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Discussion Starter · #1 ·
Good day,

I own a manual 5-speed 2009 Subaru Outback. I have been experiencing an occasional noise with the following symptoms:

1) A clunking noise when in 1st, 2nd and reverse whose frequency increases with increasing angle of turn in both the left and right directions. I believe the noise may occur in all gears, however at 3rd gear + speeds the angle of turn is too shallow to cause the noise/issue to occur.

2) The noise does not occur all the time. It seems to occur after the car has been warmed up, typically after it has been put in 5th gear and driven at speeds exceeding 80 km/h (i.e. after local highway driving).

3) There are no tears or grease leaks from any of the cv boots and both of the cv axles appear to be healthy.

Any insights would be greatly appreciated.
 

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Could be a bad outer CV joint in one of the front axles, or even a damaged front or rear differential.

But the clunking could also be symptomatic of torque binding, which appears when making tight turns and the center differential viscous coupler is damaged and binding. It's not uncommon, especially if the car has ever been towed two wheels up, or driven for a good number of miles with tires that don't have the same rolling circumference. The special viscous fluid inside the coupler is supposed to remain fluid except when there's wheel spin, but under some conditions can become permanently more sensitive to heat. It's somewhat fluid when the transmission is cool, but when the transmission it's in heats up, the warmed fluid becomes more like a gel, or glue, and causes the coupler plates to bind. The inability of the plates to move relative to each other causes torque stress in the drive train when the wheels are rotating at different rates, as is the case when turning.

Search here for "torque bind". Should be lots of posts. Also see: http://www.subaruoutback.org/forums/66-problems-maintenance/307762-viscous-coupler-test.html
 

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my guess, if it isn't present when stone cold - then shows up after 10-15 minutes of driving, AND you feel a jerking/bucking affecting the car...center differential is bad.
 

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Discussion Starter · #4 ·
I really appreciate the input from everyone!

I want to double check that I am understanding this correctly. In an all wheel drive vehicle, the center differential generates torque for the front and rear wheels, and generates different amounts of torque when the wheels need to rotate at different speeds (i.e. when turning). It consists of a set of bevel gears and a viscous coupling. The viscous coupling, consisting in part of a silicon-air fluid, transfers torque from the center diff to the rear side gear by generating a shearing force within the fluid. When the wheels are aligned, there is no difference in torque required as both the front and rear wheels are rotating at the same speed. When the wheels are angled, the difference in rotation speed, and hence torque, increases with the angle of turn. Given that the viscosity of the fluid within the viscous coupling decreases with heat, it would imply that there is an issue with either the bevel gears, or one of the plates within the viscous coupling, and it only becomes apparent when the viscosity is sufficiently low that slippage occurs.

I am trying to take this auto issue as an opportunity to learn more about my vehicle, and ideally get it fixed without breaking the bank. Any further feedback would be greatly appreciated!
 

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You're on the right track . . .

There's a number of systems used for AWD. In Subarus with manual transmission, the front-rear drive is managed by a mechanical differential with a viscous coupler. The mechanical differential provides distribution of torque to the front and rear drive at all times, the same as the front and rear differentials direct power to the two front and two rear wheels. The viscous coupler is normally supposed to play a role only when wheels lose traction and slip, causing the front and rear drives to turn at significantly different speed.

In normal operation, the viscous fluid in the coupler remains liquid (e.g. consistency of 80W gear oil). When the fluid is subject to shearing, it generates internal heat, and this causes it to become less liquid, to the point of being more like "silly putty" (anything in contact with it will stick).

The viscous coupler consists of a number of thin metal plates stacked together. Alternate plates are splined to the center differential's case and to the rear side gear of the mechanical differential. The plates are surrounded by the special fluid but these are isolated from the rest of the mechanical differential gears etc.

When going straight, both the front and rear side gears and differential case turn at essentially the same speed, so the plates are turning together as well. At this time there's no shearing action, and the viscous fluid remains liquid and has no effect. This normally applies also when turning tight corners -- the difference in rotation speed between the front and rear drives is relatively small, and although there is some shearing action, it's not at a high speed and not for a long time, so the fluid doesn't become "gelled".

When a wheel loses traction, it speeds up significantly. This results in a larger difference in the front and rear drive rotation, and more shearing of the fluid. The fluid gels, binding the alternate coupler plates together. This effectively locks the differential case and the rear side gear. With the case and rear side gear locked together, the front side gear has to turn at the same speed.

When a wheel looses traction, it takes a lot less torque to turn it, and in this type of mechanical drive system, if one wheel spins, then the torque going to the others cannot exceed the torque needed to spin that one wheel. That torque is not enough to turn the wheels that have traction, so the car doesn't move.

However, with the viscous coupler plates held together by the gelled fluid, the two side gears and differential case are locked together. This forces the front and rear drives to turn together, and therefore the torque delivered to the drives will be determined by the force needed to turn the wheels that have traction. And that is what should get the car moving.

When the car starts moving and all four wheels are turning at the same speed again, the shearing inside the coupler slows down, and the fluid should return to its more liquid state. If it doesn't, then torque binding symptoms are felt.
 

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If the center differential is responding to shear and locking up, either one wheel was turning faster than the others or the viscous coupling (meaning the silly putty itself) has failed somehow.

So... start simple. Check to make sure you don't have one odd-sized tire, or one of them over/under inflated to the point where normal driving would cause that wheel to rotate at a different speed than the others even while driving straight.

In a nutshell, the viscous coupling is supposed to enforce an even 1:1 ratio of revolutions made by the front wheels to revolutions made by the rear wheels. It is also supposed to ignore small differences while reacting swiftly to big differences.

If a viscous coupling is continually stimulated by a small difference (i.e. 1 worn-out tire + 3 new ones) it can heat up enough to lock as if responding to a momentarily free-spinning wheel. It's 100% mechanical, not smart enough to know the difference.

If it does that long enough, the media gets cooked beyond tolerance and cannot respond properly again.
 
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