Differential ON FIRE today.... - Page 2
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This is a discussion on Differential ON FIRE today.... within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Originally Posted by phoebe It doesn't make sense that it was a problem with the fluid. Like I said, I've ...

  1. #16
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    Quote Originally Posted by phoebe View Post
    It doesn't make sense that it was a problem with the fluid. Like I said, I've taken this car in for all the recommended maintenance. And it's too much of a coincidence to think that the fluid problem peaked precisely when I got a flat. No, the donut was the problem. The insurance assessor and repair shop guys agreed on that. The interesting point is raised by T.J. He hasn't had the problem after several flats. Maybe that has something to do with the size of our tires compared to the size of our donuts. I do have large tires. Maybe the donuts are all the same size. Or maybe it had something to do with the country roads I drove on. Hard question.
    The donut should be sized to the stock tires. If you have a different size than stock a problem would/could occur. Should explain this in the manual I believe.

    Last edited by Donkey; 02-05-2010 at 08:32 AM.
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    Doughnut cooked differential '04 WRX 5MT

    Add my name to the list of folks who drove less than 30 miles and cooked the diff. No fire in my case, though.

    * Car has 98K mi. Stock wheels & tires, suspension, chassis, engine, everything. All maintenance done by Subaru.
    * Always changed four tires at once. I keep the pressure up, but I run 'em till the edges wear out.
    -- Don't reply telling me about underinflation. I tend to drive it hard.

    * Got the flat in Lancaster, NH at about 8:45 PM, put the doughnut on. No problem, I've done this a million times.
    * Drove into Whitefield at << 50 MPH (15 through town, 45 the rest of the way).
    * Stopped for gas in Whitefield, NH (~9 mi). No issues. Drove <<50 MPH through Carroll & Twin Mtn into Franconia.
    * First noticed some wierdness in Franconia. Slight pull, slight vibration, faint smell.
    * Pulled off at the first exit on the Parkway. Car didn't want to start up again from the stop sign.
    * Made a right turn off the ramp (car made a creak), drove it about 25 ft to get it clear. Total distance driven: ~27 mi.
    * Jumped out, jacked up the car, checked the doughnut. Looked OK.
    * De-torqued the lugs anyways, shook the tire around & re-torqued them. Doughnuts' on fine now, but it was before too.
    * Got back on the Parkway, immediately noticed no improvement. Pulled off at the next exit (~800 ft).
    * Thought about it for a few minutes.
    * Checked the doughnut's sidewall. Says it needs >60 lbs. And ya know, I've got a compressor handy.
    * Hooked up the compressor. Doughnut's got ~30 lbs. Swore.
    * Repressurized the doughnut in ~5 mins. Waited ~25 more for the diff to cool down.
    * Drove the car the rest of the way to Merrimack (~90 miles) with no issues, besides a faint hum. Arrived ~12:30 AM.
    * Got 4 new tires in South Nashua that afternoon.
    * Drove back to Lancaster (~130 mi) that evening. No issues.
    * Drove back to VA (~668 mi) the next day. No issues.
    * Got the car checked out at Stohlmann Subaru in Herndon. Estimate is $2700 for a driveshaft & cooked diff.
    * Paid 'em for their investigation time but drove away unfixed. Can't swing the repair.
    * It's been two weeks and I can still smell that thing if the car's been parked awhile on a hot (>95F) afternoon.

    WARNING: If you're not sure about the pressure, DON'T INSTALL THE DOUGHNUT ON THE REAR AXLE!!!

    I don't know what I'm going to do with the car, partly cuz the 2011's are pretty awesome-- but I'll most likely repair the ol' girl at some point and soldier on, especially with Baby #2 on the way. One thing for sure from now on... Full Sized Spare, and check the freakin' pressure before you put the freakin' spare on.
    Last edited by swcwrx; 08-18-2010 at 07:45 PM.

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    swcwrx--

    R160's are cheap used. Buy one from a junk yard or off the forums for a $100-$200 and put it in yourself.

    Are you sure the driveline is cooked? If it is, that is the more expensive piece as it is non serviceable IIRC.

    Personally I carry a plug kit and an air compressor. From my off roading days, I know you can fix almost anything with a plug kit, and If you can't its probably best to tow it anyways.
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    Hey 06... good on the plug kit & compressor. This one wasn't pluggable (about an inch-long tear on the inside sidewall) but I didn't even have a second thought about the doughnut anyways. Just throw it on like normal, every car the last 20 years no prob with the doughnut, even this car the first couple times no prob. For me I think the full-size spare is mandatory, the way I get flats. I still have a blast, though.

    The key is the sophistication of the drivetrain. LSD's are awesome but you gotta take care of 'em.

    Car still drives fine so I'll fix it as I can... no worries. No more doughnut, though- through with that.

    Hey where can I find a driveshaft & differential for less than $2100? That was the dealer quote for the parts by themselves.
    Last edited by swcwrx; 08-19-2010 at 06:31 PM.

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    Quote Originally Posted by xsnapshot View Post
    You cant shut off the rear diff though. Even if somehow you managed to turn off the power being sent by the center diff to the rear diff, the rear diff can still get hot.

    If its a limited slip diff (which I believe the OP's was), then with no power being sent to it, a mismatch of tired size WILL start to heat up the clutches in the diff.

    FWIW I have a 2004 automatic WRX and this summer I had a bad flat. So I swapped on my spare tire and drove it home (about 10 miles) at speeds never above 45 mph. When i got home i reached under my car and felt the diff with my bare hand. It was hot, but not too hot for me to touch. My tire size is 225/45ZR17.

    I remember taking rough estimations of the distance of the center of the rim to the ground for each of the rear two, and letting a little air out of one of my tires (can't remember which one) to get the distance to match up.

    If possible (it wasn't for me) next time you get a flat, move your spare to front where you have an open differential. That's the safest move.
    I couldn't really find what I was looking for except here, so I must bring this back up. Would any of you know which cars have an open front diff? And does a mismatched wheel diameter affect an open diff less? How?
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    Quote Originally Posted by New09wrx View Post
    I couldn't really find what I was looking for except here, so I must bring this back up. Would any of you know which cars have an open front diff? And does a mismatched wheel diameter affect an open diff less? How?
    Your car has open front and rear diffs. Yes, a traditional open diff will not attempt to equalize the speed difference.

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    But isn't there still spider gears that don't ever turn unless you're going around a corner, that are now turning because one wheel's spinning a little faster than the other?

    So what about Subarus with LSD's and 18" wheels with our spares? So now you have a diff that is attempting to equalize, AND there's a bigger difference between the 24" spare and 26" oem wheels. What do they do?
    '09 silver WRX base sedan. Stock-for now...

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    Quote Originally Posted by New09wrx View Post
    But isn't there still spider gears that don't ever turn unless you're going around a corner, that are now turning because one wheel's spinning a little faster than the other?

    So what about Subarus with LSD's and 18" wheels with our spares? So now you have a diff that is attempting to equalize, AND there's a bigger difference between the 24" spare and 26" oem wheels. What do they do?
    The spider gears will only rotate in event of a speed difference between wheels, as you have mentioned. Since a standard open diff does not equalize wheel speed difference, running different size wheels (with an open differential) will cause no harm to the differential and torque, of course, will be evenly split across the wheels. With an LSD that attempts to equalize the speed difference (speed sensing), this can cause damage to the LSD. Your car only contains one LSD: a viscous coupling unit in the center differential.
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    I still have my '04 WRX -- it should hit 189K miles tomorrow morning. It was worth sinking the money into because it's such a solid car.

    Unfortunately for my old mechanical rear LSD, it was not built to withstand significantly different rotation rates for any length of time. I cooked mine in about 27 miles back about five years ago. The reason is internal friction converting kinetic energy of rotation into heat. It doesn't take long until the hot metal starts to expand, which increases the frictional force, which increases the KE-->heat conversion rate. The heating rate is not linear and so the diff temp spikes faster than you can realize it.

    I have a full-size spare in my trunk now. I recommend anyone with a 2002 - 2007 WRX go out to your car right now and donate your doughnut to your neighbor for a unicycle project. Then either buy yourself a full-size spare, or don't carry a spare. The distance you can safely drive on the doughnut is equal to the distance you can safely drive on a flat.

    HOWEVER, if you have a 2008 or later WRX, you do not have a mechanical rear limited-slip and so do not run the same risk I did. I think it's an electronically controlled, brake-based setup that emulates an LSD. Same result, probably similar performance -- but it's cheaper and more durable.

    Because of the VLSD in 2008+ WRX's, I'm guessing it's still a really bad idea to put different tires on the same axle, the reason being the differing rotation rates may be sensed as a slip, which would cause the electronics to engage the brakes, which after a few miles would most likely result in some cooked brake pads. Still better than buying a new diff.

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