Subaru manual transmissions are pretty well built and have minimal problems. When failure occurs there are usually just a couple of things that fail. The most common problem is bearing failure causing the transmission to be noisy and if it’s excessive your trans will sound as if someone has put rocks in it. The other failure that is common is the viscous coupler. In Subaru MT transmissions the viscous coupler acts like a limited slip differential, but instead of allowing slip from the left and right tires the viscous coupler allows slippage of the front to rear wheels. This is for turning only because the front and rear tires while turn at a different rate in turns. The viscous coupler will also slip if the vehicle is under a heavy load going straight and when this occurs the power will be mainly transferred to the front wheels. Most of the time viscous coupler failure is on a WRX model, where the snap ring breaks out of the viscous coupler case. When this occurs there is no power transferred to the rear wheels.
These transmissions are pretty simple to repair, so if you would like to perform your own repair I will give you some tips to help you along the repair procedure. If after reading this you decide that the equipment needed and/or this seems like a little too much…
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Do it yourself Subaru MT Transmission Repair
Do it yourself required tools
- Shop press
- Split bearing puller (large and medium size)
- Press fittings Tip: use different size exhaust pipe and old bearing races.
- 55mm 6 point socket (mid or deep)
- 44mm 6 point thin walled socket ( shallow OR deep)
- Air Tools (1/2” Impact Gun)
First off, separate the transfer case unit from the main transmission case. After that is done, the biggest part that makes this easy is how you split the case and dis-assemble the transmission. The left side of the case will hold you input shaft, counter shaft, differential and shift forks and the other side is just part of the case. Just remove two of the four bolts holding the counter shaft main bearing from the side of the case your removing (at rear of case where transfer case unit was). Always mark the shim that is behind this bearing. It may not seem necessary, but upon re-assemble you can easily mix up how this goes and the holes won’t align correctly.
Now that this is done the rest is pretty self-explanatory, remove your shafts and press on and off the bearings that need replaced. I always recommend replacing all ball bearings and seals. There are some small flat bearings and round bushing type bearings that are OK to re-use. Use your split bearing puller and array of exhaust tubing to remove and press new bearings to the shafts. I use an assortment of 1’ to 3 ½’ exhaust tubing in ½’ increments for the job and also have some roll cage tubing that is more heavy duty for pressing some stubborn bearings. Another good tip for substituting your press fittings is to keep the bearing races. They can become very handy for certain needs, especially if you have a welder. You can then weld and fabricate special press fittings exclusively for certain jobs such as this.
When you get to the differential, do not move or adjust the carrier bearing adjusters. Replace the seals from the inside of the case, and if you want to replace the large O-rings in the differential carrier bearing preload adjusters just mark them. The service manual will tell you to use special tools such as dial indicator to adjust backlash. I have been working on these transmissions for over 10 years and have never had a problem doing it this way when replacing the differential carrier bearings. Another thing to look for that I see sometimes is these bearing races sometime have a tendency to spin in the case. Especially if the bearing is worn, causing drag. To resolve this problem just take a pointed punch (I use a spring loaded one) and ping around the case where the race fits. This will cause the race to fit tight again, cool huh. No need to replace the transmission case.
Hope this was helpful