Viscous coupler OR couplers

[quazi]

I've searched here and far and wide for info on the viscous coupler.

 

Maybe my my overheated searching has caused the fluid in the diff situated between my ears to seize-up, but I can't seem to determine how many viscous couplers my '03 Forester (5sp MT) has.

 

I've read identical accounts of the clunking trouble when tight turning, warmed up and at slow speed.

 

This often points to a diagnosis of viscous coupler failure.

 

For the same symptom, some diagnose "bad V/C in center diff" and some "bad V/C rear diff."

 

'Ow many V/C's we got, mon?

 

Leaving aside whether my V/C diagnoses is correct, if we have two, how do we know which one's bad?

 

Subaru's not too forthcoming in the parts blow-up department, I've noticed, and my Haynes manual doesn't even list ONE V/C!

 

Thanks,

 

JW

[johnceggleston]

http://opposedforces.com/parts

 

the is one viscous coupler in the manual trans. it is the power distribution point for the rear wheels. when it fails you have torque bind. but usually you have it when making slow tight turns in BOTH directions, not just one.

 

a bad axle / CV joint can cause clicking, knocking, maybe even binding, in turns .

[quazi]

Thank you, johnceggleston!

 

Great link. Somehow, hours of googling and binging didn't get me there.

 

I focused on getting to the rear diff blow-up, but if the rest of that site is organized in the same way, it's golden.

 

It is binding right and left. Just like my Cornbinder will do with too much traction in 4WD. More of a scuffing, rubbery feel than a metallic clunk or tick.

 

With the design of the rear diff, changing this part looks way easier than I would have expected.

 

JW

[GeneralDisorder]

You can only buy the entire center diff assembly for about $450 from Subaru.

 

You should also buy all four of the bearings - especially the front support bearing for the diff - that go in the rear extension housing. The support bearings for the diff and transfer gears/shafts are prone to failure in those years so be prepared to do all the bearings along with the diff.

 

Sadly this is common enough that used parts are not really a good option - you would likely have to buy a whole transmission and it will almost certainly need the same bearings.

 

GD

[johnceggleston]

it is not the rear diff.

 

the part causing your problem is in the rump roast end of the trans.

 

item #41

 

http://opposedforces.com/parts/forester/us_s11/type_1/manual_transmission/mt_transfer_and_extension/illustration_2/

[GeneralDisorder]

Looks like 46 comes with 41 but you should also replace 4, 5, and 26 while you are in there. And get a new 46 if you get a used diff. Often they are all damaged but 46 goes first.

 

GD

[Fairtax4me]

What sub-model Forester? X, XS?

 

The limited slip rear diff was only available on the XS model. The X model will have an open rear diff. Barring physical internal damage, that would rule out the rear diff.

 

Torque bind symptoms are almost always due to the transmission on these.

Tire size, air pressure, and tread wear play a crucial role in the longevity of V/C in the center diff. If at ANY point the car was driven with mismatched tires, or two brand new tires mixed with two old ones, there's a good chance the center diff was damaged at that time.

[nipper]

http://www.subaruforester.org/vbulletin/f89/center-diffs-how-replace-them-46979/

 

With pictures!

[quazi]

OK---

 

My REALLY bad...

 

Dense, dense. But I've got it now. The scales have fallen from my eyes.

 

I'd like to erase my previous post so as not to confuse anybody else. The center diff, rear diff thing had me chasing my tail.

 

The XS models have limited slip diffs in the rear, but I care about this not at all, except to say that what I have in my 2.5X is an all wheel drive in the same sense that my old-school rigs are "4 wheel drive" when in reality they are 2 wheel drives: one front and one rear unless they have limited slips, or some type of locker.

 

I think this has been part of my confusion----thinking that my Subie was some totally different, mysterious machine, whose VISCOUS-Cous-cous--

COUPLER-Coupler-coupler untethered it from the physical bounds other vehicles must live by and magically managed power to all four wheels.

 

What it does is transfer power to front and rear, on wet or dry pavement, full-time, something that would tear apart a locking 4 wheel drive. Which I guess is kinda' magical.

 

Now if you had a V/C at all four corners...

 

It still looks pretty easy. Easier now, I think.

 

Thanks, all

 

JW

[nipper]

OK---

 

My REALLY bad...

 

Dense, dense. But I've got it now. The scales have fallen from my eyes.

 

I'd like to erase my previous post so as not to confuse anybody else. The center diff, rear diff thing had me chasing my tail.

 

The XS models have limited slip diffs in the rear, but I care about this not at all, except to say that what I have in my 2.5X is an all wheel drive in the same sense that my old-school rigs are "4 wheel drive" when in reality they are 2 wheel drives: one front and one rear unless they have limited slips, or some type of locker.

 

I think this has been part of my confusion----thinking that my Subie was some totally different, mysterious machine, whose VISCOUS-Cous-cous--

COUPLER-Coupler-coupler untethered it from the physical bounds other vehicles must live by and magically managed power to all four wheels.

 

What it does is transfer power to front and rear, on wet or dry pavement, full-time, something that would tear apart a locking 4 wheel drive. Which I guess is kinda' magical.

 

Now if you had a V/C at all four corners...

 

It still looks pretty easy. Easier now, I think.

 

Thanks, all

 

JW

 

now i am confuzzled

[quazi]

Sorry, nipper---

 

The Subarapture has me speaking in tongues.

[GeneralDisorder]

Actually the center viscous coupler doesn't transfer power at all. It resists the action of the center diff that allows the front and rear to turn at different speeds. The more they differ the stronger the VC resists - heat energy changes the fluid in the VC to solid and at some critical point the center diff ceases to differentiate at all.

 

So that if you have a single front wheel that's spinning it will cause the VC to heat up, eventually lock, and stop the center diff from allowing power to flow to only that wheel - causing one or both rear wheels to slowly accelerate to the same speed as the front wheel that has lost traction. It's not a transferring of power but rather a power splitting device that works based on demand.

 

GD

[Fairtax4me]

I think this has been part of my confusion----thinking that my Subie was some totally different, mysterious machine, whose VISCOUS-Cous-cous--

COUPLER-Coupler-coupler untethered it from the physical bounds other vehicles must live by and magically managed power to all four wheels.

But... it IS magical... and untethered of the physical bounds of other vehicles...

 

:-p

 

Just Youtube "Subaru vs snow" and you'll see what I mean. All 4 wheels spin all the time.

[quazi]

So why does Subaru offer a limited slip in the XS models?

 

IIRC, a standard clutch-disk type limited-slip provides around an 80/20 power split.

 

Not a huge improvement over an open diff, although someone stranded in the sticks would have a different view, I'm sure.

 

JW

[GeneralDisorder]

So why does Subaru offer a limited slip in the XS models?

 

IIRC, a standard clutch-disk type limited-slip provides around an 80/20 power split.

 

Not a huge improvement over an open diff, although someone stranded in the sticks would have a different view, I'm sure.

 

JW

 

Clutch types are rated in break-away torque not in power split and they can be "tuned" to be as hard or as gentle as you like. There is no rule governing all clutch types in that way.

 

Viscous diffs are tuned differently and are used for a different purpose. They are primarily for spirited street driving and maybe driving in the snow, etc. They aren't for crawling around in the woods as they would never get hot enough to be of any real advantage in that scenario. They have to heat up to work - that means working them hard such as auto-cross and snow/ice driving at speeds greater than 15 mph.

 

GD

[nipper]

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

 

AWD systems

 

and here is clutch type LSD

 

http://auto.howstuffworks.com/differential4.htm

[quazi]

Clutch types are rated in break-away torque not in power split and they can be "tuned" to be as hard or as gentle as you like. There is no rule governing all clutch types in that way.

 

Viscous diffs are tuned differently and are used for a different purpose. They are primarily for spirited street driving and maybe driving in the snow, etc. They aren't for crawling around in the woods as they would never get hot enough to be of any real advantage in that scenario. They have to heat up to work - that means working them hard such as auto-cross and snow/ice driving at speeds greater than 15 mph.

 

GD

 

So, if you never get on the pavement, there might be no reason to replace a burnt-out V/C and in fact might offer off-road advantage?

[nipper]

So, if you never get on the pavement, there might be no reason to replace a burnt-out V/C and in fact might offer off-road advantage?

 

That is true. Some people weld the gears together for dirt tracks or trails. Those cars are not pavementg freindly.

[quazi]

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

 

AWD systems

 

and here is clutch type LSD

 

http://auto.howstuffworks.com/differential4.htm

 

That's a cool read, nipper.

 

I had a clutch-pack and now a torsen type in my Scout's Dana 44.

 

JW

[quazi]

Looks like 46 comes with 41 but you should also replace 4, 5, and 26 while you are in there. And get a new 46 if you get a used diff. Often they are all damaged but 46 goes first.

 

GD

 

I wonder why #4 is missing from the attached parts list.

[torxxx]

well I'm gonna semi-hijack this thread lol

 

VC aside, and I'm sure GD will know the answer to this but other than getting a Gravel Spec Center Diff and the DCCD controller, is there any other form of Center Diff that we can install on the 5 speed? I'm getting really close to turning my OB into my offroad toy and the VC isnt going to cut it. Unfortunately I dont have access to any WRX 5MTs and/or the 6MT

[1 Lucky Texan]

the old XT6 I believe had a a manual locking center diff. Dunno if the internals would move over, or if the entire 5 speed is transplantable to anything modern.

 

 

you might read this too; http://www.scoobymods.com/attachment.php?attachmentid=9654&d=1346818252

Edited September 5, 2012 by 1 Lucky Texan

[torxxx]

The rx hax center diff control also. I might have one.of.those laying around at the shop. Tear down time!

[quazi]

While I'm not stoked to be replacing this part, I've got to give Subaru engineers a couple big thumbs-up.

 

First, I love the fact that two wrench sizes do most of the work: the 12mm and the 14mm.

 

Second, the exhaust is really a dream to remove. I approached this with much caution, because exhaust system bolts are notoriously difficult. But it comes off in two big sections. When you unbolt the two halves at the center support, both halves remain suspended: the front half, where it bolts to the rear half, rests in a hook, so that you don't need a jack to hold it up while unbolting the headers. This is especially sweet during assembly. Hook the aft end and swing the header end into position.

The rear half of the exhaust rests in rubber hangers. A sponge-load of soapy water helps to slip the rubber off of and back onto steel "barbs". Really a piece of cake.

 

Also, the asbestos "donut" stays with one half of the exhaust, and mine was reusable.

 

For me, there is a big "pucker-factor" going into the gearbox, but all-in-all, this was a pretty easy job; I could do it in one third the time having done it once.

 

A couple more things for future searchers: I didn't want to risk damaging the oxygen sensors, so I left them in the cat and disconnected their plugs( don't forget to unwrap the "twist-ties"). I know this seems like a no-brainer, but at first I tried to get a wrench on the sensors. If I ever have to change them, I might just pull the exhaust just to get a clean shot at them.

 

My shifter linkage was held in place with two roll pins: an inner and an outer. I drove the inner one out first, with a punch, which was pretty easy. The second one was a little tougher, I don't like the idea of pounding sideways on a shaft, so I cut piece of rod around 1" or so long and found a c-clamp with a hole in the screw pad to keep the rod in place; most are like this. I pushed it through most of the way into a socket sitting on top, then finished it with a hammer and drift. I used a hammer and drift to get them back in, large one first, then inner.

 

The big bearing that bolts to the extension case is held on with a circlip, but I still had to use a heavy-duty steering wheel puller to get it off. To get the new one on, I left the shaft in the freezer for a few hours while I took care of other business, and left the new bearing (in bag!) in the sun. It got hot enough to be hard to hold, but not hot enough to sear flesh.

Slipped right on, with just a few taps on a large socket to drive it home.

 

I took the Forester on a run around town and some freeway and then to a parking lot for slow-speed tight turn tests. No shudder, and in fact it drove better than in a long time. It had been bucking in first and second but it's much better now.

 

Anyway, thanks everyone for the input. I saved a ton of $$$ doing this at home.

 

JW

Edited September 9, 2012 by quazi
should've been SHIFTER linkage, not CLUTCH linkage

[Olnick]

Nice writeup of your process, JW. Should be helpful to others down the road. Thanx.