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Hi, I wanted to share this with the internet as I've often found such guides invaluable when fixing my old Subaru. This is primarily based off a video rebuilding a phase 2 Viscous unit, full credit belonging to jon mark on Youtube. Please reference the video HERE. The main difference being the Phase 1 transmission uses a sealed/welded viscous that cannot be opened.
On my 1995 WRX STI the center diff's viscous was seized causing extreme torque bind at all times (like a truck locked into 4x4). The replacement viscous coupler: PN 38434AA021 is obsolete/discontinued. It is increasingly hard to find New Old Stock and we are already at the point were this unit failing is a major service problem. Following this guide, we will rebuild this non-serviceable part and restore the original functionality.
This is an intermediate skill level job that won't require removing the transmission. This will cost around $100, primarily due to machinist labor work. If you have access to a lathe and welder (high skill weld) you can perform the whole thing for around $15. Either way you want a bottle of Silicone Viscous differential fluid. Example 100k fluid. There are multiple specs available, jon mark has documented 100k and 200k fluids, I decided in my cold winter usage I wanted 100k.
Step 1: Remove center differential assembly (more detailed guides available elsewhere)
Start by draining the trans and removing your center exhaust section, driveshaft then shift linkages. Support the trans housing and remove the main transmission brace. Remove the rear housing extension as shown above.
*you may need an oxy-acetylyene / MAP torch if the linkage is seized after punching out the roll pin.
*the extension housing is located by 2 dowels and won't easily pop off after removing the bolts. It's easiest to thread into the housing and use a small slide-hammer to pop it free
Remove the center diff assembly along with the upper shaft (they go in/out as a pair)
Step 2: Separate the Viscous unit from the Center differential
First, check that large bearing for roughness when spun. Replace it if required for good maintenance.
A large snap ring should release easily from the center diff allowing you to pull it apart into 2 sections revealing what we are here for: the viscous coupler. Hang onto the 2 circular bearing segments and washer as that viscous comes out. Toss them into the diff housing for reassembly later.
This is the phase 1 unit that is welded shut. Looking at the above picture, I'm going to call the wider end the "top". You are going to cut the top off in order to service this guy.
Step 3: Cut the Viscous coupler open on a lathe
I had the machinist cut right on the Subaru factory weld near the top. You can use this photo to show your machinist the cut location and depth. As pictured above the unit will now pull apart by hand. Don't open it until your ready to clean, lube and assemble the unit in 1 shot. There are many disks inside and the order is important.
Step 4: Rebuild the Viscous coupler
With the top removed, you can see the burnt/failed fluid. One part at a time remove, clean and layout in order. I cleaned by first wiping with a rag then scrubbing with solvent followed by washing off the solvent with brake clean. Don't wipe, allow the brake clean to evaporate instead of introducing rag lint.
There are many disks and cleaning will take some time. Note we have 3 different parts: an outer disk, an inner disk and a spacer ring. The rings don't lock into a groove like a traditional snap-ring, they are easy to remove and install. I found my rings came out a bit mixed up. After laying it all out, move some rings around if need be. You want a consistent stack in a repeating order.
With the viscous housing empty and clean, you can pop the center gear out if you like. It's held in by tension from a dynamic seal. I removed the seal, cleaned it off and used a spray silicone lubricant. there is a similar seal in the "top" section as well. If your seals aren't pliable and feel hardened, it may be best to not disturb them.
Time to lubricate and assemble the stack, I don't have a picture as the fluid is very sticky and glue-like expect it to be messy. You should watch jon marks video on he did it for reference. In my case I lubed each disk as I placed them into the housing with a thin layer so all faces were covered. There is a master spline on the each disk, I made a paint mark for the outer and another for the inner disk on the housing so I could align them all the same. Having the alignment the same better allows the thick fluid to move between layers during assembly. At around 50% assembled I added half the bottle followed by the remainder at 75%. After placing a disk, press hard for 5-10 seconds to squish the fluid and allow the disks to stack tightly.
Once all the disks are installed we just need to fit the "top" back on. jon mark had trouble closing up his phase 2 unit at this point. You can see in the above picture I've driven the 2 balls with a hammer and punch from the inside to remove them. This was key in getting air out for the phase 2 rebuild and closing it up. I didn't have trouble with my phase 1 unit but it's easy to do. You then reinstall balls by dropping them back in and peening the edges over to lock them in place just like Subaru originally did.
Step 5: Have the Viscous Coupler welded back together
We want to maintain the same height of our viscous when we are done. That is 88mm total height or 35.2mm on the main body that holds the clutches. By cutting it open we've lost material and want to provide the machinist the correct height. I made some 3D printed brackets you can use to hold the unit at 35.2mm. I suggest printing 4 of them.
The machinist can tack weld the unit with the brackets setting the height then discard them and fully weld the seam. You may also want to have the weld machined back down if it's "higher" than the original dimensions or a bit rough in places.
Pictured above is the completed viscous coupler, ready to go back in. You might try (like I did) to spin the unit manualy. Clamp in a soft vice and find something you can wedge in the upper splines. Use leverage to turn the inner core proving the unit now can internally move again (it takes a decent amount of torque to overcome).
Step 6: Reassemble your car
After cleaning up the case halves to bare metal, you can use a Subaru gasket or just apply RTV and allow adequate time before filling the trans back up after assembling everything in reverse order.
Final: Test drive to confirm you no longer have a torque bind condition by driving 20 minutes and then performing a few tight turns in a dry parking lot.
While this might look intimidating it's not terribly difficult. If you are stuck trying to find a new viscous like I was I'd hope you reference this guide and that it serves you well in getting your Subaru AWD back in good form!
Thanks to jon mark for blazing this trail.
On my 1995 WRX STI the center diff's viscous was seized causing extreme torque bind at all times (like a truck locked into 4x4). The replacement viscous coupler: PN 38434AA021 is obsolete/discontinued. It is increasingly hard to find New Old Stock and we are already at the point were this unit failing is a major service problem. Following this guide, we will rebuild this non-serviceable part and restore the original functionality.
This is an intermediate skill level job that won't require removing the transmission. This will cost around $100, primarily due to machinist labor work. If you have access to a lathe and welder (high skill weld) you can perform the whole thing for around $15. Either way you want a bottle of Silicone Viscous differential fluid. Example 100k fluid. There are multiple specs available, jon mark has documented 100k and 200k fluids, I decided in my cold winter usage I wanted 100k.
Step 1: Remove center differential assembly (more detailed guides available elsewhere)
Start by draining the trans and removing your center exhaust section, driveshaft then shift linkages. Support the trans housing and remove the main transmission brace. Remove the rear housing extension as shown above.
*you may need an oxy-acetylyene / MAP torch if the linkage is seized after punching out the roll pin.
*the extension housing is located by 2 dowels and won't easily pop off after removing the bolts. It's easiest to thread into the housing and use a small slide-hammer to pop it free
Remove the center diff assembly along with the upper shaft (they go in/out as a pair)
Step 2: Separate the Viscous unit from the Center differential
First, check that large bearing for roughness when spun. Replace it if required for good maintenance.
A large snap ring should release easily from the center diff allowing you to pull it apart into 2 sections revealing what we are here for: the viscous coupler. Hang onto the 2 circular bearing segments and washer as that viscous comes out. Toss them into the diff housing for reassembly later.
This is the phase 1 unit that is welded shut. Looking at the above picture, I'm going to call the wider end the "top". You are going to cut the top off in order to service this guy.
Step 3: Cut the Viscous coupler open on a lathe
I had the machinist cut right on the Subaru factory weld near the top. You can use this photo to show your machinist the cut location and depth. As pictured above the unit will now pull apart by hand. Don't open it until your ready to clean, lube and assemble the unit in 1 shot. There are many disks inside and the order is important.
Step 4: Rebuild the Viscous coupler
With the top removed, you can see the burnt/failed fluid. One part at a time remove, clean and layout in order. I cleaned by first wiping with a rag then scrubbing with solvent followed by washing off the solvent with brake clean. Don't wipe, allow the brake clean to evaporate instead of introducing rag lint.
There are many disks and cleaning will take some time. Note we have 3 different parts: an outer disk, an inner disk and a spacer ring. The rings don't lock into a groove like a traditional snap-ring, they are easy to remove and install. I found my rings came out a bit mixed up. After laying it all out, move some rings around if need be. You want a consistent stack in a repeating order.
With the viscous housing empty and clean, you can pop the center gear out if you like. It's held in by tension from a dynamic seal. I removed the seal, cleaned it off and used a spray silicone lubricant. there is a similar seal in the "top" section as well. If your seals aren't pliable and feel hardened, it may be best to not disturb them.
Time to lubricate and assemble the stack, I don't have a picture as the fluid is very sticky and glue-like expect it to be messy. You should watch jon marks video on he did it for reference. In my case I lubed each disk as I placed them into the housing with a thin layer so all faces were covered. There is a master spline on the each disk, I made a paint mark for the outer and another for the inner disk on the housing so I could align them all the same. Having the alignment the same better allows the thick fluid to move between layers during assembly. At around 50% assembled I added half the bottle followed by the remainder at 75%. After placing a disk, press hard for 5-10 seconds to squish the fluid and allow the disks to stack tightly.
Once all the disks are installed we just need to fit the "top" back on. jon mark had trouble closing up his phase 2 unit at this point. You can see in the above picture I've driven the 2 balls with a hammer and punch from the inside to remove them. This was key in getting air out for the phase 2 rebuild and closing it up. I didn't have trouble with my phase 1 unit but it's easy to do. You then reinstall balls by dropping them back in and peening the edges over to lock them in place just like Subaru originally did.
Step 5: Have the Viscous Coupler welded back together
We want to maintain the same height of our viscous when we are done. That is 88mm total height or 35.2mm on the main body that holds the clutches. By cutting it open we've lost material and want to provide the machinist the correct height. I made some 3D printed brackets you can use to hold the unit at 35.2mm. I suggest printing 4 of them.
The machinist can tack weld the unit with the brackets setting the height then discard them and fully weld the seam. You may also want to have the weld machined back down if it's "higher" than the original dimensions or a bit rough in places.
Pictured above is the completed viscous coupler, ready to go back in. You might try (like I did) to spin the unit manualy. Clamp in a soft vice and find something you can wedge in the upper splines. Use leverage to turn the inner core proving the unit now can internally move again (it takes a decent amount of torque to overcome).
Step 6: Reassemble your car
After cleaning up the case halves to bare metal, you can use a Subaru gasket or just apply RTV and allow adequate time before filling the trans back up after assembling everything in reverse order.
Final: Test drive to confirm you no longer have a torque bind condition by driving 20 minutes and then performing a few tight turns in a dry parking lot.
While this might look intimidating it's not terribly difficult. If you are stuck trying to find a new viscous like I was I'd hope you reference this guide and that it serves you well in getting your Subaru AWD back in good form!
Thanks to jon mark for blazing this trail.
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