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.
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:
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!
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.
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).