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99 Forester L
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Discussion Starter #1 (Edited)
I purchased a 99 Forester a few months ago with a bad center diff. As soon as the transmission had heated up enough, I was met with the nasty binding, a good sign of a bad diff. I headed to the boards to understand the issue a little more, and to figure out how to fix it myself. This thread is the result of everything I've learned so far. By no means am I an expert, nor do I guarantee that what I offer here is 100% accurate. I've done a lot of searching, and a lot of asking, and with that disclaimer I present to you my findings.

Most 5 speed Subaru transmissions contain a viscous center differential to provide a 50/50 power split between the front and rear wheels. In the event of loss of traction, the viscous unit inside the differential is responsible for sending more power to wheels where the slippage occurs.

Diagram of your typical Subaru Center Diff and how it goes into the transmission assembly (number 29 in the pic):


I won't go into the science on how a viscous diff works, but I will try and explain how and why it can go bad.

What are the main signs of a "bad" center diff?
Clunking and banging of the drivetrain during slow tight turns when the transmission is warmed up seem to be the most common sign.

What are the main causes of diff failure?
Doughnuts or driving the car with mismatched tires (different levels of tread or brands). These two problems are responsible for overheating the differential

How do those things cause the diff to fail?
When the front and rear wheels are spinning at different speeds, the fluid inside the diff heats up. If it overheats, this fluid will break down and wear out. The clunking and binding is a result of worn fluid trying to do it's job, and then failing and releasing. It grabs, slips, grabs again, and slips.

Aside from the binding, how can I tell if I have a bad differential? Can I tell if the transmission is out of the car?
All you can do to check the center diff is perform a visual inspection. Sometimes if the diff has been "cooked" the metal will turn a different color due to the heat, like those exhausts with a burnt tip. Aside from that, there's nothing you can do. All the damage is done inside the viscous unit itself. It's completely sealed so you're stuck.

How many different viscous diffs are there?
There are two different center differentials. One for phase 1 transmissions (91-98 Subaru 5MT), and one for phase 2 transmissions (99+). These differentials can't be swapped between phases, so if you try and get a used one, be sure of what transmission it came out of.


How do you know they won't work?
Aside from an obvious physical difference, the inside of the differentials are structured very differently. There are splines inside the diff that allow the transfer gear shafts to send power. P1 and P2 transfer gear shafts are splined differently, therefore so are the diffs.

Is there any other option?
It is possible to swap center diff's between phases as long as the corresponding tail shaft AND transfer case is used as well. That means that the main shaft from the transmission between phase 1 and phase 2 IS the same. It's just everything else that's different. The entire assembly can be seen above in the first diagram labeled 3-1. You need everything that's pictured there.

What about 02+ WRX differentials?
These are also phase 2 and will work in any 99+ transmission. There is some misinformation floating around that it won't work because of the 1:1.1 transfer gears. These gears are NOT in the center diff, so you'll be fine.

Ok, my center diff is toast. How can I fix it?
You have three options:
1. Replace the entire transmission
2. Replace the viscous unit inside the center diff
3. Replace the entire center diff

I chose to replace the entire center diff because I found one locally at a good price ($150 bucks baybee!). Here's a brief how to on taking the old one out. These pictures are from a 1996 Phase 1 transmission. The teardown is the same, as I can confirm after replacing the diff in my Forester. This tranny was just for practice.

The center diff can be replaced with the transmission still in the car. The only additional steps you'll need to add would be to remove the exhaust, driveshaft, and disconnect the shift linkage from the transmission.

Tools needed:
14mm socket
hammer
punch
screwdriver or chisel to help split the extension from the case

1. Drain the transmission fluid. Remove the shift linkage knuckle pictured. There are two roll pins, one inside of the other. I removed both together.


2. With the pins out, you may need to tap off the knuckle with a hammer. You'll be left with this:
Knuckle



3. Remove all the 14mm bolts that hold the extension onto the transfer case housing.



4. The assembly should come right out of the transfer case. You might need to help it, but ours pretty much fell out. Be careful not to tip things forward while removing, the diff can fall out! You'll be met with this:


5. In order to get the diff off of the shaft it slides onto, you'll need to remove the shaft that the transfer gear is on. Again, this slides out, no force should be required.


6. Once that shaft is out, grab the center diff and slide it off the shaft.


This is what the tail housing will look like with the transfer gear shaft and center diff removed. The shaft the center diff slides on to shouldn't come out of the housing.


7. Put your new, good diff back on that shaft, and slide the transfer gear shaft back into the housing. Make sure the transfer gears mesh together correctly. I've provided some pictures of how it goes back together, minus the center diff.



8. Slide everything back into the transfer case, put the bolts back in, and reinstall the linkage. Don't forget a replacement gasket or some hi-temp silicone to prevent gear fluid from leaking out. It also wouldn't be a bad idea to put assembly lube or fresh gear oil on your parts to make sure everything gets lubricated and slides together easily. Chances are your new center diff has been cleaned off of all it's old gear oil and is dry. It's a good idea to lube it up before getting it together. That should be it though!
Installation Notes
In my experience, replacing the diff with the trans in the car took me roughly 4 hours total work time. I did have the advantage of previously disassembling a tailshaft from a transmission that wasn't in a car for practice, which did help immensely. Hardest part of the removal was getting the linkage knuckle out. The double roll pin is extremely stubborn and was hard to budge. Ever harder to remove was actually getting the knuckle off of the shaft. I was able to get mine to budge after heating it with a torch, and twisting it side to side with a wrench until it broke free. Once it was loose, it slid off. Neither piece is splined. Aside from that, it's pretty easy work. When reassembling, make sure that no gear oil gets on your silicone, if it does your seal probably won't work.

So what does the inside of the center diff look like?
Here's a picture of the internals of a phase 1 diff. I can't find one for a phase 2, but they're quite similar so nevermind.


Number 26 in that diagram is the actual Viscous Unit. This is the piece that contains the fluid. Here are some actual pictures of it:



Notice that one side has teeth that the spider gears turn on. In my research I've found a few instances where the spider gears go bad, and don't allow the diff to function. I'm not exactly sure how often this occurs or why it does, but I'll update the more I find out.

So can I take my differential apart and fix it?
According to Subaru, you can't take the phase 2 center diff apart. This may be the reason I haven't found a parts breakdown of the diff internals:


Now, you can take it apart, but you can't buy internal replacement parts. If you were to upgrade to a STI center diff in phase 2, you get the entire assembly. For phase 1, you can purchase upgraded STI viscous coupling units without the entire assembly. A part number for example:
ST3850055010 = Centre Viscous diff Ver.5/6 STi 20kgf m/100rpm

If you can take both units apart, then why doesn't Subaru say you can fix it?
From Rallispec: The newer center diffs have the viscous unit integrated into the diff carrier (and therefore you have to replace the entire carrier assembly) whereas on the earlier models the viscous unit could be removed from the carrier and replaced separately.

Extra Materials:
Factory Service Manual Transfer Case removal:
MSA5T0124A27669.pdf -- FileFactory.com - free file hosting --

I'd like to thank the SEARCH button, RalliSpec for their help with the center diff parts, and DS1 Motorsports for answering some of my previously unanswered questions.

If anyone has anything to add to this writeup, or if I missed anything, please let me know. I had posted this writeup on RS25.com: Subaru Impreza GC8 & RS Forum, but felt I should share it with the other Forester owners on here as I've read a lot about center diff issues. My Forester has 180k miles on it, and was run for who knows how long on mismatched tires. The old center diff was shot with a warped retainer C-Clip, and a WASTED Diff bearing (part #28 ).
 

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99 Forester S
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Thank you so much. I have waited for a long time for someone to even acknowledge this problem. 4 hours to do the job with the transmission in the car, easier than many other things. I'm not clear, though, can one purchase a new viscous unit from Suby or the aftermarket? Do you have a recommended pt number?
PMD
 

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99 Forester L
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Discussion Starter #6
Thank you so much. I have waited for a long time for someone to even acknowledge this problem. 4 hours to do the job with the transmission in the car, easier than many other things. I'm not clear, though, can one purchase a new viscous unit from Suby or the aftermarket? Do you have a recommended pt number?
PMD
You can go to the dealer, but I opted not to.
Part # 38913AA101 - Careful though, it's expensive! List price is around $500. I'm not sure if that price includes the diff bearing (#28 in the diagram).

What I did, and what I suggest you try and do is find some kid who blew up his WRX transmission and is trying to sell a broken transmission for parts (NASIOC or Ebay). That center diff is probably still good, and can be had for cheap. I got mine for $150 locally. I found another online for $300, so if you search for a used one you might find a good deal.

The install is really not that hard if you take your time. The only regret I have so far after the install is not having purchased the gasket and only using silicone sealant. I think I have a small leak.

Thanks for all the kind words guys!
 

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2004 Forester XT Premium 4EAT
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Awesome write-up... I've added a link to this thread to the how-to knowledge base.
 

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2001 Forester L
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hmm?

I recently purchased a '01 Forester L and i believe i have this same problem. what i am wondering is if it is a "fix immediately" thing or an "annoying but technically not hurting anything" sort of thing

thoughts?
 

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2001 SOLD!
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Nice thread. More technical writing than me :lol: I just take apart and take pictures!!!!:bling:
 

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99 UK S-turbo
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Great post, but looks like an amalgamation of different forum posts with the usual suspects

In the event of loss of traction, the viscous unit inside the differential is responsible for sending more power to wheels where the slippage occurs.
Er no, that will be sending it to the wheels were its NOT occuring I think!

What are the main causes of diff failure?
Doughnuts or driving the car with mismatched tires (different levels of tread or brands). These two problems are responsible for overheating the differential
How do those things cause the diff to fail?
When the front and rear wheels are spinning at different speeds, the fluid inside the diff heats up. If it overheats, this fluid will break down and wear out. The clunking and binding is a result of worn fluid trying to do it's job, and then failing and releasing. It grabs, slips, grabs again, and slips.
Nope, yes doing the above CAN damage the diff, but the fluid doesn't break down at all, that is why the fluid chosen is used, however if the diff overheats the fluid can leak out (acts like open diff), of a seal can fail, however the failure mode mentioned is where the diff has overheated and gone into 'hump' this is what its designed to do in extreme situations and the meatal playes inside deform and come into contact locking the diff, however if this happens to often the plates can deform and hit each other when not meant to, the fluid interaction logically cannot create the condition described.

Aside from the binding, how can I tell if I have a bad differential? Can I tell if the transmission is out of the car?
All you can do to check the center diff is perform a visual inspection. Sometimes if the diff has been "cooked" the metal will turn a different color due to the heat, like those exhausts with a burnt tip. Aside from that, there's nothing you can do. All the damage is done inside the viscous unit itself. It's completely sealed so you're stuck.
Not so, you can jack and wheel and measure toque to trun via a nut and torque wrench on the driveshaft nuts, I don't have torque data for scoobs (I do for Fords who put it in their service manuals), but its easy to X-check with a known/believed good car, also the MRT website details using a lathe for a similar (if a lot less conveniant but more accurate) check.

Notice that one side has teeth that the spider gears turn on. In my research I've found a few instances where the spider gears go bad, and don't allow the diff to function. I'm not exactly sure how often this occurs or why it does, but I'll update the more I find out.
Certainly a good first check, it could be something lodged inside, or it could be more serious and mean replacing the diff anyway!

Good write up on the swap.

Simon

Sources of my info were at Ricardo when I worked there, they bought out FFD who developed the VC in the first place and Ricardo still own the patent(s) and control a lot of the related IP.
 

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I still don't understand how power gets back to the front wheels, this diff only controls the output heading to the rear.

The front wheels must be hard wired to the gear box and engine which means all subarus are FWD with a clutch/viscous join for the rear wheels only?

I thought the centre diff would be like a transfer case and coupling that split the power in both directions form one source (ie like a rear diff which takes input shaft power and splits it left AND right at the same as controlling slip.) This set up would be like have power going to one wheel only with a transfer gear coming off the shaft with power to a coupling to the other wheel only. Meaning that you drive only one wheel with the power or both wheels via the coupling but never just the coupling wheel.

Ie Never can drive just the rear wheels in an AWD Subaru.
 

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Nope all wrong I'm afraid, the output from the gearbox drives the diff 'outer cage' the rear output goes to the rear, the front out goes through the front prop shaft which is INSIDE the gearbox out/diff input shaft and heads back to the front diff. If what you said were true there would be no need for gears inside the rear power takeoff.

Note this shaft inside a shaft is an identical arrangement to Audi Quattro (pre A/S5 anyway) Alfa 4x4 and Porsche911 4wd cars (Ok reverse front and rear referances).

Simon
 

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99 Forester L
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Discussion Starter #13
Great post, but looks like an amalgamation of different forum posts with the usual suspects



Er no, that will be sending it to the wheels were its NOT occuring I think!


Nope, yes doing the above CAN damage the diff, but the fluid doesn't break down at all, that is why the fluid chosen is used, however if the diff overheats the fluid can leak out (acts like open diff), of a seal can fail, however the failure mode mentioned is where the diff has overheated and gone into 'hump' this is what its designed to do in extreme situations and the meatal playes inside deform and come into contact locking the diff, however if this happens to often the plates can deform and hit each other when not meant to, the fluid interaction logically cannot create the condition described.


Not so, you can jack and wheel and measure toque to trun via a nut and torque wrench on the driveshaft nuts, I don't have torque data for scoobs (I do for Fords who put it in their service manuals), but its easy to X-check with a known/believed good car, also the MRT website details using a lathe for a similar (if a lot less conveniant but more accurate) check.


Certainly a good first check, it could be something lodged inside, or it could be more serious and mean replacing the diff anyway!

Good write up on the swap.

Simon

Sources of my info were at Ricardo when I worked there, they bought out FFD who developed the VC in the first place and Ricardo still own the patent(s) and control a lot of the related IP.
Thanks for the review - I'v neglected to check back here over the past few months, but as it's getting into snow season again, I'll frequent the boards a bit more.

Your explanation on diff failure makes much more sense than just a fluid breakdown as I was previously told. It was explained to me that the fluid within the sealed diff was silicone based, and extensive heating caused the fluid to "tear" and no longer lock the plates. Warped plates make a lot of sense given how thin they are.

Checking the differential via torquewrench on an axle nut is a good idea, and your mentioning is the first I've heard of it. I'm not exactly sure how you'd actually perform the test though. It's interesting that the Subaru FSM makes no mention of this or any method for diagnosing a diff failure. I'm curious as to how high the breakaway torque would be.

Again, I appreciate the feedback, I'll work on revising the details.

Nate
 

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1999 Forester
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Really appreciate your write-up on this problem.

Our Forester had been exhibiting signs of a failed dif for over a year, and we just put up with it. It gets really bad after highway driving and the fluid in the coupler heats up.

Based on your info I went looking for the part, which now lists new at the dealer at around $630. Picked up a blown '03 WRX tranny with all the parts, and pulled the part in question very easily thanks to your pics. Originally I intended to do the install myself, but after speaking with a quality local shop regarding our clutch job which was also due I managed to get them to agree to just drop in the viscous coupler at no extra charge while my transmission was out of the car. I always try to group my maintenance work like this and really lucked out on this one.

This write-up helped with the entire process immensly and I appreciate the work you did to post it.
 

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2001 Forester
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Nice write-up. I'm fairly certain that my diff is bad; I'll check it as soon as the weather breaks. 267,000 miles and a few too many donuts. It's about time.
 

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2000 Subaru Forester
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Central Dif

Hey
My Forester '00 5 spd, had those strange noise and wheel blocking sensation after being driven for over 50 miles with speed over 60m/h.
So I bought used central dif and had my mechanic exchange it.
Sadly this did not rectify my problem.
The question is can you bench test this part (central dif)?
I have a feeling that I was sold a bad dif for exchange.
 

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How many different viscous diffs are there?
There are two different center differentials. One for phase 1 transmissions (91-98 Subaru 5MT), and one for phase 2 transmissions (99+). These differentials can't be swapped between phases, so if you try and get a used one, be sure of what transmission it came out of.
[Question Deleted]

I found the answer to my question by reading and searching thru the forums a bit more..

An Automatic doesnt use a viscous differential.. it uses a Multi-Plate Transfer Clutch (MPT).

By the way... Great Write-up from the original poster!!
 

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Discussion Starter #18
Updated the thread with new compatibility details. It's possible to mix and match center diffs between transmission phases as long as the transfer case and tailshaft are included. So you can have a P2 transmission, and swap in a P1 transfer case, center diff, and tailshaft. It works the other way around as well!
 

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Hi all,

I just purchased a new 1997 Forester (Automatic, Turbo). I'm getting 'the shuddering' when turning in tight circles from cold, is this likely to be the centre differential too?

Sleepy Floyd's post seems to imply that Automatic's don't use a differential at all but something else completely.

I must admit I'm at a bit of a loss!

My mechanic says the center differential is bad, from what I've read this is phase 1 differential (unless my interpretation of Sleepy Floyd's post is correct in which case I don't know what the hell is going on!).

There's another thread on here (I can't post the link as I'm not a premium member yet) with a title of 'Mine vibrates and shudders when turning in car parks' that describes my symptoms but pointing at the rear differential as the problem.

We've spent all our savings on getting this forester so I'm desperately trying to avoid shelling out $500 on the wrong part.

Any advice is hugely appreciated!!

Thanks

Bob
 

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Hi all,

I just purchased a new 1997 Forester (Automatic, Turbo). I'm getting 'the shuddering' when turning in tight circles from cold, is this likely to be the centre differential too?

Sleepy Floyd's post seems to imply that Automatic's don't use a differential at all but something else completely.

I must admit I'm at a bit of a loss!

My mechanic says the center differential is bad, from what I've read this is phase 1 differential (unless my interpretation of Sleepy Floyd's post is correct in which case I don't know what the hell is going on!).

There's another thread on here (I can't post the link as I'm not a premium member yet) with a title of 'Mine vibrates and shudders when turning in car parks' that describes my symptoms but pointing at the rear differential as the problem.

We've spent all our savings on getting this forester so I'm desperately trying to avoid shelling out $500 on the wrong part.

Any advice is hugely appreciated!!

Thanks

Bob
The auto uses a multiplate clutch and not a center diff. This consists of the MPT unit (which is basically a controlled center diff), however its more likely that the transfer duty solenoid (the part that controls the MPT) has failed, both are in the tail housing of the transmission similar to the center diff on a manual trans forester.
 
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