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Certainly could be that the grooves are hanging up the apply piston from engaging AWD - just as easily as the grooves could keep it from disengaging. Probably depends on driving habits that led to the grooves in the first place - the auto's are basically 90% front wheel drive till they sense wheel slip so one could envision a vehicle that is driven very carefully and rarely has occasion to engage AWD. The grooves would form in the unlocked position and may hang up trying to engage till the duty-c ramps down to a lower state where enough pressure is held on the piston to overcome the sticking in the grooves. In all cases I'm aware of the duty-C solenoid dumps line pressure away from the lockup piston - so that a failure of the solenoid will result in full locked 4WD operation and the driver will experience really severe torque bind. If installing the fuse alleviates the problem then the duty-C is working. In such cases the torque bind is almost universally drum wear groove related except in rare cases where the vehicle has been run with severely different tire sizes or amounts of wear which can result in a burnt up clutch pack. New drums can be bought for some models - we recently got some new ones for a 2006 Forester XT where the customer wanted new rather than dressing the grooves. Some others may be NLA. Typically do have pretty good luck just filing down the edges of the grooves - usually this results in a 90% reduction in toque bind symptoms. On the older models prone to Duty-C failure we typically will replace the solenoid while in there. I will usually buy a clutch pack just in case but 90% of the time I return it. Have also seen a few (90's cars) with Duty-C driver failure inside the TCU. This is rare but shouldn't be discounted. They may or may not throw a code, but in the few cases I've seen the Duty-C solenoid and wiring checks out but the code is present and changing the solenoid doesn't help. Swapping out the TCU fixes these cars. Haven't tried going board level and checking for cracked solder joints or bad driver MOSFET's but given the age it's certainly a possibility. GD

Let me know if you need pictures or parts. I took the entire double unit down and all seals carefully removed as well. There’s a few drains and some T into others’s duct. Haven’t gone to see the exit for the front tubes yet but you were already down that path. Just about to roll this car out for scrap.