How To: Center Diffs

Flywheel, Clutch, Transmission, Axles, etc...

Moderators: Helpinators, Moderators

n2x4
Fourth Gear
Posts: 1522
Joined: Tue Oct 18, 2005 3:52 pm
Location: Massillon, Ohio

How To: Center Diffs

Post by n2x4 »

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):
Image

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.
Image

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 virtually the same in a Phase 2 trans. 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 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.
Image

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

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

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:
Image

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.
Image

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

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.
Image

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.
Image
Image

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.
Image

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:
Image
Image

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:
Image

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:
http://www.main.experiencetherave.com/s ... A27669.pdf

If anyone has anything to add to this writeup, or if I missed anything, please let me know. The last thing I want to do is spread bad info!
Last edited by n2x4 on Wed Apr 14, 2010 3:06 pm, edited 1 time in total.
SUBARUEHS Racing
BSOD2600
Fourth Gear
Posts: 1636
Joined: Tue Sep 12, 2006 6:49 am
Location: Seattle, WA

Post by BSOD2600 »

Very nice writeup!

'11 WRX Limited
'94 SS | 3" TBE, 07 TMIC, TD05H-16G, Revtronix Stage 2, Walbro -- Sold
'94 TW | R.I.P.
skid542
Fifth Gear
Posts: 2857
Joined: Fri Dec 26, 2003 6:37 am
Location: North Idaho

Post by skid542 »

Agreed. Thanks for the information! This will make a great resource for the BBS.
Lee

93' SS, 5mt swapped, 182k, not stock...
96' N/A OBW 5sp, 212k, Couple mods... RIP
99' N/A OBW, 4eat, mostly stock.
n2x4
Fourth Gear
Posts: 1522
Joined: Tue Oct 18, 2005 3:52 pm
Location: Massillon, Ohio

Post by n2x4 »

Thanks guys, just spreading the wealth. For some reason there's not a lot of center diff info out there. I know I ran into that issue with my Forester so I knew I'd better figure it out and share my findings. I'm happy to report that the Forester has several thousand miles on a low mile WRX center diff and there are no more issues!
SUBARUEHS Racing
AWD_addict
Fifth Gear
Posts: 2774
Joined: Sun Nov 23, 2003 12:22 am
Location: Western WA!

Post by AWD_addict »

Good info, thanks!
90 L+ wgn
Legacy777
Site Admin
Site Admin
Posts: 27884
Joined: Tue Oct 15, 2002 11:37 am
Location: Houston, Tx
Contact:

Post by Legacy777 »

Very nice writeup!

Thanks


BTW, fabrication of some punches similar to these, help in installation of the shifter joint.

http://main.experiencetherave.com/subar ... P_4188.JPG
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
tturnpaw
First Gear
Posts: 208
Joined: Sat Dec 27, 2008 4:15 am
Location: Marysville, WA

Post by tturnpaw »

Good writeup!! Also, spider gears most likely fail from slippage at speed. For example, you have an open differential rear, and one tire spins at high speeds will equal chipped teeth and failure. Another example is high levels of torque but this is less likely. Clutches fail for the same reason.
1-3-2-4
Second Gear
Posts: 308
Joined: Sun Nov 06, 2005 6:21 am
Location: Greenwich, Connecticut
Contact:

Post by 1-3-2-4 »

good info!
adema2626
Second Gear
Posts: 330
Joined: Thu Jul 27, 2006 12:39 am
Location: Oregon

Post by adema2626 »

Nice write up very in depth. Should weld those spider gears before you put it back in to make it rwd :)
-Aaron
93 Imp wag, 94 Imp wag WRX swap, 99 Imp Sedan WRX swap, 01 Imp Outback Sport, 05 Outback XT, 00 Outback, 98 Outback, 93 Leg touring, 93 Leg wag, 94 Leg wag lifted ej22t swap, 79 Brat 4x4 EA81 4spd d/r swap, 97 Crown Vic lifted
Legacy Rally Guy
Third Gear
Posts: 586
Joined: Tue May 23, 2006 2:30 pm
Location: Northern Missouri

Post by Legacy Rally Guy »

VERY nice... I'll be doing this here pretty soon!

THanks
1991 Legacy N/A- Being built to withstand zombie apocalypse!
2004 WRX STi- (GT3076r, 850cc Deatchwerks, Unequal Length Headers, FMIC, Alum Driveshaft, Camber Kit, Daily Driver)
Soul Shinobi
Second Gear
Posts: 496
Joined: Fri Apr 14, 2006 8:49 pm
Location: Nashua, NH, USA

Post by Soul Shinobi »

Excellent writeup!
adema2626 wrote:Nice write up very in depth. Should weld those spider gears before you put it back in to make it rwd :)
This is EXACTLY what I wanted to ask about. Where exactly would I want to weld to disable the front wheels entirely?

EDIT: Looking at the second exploded view diagram, couldn't you just weld the rear-gear (30) to the case (33)? Or, alternatively, removed the VC unit (26) and weld the spider gears (28 ) to the rear-gear (30) and/or case (33).

Image
Last edited by Soul Shinobi on Sat Sep 05, 2009 3:38 am, edited 2 times in total.
-Nick
1992 BC Legacy L Sedan AWD 5MT 272,000 - Wish you the best
1998 BK Legacy L Wagon 5MT 234,000 miles - RIP
2002 SF Forester S Wagon 5MT 215,000 miles - Current winter sleigh
1986 FC Mazda RX-7 GXL Coupe 5MT 155,000 miles - Summer cruiser
1-3-2-4
Second Gear
Posts: 308
Joined: Sun Nov 06, 2005 6:21 am
Location: Greenwich, Connecticut
Contact:

Post by 1-3-2-4 »

the only way to get the shift linkage knuckle out is to drop the tranny right? Replacing the center diff by going what's posted here seems way too easy..

However at 209,300 miles it seems my center diff is still pretty good.. I've played with it in the rain and I can get sideways if I push it. I'm not sure of the life ratings of them however.
Soul Shinobi
Second Gear
Posts: 496
Joined: Fri Apr 14, 2006 8:49 pm
Location: Nashua, NH, USA

Post by Soul Shinobi »

1-3-2-4 wrote:the only way to get the shift linkage knuckle out is to drop the tranny right?
No, I've done it twice on the car. It helps to leave the shifter in 5th gear, this is very helpful! You have to punch out the smaller center roll pin to relieve pressure on the bigger one, then punch out the bigger one. The real pain is then getting the linkage off its shaft, I used a torch both times, hitting it off with a chisel in between heating. Not easy. A teacher of mine who was a Subaru tech said he fabricated a tool for hitting it off, a long chisel made from a lateral link. If you've got a long rod (~1.5+ feet) like that it's easier to hit it from the front of the car toward the back.
-Nick
1992 BC Legacy L Sedan AWD 5MT 272,000 - Wish you the best
1998 BK Legacy L Wagon 5MT 234,000 miles - RIP
2002 SF Forester S Wagon 5MT 215,000 miles - Current winter sleigh
1986 FC Mazda RX-7 GXL Coupe 5MT 155,000 miles - Summer cruiser
beatersubi
Fifth Gear
Posts: 2385
Joined: Tue Mar 13, 2007 12:56 am
Location: 10-9-oh(!), wa

Re: How To: Center Diffs

Post by beatersubi »

Very good and helpful info. Can we get a sticky?
93 legacy wagon L, 22T swapped (TW imitator) now with five forward speeds. (Gone, but never forgotten)
johndrivesabox wrote: Rally, my kyboard is brok, his has nohing o do wih h liquor.
Originalcyn wrote:Apparently everyone hates Gabe.
1-3-2-4
Second Gear
Posts: 308
Joined: Sun Nov 06, 2005 6:21 am
Location: Greenwich, Connecticut
Contact:

Re: How To: Center Diffs

Post by 1-3-2-4 »

agree this should be a sticky
Legacy777
Site Admin
Site Admin
Posts: 27884
Joined: Tue Oct 15, 2002 11:37 am
Location: Houston, Tx
Contact:

Re: How To: Center Diffs

Post by Legacy777 »

added to the sticky compilation thread
http://bbs.legacycentral.org/viewtopic. ... 59#p264859
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
1-3-2-4
Second Gear
Posts: 308
Joined: Sun Nov 06, 2005 6:21 am
Location: Greenwich, Connecticut
Contact:

Re: How To: Center Diffs

Post by 1-3-2-4 »

Thank you Josh
n2x4
Fourth Gear
Posts: 1522
Joined: Tue Oct 18, 2005 3:52 pm
Location: Massillon, Ohio

Re: How To: Center Diffs

Post by n2x4 »

Woo - Stickyyy!
SUBARUEHS Racing
Burnera
In Neutral
Posts: 41
Joined: Fri Dec 12, 2008 11:23 pm

Re: How To: Center Diffs

Post by Burnera »

Is the center differential the same from the 5MT to the 4EAT?
James614
Fourth Gear
Posts: 1744
Joined: Sun Jun 10, 2007 4:35 am
Location: Arkansas

Re: How To: Center Diffs

Post by James614 »

Short answer is no. Long answer is the 4eat doesn't have a center diff, it uses a clutch pack to transfer power from the front wheels to the rear. The clutches are controlled by the "Duty C" solenoid. Problems with the 4WD in a 4EAT are usually a result of this solenoid failing. If you search on here you can find threads dedicated to Duty C diagnosis and replacement.
93 Touring Wagon (EJ20G 5spd Swap) -- Finally back and running strong as ever!

05 Outback 2.5XT 5spd -- Now the wife can have her SUV and get in on the turbo Legacy goodness at the same time.
Burnera
In Neutral
Posts: 41
Joined: Fri Dec 12, 2008 11:23 pm

Re: How To: Center Diffs

Post by Burnera »

James614 wrote:Short answer is no. Long answer is the 4eat doesn't have a center diff, it uses a clutch pack to transfer power from the front wheels to the rear. The clutches are controlled by the "Duty C" solenoid. Problems with the 4WD in a 4EAT are usually a result of this solenoid failing. If you search on here you can find threads dedicated to Duty C diagnosis and replacement.
Thanks for the answer. The clutches on my 4EAT are shot, i can feel them stutter and slip all the time.
I put in the FWD fuse and it's silent now, so i know what the problem is.
I plan to get a 4EAT with the VTD center diff.
I know if i put it in, i'll have to leave the FWD fuse in, since my TCU/ECU will have no idea whats up, and the VTD defaults at 45/55 torque split with no computer input.
From what i've been reading, this works, but do you have any damning info?
James614
Fourth Gear
Posts: 1744
Joined: Sun Jun 10, 2007 4:35 am
Location: Arkansas

Re: How To: Center Diffs

Post by James614 »

You need a JDM WRX transmission from 93-96 for this (and some older twin turbo Legacy models, and some SVX/Alcyobe models, but the VTD option on those was likely not 100% from what I was able to tell in my previous research). The phase 2 USDM models cant be controlled by the TCU. Good news for that is the JDM units are often sold as a swap including the proper TCU which should be plugin compatible. The key to finding the right phase one trans is a trabs code starting in "TZ102Y" vs the TZ102Z" you see normally. Also, the JDM WRX is 4.11 FD, so your rear diff needs to match that.

I wanted to do this a few years ago, but a great deal on a 5spd swap fell into my hands and I went that route instead. But one of these days I would love to have it on another car.
93 Touring Wagon (EJ20G 5spd Swap) -- Finally back and running strong as ever!

05 Outback 2.5XT 5spd -- Now the wife can have her SUV and get in on the turbo Legacy goodness at the same time.
James614
Fourth Gear
Posts: 1744
Joined: Sun Jun 10, 2007 4:35 am
Location: Arkansas

Re: How To: Center Diffs

Post by James614 »

And oh yeah, the phase 1 VTD transmissons are 36:64 torque split. ;)
93 Touring Wagon (EJ20G 5spd Swap) -- Finally back and running strong as ever!

05 Outback 2.5XT 5spd -- Now the wife can have her SUV and get in on the turbo Legacy goodness at the same time.
Burnera
In Neutral
Posts: 41
Joined: Fri Dec 12, 2008 11:23 pm

Re: How To: Center Diffs

Post by Burnera »

Has anybody done it with usdm WRX stuff and failed?
mike-tracy
Moderator
Moderator
Posts: 5000
Joined: Tue Aug 25, 2009 7:52 am
Location: Des Moines, Wa

Re: How To: Center Diffs

Post by mike-tracy »

Burnera wrote:Has anybody done it with usdm WRX stuff and failed?
If you're trying to find someone who has done it, maybe look at newer cars that came with both phase 1&2 transmissions in the same body. Such as the 1st generation impreza, and the 2nd gen legacy.

Even if you could rewire your whole car just to run the WRX tranny, would you really want to lose the functions tied into the phase 1 transmission ecu? Such as, ABS, Cruise Control, and other functions? Or were you thinking about swapping and rewiring those in as well, from the WRX?
1992 Legacy SS 5mt, build in progress
Josh Colombo wrote: Mon Jan 14, 2002 10:23 am Wait....I'm confused now.
Post Reply