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2002 Subaru Outback H6 L.L.Bean
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Discussion Starter · #1 ·
How do you go about testing the LSD rear in the LL Bean models? I have mine jacked up ad when I pin one tire the opposite spins in the same direction. Is this normal?
 

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when I pin one tire the opposite spins in the same direction.


This seems to be a characteristic of the LSD, which I attribute to the viscous coupling seals between one of the side gears and the internal differential case, as well as the drag within the viscous coupler itself.

A further test would be to somehow prevent the drive shaft from turning, and then turn one rear wheel. Or, raise only one rear wheel, and try to turn it (drive shaft free to turn). In the first case the opposite wheel should turn in the opposite direction, but in both cases it will be very(!) hard to "spin" the wheel, especially when compared with the front wheels.

Although the rotation test can be used to identify differentials equipped with a limited slip viscous coupling, it could give a "false positive" result if the differential is damaged internally. In addition, it only verifies the probable existence of a LS differential; it does not verify if the LS viscous coupling will actually work properly when one wheel loses traction.
 

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OBW H6 VDC, Tribeca, XT6
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it only verifies the probable existence of a LS differential; it does not verify if the LS viscous coupling will actually work properly when one wheel loses traction.
folks that do a lot of Subaru's are suggesting that these VLSD units don't last very long, they essentially fail to an "open" or non-LSD diff. so like he said, even if it's passing that test as an LSD it might not mean it's really gaining you any traction.
 

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01 Outback H6 VDC, 97 GT wgn w/ ej22, 98 OBW w/ej22
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i have read just what gary said, the Vlsd tends to fail to an open setup.

but we all know that the center Viscous coupling tends to fail to a locked, binding set up. this seems inconsistent to me.
(has any one ever experienced a center diff failure on a manual trans where there was no power to the rear??)

are the designs so different that fail in opposite directions?? did the engineers plan on these failing in different directions?
 

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2002 Subaru Outback H6 L.L.Bean
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Discussion Starter · #7 ·
That's why I'm asking because I'm afraid mine might be the cause of the binding I'm experiencing when I turn. I had one wheel on the ground and the other I was able to turn so I guess it isn't stuck locked. I am interested in finding out a way to test it out though to be sure.
 

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are the designs so different that fail in opposite directions?? did the engineers plan on these failing in different directions?
The viscous couplers inside the two different differentials are pretty much the same mechanically, at least according to the service manual diagrams. But their locations could be the distinguishing factor.

The center differential is exposed to continuous heat from the transmission (and indirectly the engine) in which it's contained. Consequently, "cooking" of the fluid is possible. (See the link to "The Viscous Coupling" in the post I linked above.) Cooking of the fluid can lead to premature, or permanent gelling, which leads to the locked, or binding, situations for the center differential.

In contrast, the rear differential tends to remain relatively cool except, perhaps, when unduly stressed due to extended spinning of a wheel. Cooking of the fluid would be rare. Instead, the more common failure would be due to wear or failure of the seals or related surfaces that separate the silicone fluid from the differential gear oil. Once these have failed, the fluids mix and there's no coupling effect.

In a viscous coupling, heat due to shearing between the coupler plates is what causes the silicone fluid to "gell", thereby providing the connection between the plates. But heat also causes the fluid to expand. Given that the coupler is a "sealed" area, if the pressure gets too high, it could cause the seals to fail. This should not normally happen if the VC is working properly -- the initial heat of shearing should cause the gelling and locking of the plates before the expansion and pressure reaches a critical level. When the fluid gels, the difference in plate speeds is reduced, and this will limit further heating. But, for example, if there's two markedly different size tires/wheels on the axle, and they both have good traction, the inability of the fluid to bring the plates into sync could lead to excess heat, failure of the seals, mixing of fluid, and loss of LS function.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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I've read the center diff referred to as a LOCKING viscous unit. As in, slippage causes a lock to 50/50 distribution of torque. The rear can (I'm supposing) transfer somewhat linearly from 1% on up.

maybe it is a different design than the rear.
 

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That's why I'm asking because I'm afraid mine might be the cause of the binding I'm experiencing when I turn. I had one wheel on the ground and the other I was able to turn so I guess it isn't stuck locked. I am interested in finding out a way to test it out though to be sure.
Ah, binding on turns. With a Subaru automatic transmission this could be due to the multi-plate AWD transfer clutch remaining engaged when it should be at least partially released.

I believe your LLB has the 4-speed AT, with the Multi-Plate Transfer clutch. "Torque bind" is what could be happening, and a search here for this will provide a lot of reading, helpful suggestions, and even DIY directions, ranging from changing the ATF through replacing the AWD control solenoid and clutch plates.
 

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The rear can (I'm supposing) transfer somewhat linearly from 1% on up.
Why would engineers want to have, say, 25% transfer? In both cases, the objective is to have both outputs (front and rear drives, or left and right axles) turning at the same speed.

In the case of the rear differential, if one wheel has good traction, and the other not, the VC action slows the spinning side. By doing this, torque is built up in the drive train and therefore the wheel with traction is able to move the car.

When as a result of the VC action, the wheel without traction is no longer able to spin faster than the one with traction, it can be said that the VC is "locked", but in fact the VC fluid is just maintaining enough friction between the plates of the coupler to prevent the wheel with less traction from breaking away. It's a dynamic process.

The same applies to the center differential's VC.

A VC can "lock", but this is more often due to shifting of the plates such that they come in physical contact. This is not an intended situation, but can happen under extreme conditions.

There's descriptions of the center VC and the rear differential VC action in the "Mechanism and Function" section of the factory service manual, I believe beginning with 2002 editions, or thereabouts.
 

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OBW H6 VDC, Tribeca, XT6
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That's why I'm asking because I'm afraid mine might be the cause of the binding I'm experiencing when I turn.
you have torque bind, you can ignore the rear differntial for assessing this issue.

first immediate steps:
1. do the tires all match in size and tread depth?
2. has the transmission fluid ever been changed?

if the tires check out the first step is to change the fluid. this requires 3 drain and refills unless you pay to have it flushed - your choice.

place the FWD fuse in the fuse holder...should be around the passengers side front strut tower with a "FWD" cover on it. remove cover and install any 10, 15, or 20 amp fuse. or read your owners manual for where the FWD fuse holder is located.

there are two failure modes and the fuse will relieve the torque bind for one of them. let us know if that makes it go away or not.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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what ^^^ he said.

the CENTER diff in your car is not really a differential. It can cause the binding you're feeling.


the REAR diff is likely a viscous couple diff (good or bad with age as many folks contend) but is not likely to be contributing to what you're feeling.

It's less likely but possible you have a bad u-joint or other problem.
 

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Why would engineers want to have, say, 25% transfer? In both cases, the objective is to have both outputs (front and rear drives, or left and right axles) turning at the same speed.

In the case of the rear differential, if one wheel has good traction, and the other not, the VC action slows the spinning side. By doing this, torque is built up in the drive train and therefore the wheel with traction is able to move the car.

When as a result of the VC action, the wheel without traction is no longer able to spin faster than the one with traction, it can be said that the VC is "locked", but in fact the VC fluid is just maintaining enough friction between the plates of the coupler to prevent the wheel with less traction from breaking away. It's a dynamic process.

The same applies to the center differential's VC.

A VC can "lock", but this is more often due to shifting of the plates such that they come in physical contact. This is not an intended situation, but can happen under extreme conditions.

There's descriptions of the center VC and the rear differential VC action in the "Mechanism and Function" section of the factory service manual, I believe beginning with 2002 editions, or thereabouts.
Yeah, every drawing I've seen shows the viscous units drawn basically the same way, internally, so, I'm unclear as to why there's the word 'locking' used for soob MT center diffs.


here's a fun video;
 

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. . .very drawing I've seen shows the viscous units drawn basically the same way, internally, so, I'm unclear as to why there's the word 'locking' used . . .
I would look at the context; that is, where the word "locking" is used.

The video is more than fun, it's quite interesting. In part, it reinforces the importance of not towing AWD vehicles, including Subarus, improperly.

The use of the "rolling rollers frame" is quite good -- kudos to the builder.

The 325i being tested has a planetary center differential (similar to Subaru's VTD AWD used in the 5-speed AT and some earlier 4-speeds) and a rear differential. Both use viscous couplngs to control the front-rear or side-to-side output rpm, as does the Subaru. The problem with the first test car's center VC is apparent when the rear wheels spin fast but the fronts move much more slowly. The VC isn't controlling the relative speeds of the front and rear drives. The same test could be done on a Subaru with the MT or 5AT.

However, for the Subaru 4-EAT and CVT with the multi-plate transfer clutch (and no mechanical center differential), the roller frames would have to be under the front wheels, so that they could spin freely as if not having traction. The test would be to see if the rear wheels push the car along at close to the same wheel rotation speed as at the front. If not, the AWD clutch isn't functioning properly.

The roller test, unfortunately, wouldn't seem to be applicable to test if the center VC or multi-plate clutch is engaged when it shouldn't be.

In any event, SubaruDan1 has to look to the AWD system, unless that has already been eliminated as the cause of the binding.
 

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With the car jacked up, the wheels are going to spin together. If you are getting chirping or tire squeal on turns, it is possible the differential is locked up. But, I would check the alignment first. Subaru diffs only fail due to abuse or lack of maintenance.

FYI :

Quick vid of the ramp test.
 

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Discussion Starter · #17 ·
The only reason we suspect the diff is because the noise is comming from the rear of the car instead of the center as far as we can tell.
 

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The only reason we suspect the diff is because the noise is comming from the rear of the car instead of the center as far as we can tell.
some folks experience noise/jerkiness from bad driveshaft components. I suppose that cpuld also feel like it's from the rear of the car.

Have you looked on the magnet on the rear diff's drain plug? Should be no more than - I'd say - 1/2 teaspoon of sludge. certainly no 'chunks'.

take the fill plug out FIRST so you will be assured of being able to refill the diff.

of course, it's common for a bad wheel bearing to sound worse in sweeping turns - as the car rolls on it's suspension, force is vectored the side of the car on the outside of the turn - if that's the side with a bad bearing, it will be louder. But I doubt a bad bearing would lead to a jerky/binding feeling.
 

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The only reason we suspect the diff is because the noise is comming from the rear of the car instead of the center as far as we can tell.
Perhaps some more detail about the "noise" might help. (I don't see it mentioned earlier.)

Noises tend to be deceiving; it can be difficult to locate where a noise is coming from when sitting in the driver seat.

In the event the AWD multi-plate clutch is malfunctioning, the torque bind on turns will cause a wheel to hop, and the effect could be felt throughout the drive train (as 1 Lucky Texan noted).

In an earlier post you described how one raised rear wheel would turn and concluded that the differential isn't "locked". If that's the case, and presuming it doesn't change when driving, it's not going to cause binding.

Does the binding symptom (or noise) appear as soon as the car is started from cold and moved, or does it appear only after driving for some time (after the drive train might warm up)?

A possible test to eliminate, or identify, the AWD as being the case:

I believe your 2002 H6 has an AWD disabling feature -- a fuse that can be inserted to turn off the AWD. This should be described, along with the location of the FWD/AWD socket, in the Owners Manual, probably in the section on replacing a flat tire. My suggestion would be to insert a fuse in the socket. A warning light, either FWD or AWD, should come on in the instrument panel. This, in effect, makes the car front wheel drive only. With the AWD disabled, drive the car through the turns that in the past would result in the binding and see what happens. If the binding is gone, then it's the AWD clutch mechanism; either the clutch itself, or the control solenoid and/or transfer valve. (If the binding remains, it could still be the clutch, but I'd start with the test first.)
 

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At 122k, my rear LSD still works beautifully.

I think some degree of noise from it is normal. For instance, if I take a sharp corner (like a u-turn) from a stop, I can hear a faint groan type of noise from the back. I get the same noise for a split second if I'm on a muddy hill and one back wheel slips. I've heard the same noise out of every rear LSD equipped Outback I've driven. It doesn't really bind (figure 8 test goes fine...need almost no throttle), but it does make noise under extreme circumstances like that. During normal daily driving I rarely (if ever) hear it.
 
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