legacy clutch-pack failure .. any guesses?

[Guest goodsonr]

Hi All

 

I have a 95 Legacy (automatic) with about 140K. I had a failure that involved the clutch pack. Searching the archives, I couldn't find anything similar, so thought I'd ask.

 

The tale of woe is ...

 

driving along the highway, the rear-diff went up in flames (literally). A new one was installed which lasted all of 80km. Turns out the clutch pack was locking the rear into full-time 4-wheel all the time so at highway speeds.. too much torque to the rear which burned-it-up.

 

So far, the dealer hasn't said why/how the clutch pack failed (nor do I know if they even tried .. may just be doing the usual "throw in a new one and hope it works").

 

I searched the archives and found many references to "torque bind" on low speed turns, often caused by tires being different sizes.. but I didn't see any mention of a complete failure at all speeds. I'll admit one sin here .. last year I replaced only 2 of 4 tires. Put the new ones on the front and the best of the remaining on the back. I see now this would lead to the "torque bind", but I have a hard time believing it could cause the entire clutch pack to fail (especially since this car is driven mostly on the highway .. so mostly straight line).

 

Any guesses from the experts out there what might have happened.. just my luck or ???

 

Tks for any knowledge

 

Ron

[Guest Commuter]

I find it strange that the rear differential would go. I would think that you'd have an open diff in that car.

 

Tires of different sizes.. Yes, definitely damaging to the center clutch pack. Straight line is not the issue. It is the differential in speed of the front axle compared to the rear axle. Highway speeds just make things that much worse since everything is turning that much faster.

 

The clutch pack in the auto is only meant to tolerate slight and momentary slip. Good enough when a wheel starts to spin and it tightens up to give you the "AWD". Good enough to allow the necessary slippage when you turn corners. But not good enough to take constant slippage that happens when tires are not the same size.

 

I saw a post on the old i-club where a guy had burned up two VC (viscous couplings, he had a manual) before they realized that he had 1.5" of difference in circumference between his front and rear tires! Subaru's don't take well to this... especially the automatics.

 

Commuter

[Guest intrigueing]

With the unmatched tires did it bind significantly in turns? Engaging AWD at high speeds on dry pavement can cause Buku heat and wear.

[Guest goodsonr]

"Engaging AWD at high speeds on dry pavement can cause Buku heat and wear."

 

can .... and did.

 

I can't say I noticed any binding in turns. If there was, it was subtle enough I didn't think too much about it.

 

when I say the rear-diff went up in flames (literally) it was just that .. flames. The seals melted, fluid leaked out and smoked .. and something caught on fire.. either the fluids or the housing. Just glad the people who stopped behind me had an extinguisher.

 

Well .. if its as stupid a thing as different tires (same rated size..but some old / some new) then shame on me and I'll just add the cost of new ones to the already stratospheric repair bill. Still, we are talking about a car that has < 10K in the last year since the tire-change.

 

..sigh so where is that fuse to turn the car into a 2WD (like my '91 had).

 

Thanks for the responses

 

ron