Internal and external changes require viewing the 4EAT Phase 2 as an entirely new automatic transmission. Because it is unlikely that you will be disassembling a Phase 2 transmission in the near future, this article will concentrate on principles of operation and failure diagnosis.
The 4EAT Phase 2 (introduced on 1999 model year Subaru vehicles) provides the same type of electronic control used by prior model year vehicles and shares many of the same diagnostic procedures. However, there have been internal and external changes that require this 4EAT to be viewed as an entirely new automatic transmission. Additionally, beginning with the 2001 model year, an enhancement to the all wheel drive transfer section was introduced. This enhancement, called Variable Torque Distribution (VTD), is included on all Subaru vehicles with Vehicle Dynamic Control (VDC). VTD is designed to smoothly transfer and divide the power from the engine to the wheels.
Several mechanical changes have been made for the 4EAT Phase 2 automatic transmission. These changes include: a new external oil filter, an additional speed sensor (for a total of three instead of just two), three new solenoids and the elimination of the 3-4 one-way clutch.
The external oil filter requires no scheduled maintenance, although it can be changed if necessary. The new filter is in addition to the metal valve body screen found on previous models. The screen is still present and can be flushed when the 4EAT is disassembled.
There are three speed sensors instead of the previous two: one for the front output shaft, one for the rear, and a new one that monitors the input speed of the torque converter turbine. All three speed sensors are now located on the outside surface of the transmission case, reading the rotational speeds of internal components and improving transmission characteristics.
The speed sensor for the torque converter tur-bine allows the Transmission Control Module (TCM) to calculate the actual gear ratio in real time, by dividing the turbine speed by the output speed. To avoid shift shock and sluggish shifting, the rate of change of the actual gear ratio needs to be kept within a certain range. The TCM controls the duty ratios for the 2-4 brake, the high clutch, and the low clutch so that the rate of change occurs within the target range.
Externally, the number of bolts in the 4EAT Phase 2 torque converter housing area have increased to match the increase in the number of bolts in the bell housing of the engine.
Internally the Brake Band and Servo Mechanism have been deleted and in its place an additional clutch pack is used as a holding member for 2nd and 4th gears. Also, the remaining clutch assemblies and the valve body have been redesigned, requiring new disassembly and assembly procedures.
Because it is unlikely that you will be disassembling and servicing a Phase 2 transmission, this article will concentrate on transmission principles of operation and failure diagnosis. Detailed trans-mission disassembly and assembly information can be found in the service manual.
(Download the PDF to read more.)