the driveline on AWD, past the allowed slipping point, the weakest link will break. If the front wheels are turning/slipping respectively faster than the rear wheels on a locked AWD system, force will be applied to the rear drive. The drive shaft to the rear drive wants to go faster than the wheels are traveling thus putting pressure on the diff.
Anyhow, this is my guess….
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
09-13-2011, 17:40
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.
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.
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
http://i00.i.aliimg.com/photo/v0/110744974/DIFFERENTIAL_SPIDER_GEAR_SET.jpg
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.
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
09-13-2011, 19:53
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.
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
09-13-2011, 20:02
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.
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.
Why wasn't the center diff damaged then?
How does binding in the center diff translate to binding in the rear diff?
It takes time to hurt it. It wouldn't happen over night, sorry, I should have also said that.
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.
Thank you, I may call them tomorrow.
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.
Thank you for the video. I understand how a differential works, for the most part. EDIT: I've actually seen that video before, it is very good. www.howstuffworks.com has a great tutorial on differentials as well.
It takes time to hurt it. It wouldn't happen over night, sorry, I should have also said that.
The damage happened by driving from Durango to Pagosa Springs, which is about 60 miles.
I feel like I need to point out again that ONLY the rear diff was affected. The fluid got so hot it melted the bushings holding the rear diff on. The different sized tire was on the FRONT axle.
Okay, I'm not trying to be difficult here, and I very much appreciate all the input I've received so far. I'm trying to understand exactly what happened. I have to explain to the owner of the car exactly how this damage was caused, and be able to refer to a specific action that caused the damage.
I'm too tired tonight to think about this more. Thanks everyone so far.
blackford76
09-14-2011, 02:56
Stu, the rear fried because it normally only works at low speeds, and short amounts of time, Subies are basicly front wheel drive, with rear assist. The smaller tire freaked out the awd, making it work constantly, which it wasn't designed to do. The rear diff is the weakest part of the whole system. AWD is not 4WD, it is not durable at all. My wife has a grand cherokee with the bastard awd, it gets new driveshafts every 7k miles, and either front or rear diff replacement at roughly 14k intervals. Where people who bought 4wd jeeps go hundreds of thousands of miles without repairs.
So even though that part of the drivetrain is still spinning while the car moves, it is different being under power?
blackford76
09-14-2011, 10:53
So even though that part of the drivetrain is still spinning while the car moves, it is different being under power?
Yes, without a load, it builds almost no heat, being under load the heat rises, being under constant load it burns up because it isn't supposed to be under load for any length of time.
I think I've come to my conclusion about this. Thanks for the input everyone.
clublights
09-15-2011, 12:51
I think I've come to my conclusion about this. Thanks for the input everyone.
So for those of us following this............
What is your conclusion >?????
Well, I can't really go into how I'm involved, I'm not really, but it is just that there is no conceivable way for me to say 100% that this damage was NOT caused by the odd sized spare. So basically, I will tell the lady that the damage was caused by the spare, because I can't prove that it wasn't caused by something else.
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