The full-time 4WD transaxle is based on the selective 5-speed transaxle, the overall case design and operation are the same. The basic difference between the full-time transaxle and the selective transaxle are the components of which are used to accomplish the 4WD action.
As mention previously, the selective transaxle uses a vacuum actuator, transfer shift fork and synchronizer to engage the transfer case. Full-time transaxles use one of two methods of transfer engagement. In Loyale STD., and XT models, the full-time 4WD transaxle is designed on the basis of the selective 4WD unit. However, a center differential unit and locking mechanism was added to the rear end transfer section, (taking the place of the shift fork). The center differential is locked and unlocked by a vacuum actuator when the differential lock switch located on the console box is operated. When the center differential is locked, the driveshaft is coupled directly to the front and rear wheels. This provides a maximum drive equivalent of that of the 4WD mode of the selective 4WD unit.
The second method used by the full-time transaxle is the use of a viscous coupling between the transfer case and center differential. This is used in Impreza and Legacy models. With this form of transfer engagement, the use of a vacuum actuator is not required. This type of transaxle may also be referred to as All Wheel Drive (AWD).
This transaxle is a compact, full-time transaxle that utilizes a center differential provided with a viscous coupling at the rear of a transfer unit. The viscous coupling serves as a differential action control. The center differential utilizes a highly reliable, bevel gear. It not only delivers an equal amount of drive power to both the front and rear, but controls the difference in rotating speed between the front and rear wheels. A viscous coupling and center differential gears are located in the center differential case to connect the front and rear wheel driveshafts. With this arrangement, the transfer system realized a compact construction. In addition, the viscous coupling serves as a differential action control to eliminate a mechanical lock mechanism.
In general terms this particular transaxle provides a constant 4 wheel drive action at all times, controlling distribution of power to all 4 wheels as required under varied driving conditions.