Transfer clutch burn out is not uncommon with Subaru transmissions. Normal wear and tear, sealing ring failure or molded piston failure (especially with glycol intrusion) would be a few items on the list.
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One possibility is a problem with the integrity of the spline area where the CV joint connects to the rear differential (figure 1). This may cause the transfer clutch to be over active. Another area to look for is the bore plug that retains the transfer clutch valve.
Using the same hydraulic schematic for the 4EAT Phase 2 Version 2 used in the “Seven Sisters” article (figure 2), oil from the solenoid used to stroke the transfer clutch valve goes between the face of the valve and the retaining bore plug. Should that bore plug wear around the sealing wall due to vibration, the solenoid cannot control the stroke of the valve correctly. This may prevent full line pressure from reaching the clutch.
Once clamping pressure is compromised, the clutch assembly will be incinerated in due time. There is no aftermarket fix at this time. A bore plug from a spare valve body would need to be acquired or, carefully run a pipe cutter around the plug making a slight groove. The flared groove will provide a new seal. Be sure that is not overdone making instillation and removal near impossible without causing damage.
Once the groove is made, carefully tap the plug in far enough to get the keeper installed. Over traveling the plug deep into the bore interfere with the valve’s proper position.