This has been a hot topic of late, as more and more older Subaru Legacies reach high mileage and drivetrain wear becomes a factor. One of the most common problems in AWD 4EAT automatic transmission equipped cars is the dreaded "torque-bind".
The most common symptoms of this problem are a shudder throughout the whole car, but more pronounced beneath the seat, when accelerating in a low-speed turn. The shudder will stop if power is removed and disappears when speed goes beyond the threshold of most tires or the wheels are straightened out.
Recently i've found out that the early model Legacies don't actually have the wear problems listed below, but frequently have a malfunctioning solenoid in the same area that causes pressure problems and very similar symptoms. Replacing this should fix the problems in pre-95 models.
The solution to the problem is relatively simple and comprehensive but first you should know what causes this problem.
The evolution of the problem and the solution were shown to me by a Master Technician at Subaru of Ogden in Ogden, Utah.
In the rear of the transmission is a watermelon sized aluminum housing that contains several valves, bushings and seals. The rear driveshaft that couples with the rear differential. The problem occurs in here, and the replacement of this cures it.
Inside of the housing the rear shaft spins and has teflon bushings and fluid seals for the AWD system. This is where much of the rear power is transferred. The problem occurs when these bushings wear against the aluminum. For some reason Subaru, or the transmission manufacturers, decided not to have a bearing or liner in lieu of using the housing itself as the wear surface.
This is where the problem and solution lies.
The bushings and seals wear the aluminum down after time. This allows the pressurized ATF to leak from one chamber to the next. The torque meant to be transferred to the rear ends up being greatly reduced and the fluid transaction causes the shuddering.
Thankfully the fix is relatively inexpensive and doesn't require the transmission to be removed.
At Subaru of Ogden i got to see a 95+ Legacy in the midst of this operation. The rear driveshaft and exhaust rear of the Catalytic Convertor have to be removed and the rear transmission housing needs to be removed.
The housing is completely replaced with a new one. New internal components are included and apparently the bushings and seals on the driveshaft are also replaced. The most important piece of the solution is the addition of a steel liner to protect the aluminum housing against wear. This should ensure that this problem doesn't occur for the remaining life of the car.
The procedure was quoted as $750 by Subaru of Ogden.
An updated case with the steel liner was put into prodution and installed in all new cars in mid-1997.
You may never have this problem but if you do, there is a relatively easy and inexpensive solution to a more and more common problem.