ystem
Hydraulic Control System
Self Diagnostic System
Troubleshooting/Adjustments he 4EAT Transmission is a 4 speed, microprocessor-controlled transmission
that was first introduced in 1987.5 Subaru models. It is not a 3 speed trans-
mission with overdrive. It features a lock-up
torque converter which locks up in all forward gears
except 1st. It has been offered in FWD or Full Time
AWD configurations.
The 4EAT transmission has undergone continuous
development and improvement since its introduction.
Several unique 4EAT features have been
added, changed, or deleted from various
Subaru models over the years. Well provide
you with a brief overview of these features here. For a
more thorough description of specific 4EAT operating
characteristics or features, refer to the applicable
Subaru service manual for the year and model Subaru
you are working on.
Some Subaru models were equipped with a
1-HOLD button. These include:
1987.5-91 XT and XT6 models
1988-89 L series
1990 Loyale
Manual 1st gear is activated when the 1-HOLD
button is depressed and the shifter is in manually selected 2 to provide engine
braking. The transmission will upshift through 2nd and 3rd if necessary, in order to
prevent the engine from over-revving. The 1-HOLD indicator is displayed on the
combination meter when the button is activated. When the computer overrides the
1-HOLD gear selection the display will change.
Some models have a POWER light located to the left of the tachometer (right on
Legacy). It is activated momentarily whenever the vehicle is started. The POWER
mode is selected by the computer based upon how quickly the gas pedal is
depressed. This changes the performance characteristics of the transmission. i.e., it
delays upshifts and may downshift if necessary. When selected, the computer turns
the POWER light ON.
An enhanced version of the original 4EAT was introduced with the 1990 model
year Legacy. Although similar in design to the original 4EAT, the shift quadrant is dif-
ferent. The Legacy 4EAT has a seven position quadrant: P-R-N-D-3-2-1. The 1-
HOLD button has been deleted and a manual button was added. Subaru vehicles
equipped with the manual button included:
1990-94 Legacy
1993-94 Impreza
1992-97 SVX
When the selector is in 3rd range with the manual switch ON, the transmis-
sion will start in 2nd gear and shift to 3rd. In 2nd range manual, the transmission
starts and stays in 2nd gear, but will upshift to 3rd gear at 6500 RPM to prevent
damage to the engine. In 1st range manual the transmission stays in 1st gear and
also will upshift to second at 6500 RPM to prevent damage to the engine.
Additionally, on AWD vehicles, the TCU applies a more aggressive AWD map
when the selector is in the 1st position, manual switch ON or OFF.
These changes result in improved driveability on low friction road surfaces.
The TCU monitors various engine and vehicle inputs, i.e., throttle position and
vehicle speed, etc. It also controls the electronic shift solenoids in the transmission.
Refer to the appropriate model year Subaru service manual, section 6-3, for the
location of the TCU.
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T
O v e r v i e w
Shift Quadrant
Selector Lever
TCU Features and Major Components
The 4EAT features a double planetary gear set, a lock-up torque converter, and
a variable displacement oil pump. The AWD system includes a Multi-Plate
Transfer Clutch (MPT).
The electronic control system is designed to reduce shock during shifting,
improve driving performance, and improve fuel economy. A self diagnostic system
is incorporated in order to improve serviceability and reliability.
The shifter is quieter because of the compact push/pull cable design.
The electronically controlled Multi-Plate Transfer (MPT) System provides for
controlled transfer clutch torque. It is designed to slip in order to eliminate torque
bind on cornering.
This MPT System is equipped with a transfer hydraulic control system, including
a duty solenoid and multi-plate clutch. This assembly is located in the transfer case
on the rear of the transmission.
On later 4EAT models, the transfer apply piston has been
modified and a transfer piston seal has been added to improve
the torque split control. This prevents the MPT clutch from
further applying during high speed driving.
The modified MPT clutch assembly cancels the centrifugal
pressure build-up effect which occurs behind the clutch
apply piston. It uses the lubricating pressure between the
piston seal and the apply piston to ensure a positive release
of the MPT clutch.
Lock-Up Torque Converter
The torque converter is designed to
match a wide range of engines from large
to small displacement. It is also designed
to improve acceleration from a stop and
reduce fuel consumption.
The torque converter has an electroni-
cally controlled, hydraulic lock-up clutch
system that prevents slip loss during
medium to high speed operation. This
system replaces the previous centrifugal
lock-up type clutch.
There is a friction surface on the back
of the lock-up clutch (piston) which locks
against the back of the impeller housing.
Clutch engagement shock is minimized in part, because of the torsional clutch
dampers and the wave spring/friction washer combination.
The lock-up operation is controlled by the TCU which then regulates Duty
Solenoid B mounted on the lower valve body. This solenoid controls the lock-up
valve located in the transmission upper valve body. Finally, the lock-up valve
activates the lock-up clutch (piston) located in the torque converter.
Operating Modes
The TCU regulates the cycle of Duty Solenoid B. When the duty solenoid
operates at five percent duty, i.e., substantially more OFF than ON, pilot
pressure is directed to the lock-up control valve.
In this condition, the control valve is pushed UPWARD by the combined pilot
pressure and spring force. This allows regulated hydraulic pressure to enter the
lock-up release circuit.
Th
e
6
MPT Clutch
Piston and Cover
Lock-up Clutch
Duty Solenoid B
Lock-up Valve
Torque Converter Cut-away
Shift Cable
MPT Assembly
Subaru 4EAT Transmissions
Torque Converter Components
Torque Converter
End W
r
ench The release pressure then pushes the lock-up clutch (piston) rearward and the
lock-up clutch is released from the impeller cover. On the other hand, oil drains
through the apply circuit to the oil cooler in the radiator.
When Duty Solenoid B operates at 95 percent duty, i.e., substantially more
ON than OFF, it reduces pilot pressure to the control valve.
In this condition, the control valve is pushed DOWNWARD due to the reduced
pilot pressure. As a result, regulated hydraulic pressure is directed to the lock-up
apply circuit and the release circuit drains.
The apply pressure then pushes the lock-up clutch (piston) forward which
engages the lock-up clutch with the impeller cover. When engaged, the transmis-
sion is coupled directly to the engine.
Oil Pump Assembly
A variable rate vane-type pump is used for optimum flow rate control with
minimum energy loss. In addition to pressurizing the oil, the pump provides
lubricating oil for the torque converter, the valves, the clutches, the brake
and the band.
The pump consists of the following components: Rotor Vanes Control Piston Vane Rings Cam Ring Return Spring Seal Ring Oil Pump Cover
NOTE: THE ROTOR, VANES, CAM
RING AND CONTROL PISTON ARE
ALL SELECTIVE.
The pump rate is variable because
of the cam ring eccentricity. The
eccentricity is adjusted automatically
corresponding to pressure from the
regulating valve acting upon the
control piston.
During low speed operation,
filtered ATF is drawn into the pump
suction port. The pump is driven
directly at engine speed and the ATF
is then compressed by the rotor vanes
and discharged through the delivery
port in the oil pump cover. The pres-
surized ATF then flows to the rest of
the transmission case.
During high speed operation, as the engine speed increases, the delivery rate
normally increases. However, feedback pressure generated from the regulator valve
is applied to the control piston which pushes down the cam ring. This changes the
relationship between the cam ring and the rotor. In this way, the pump delivery
rate remains constant at a preset value.
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Oil Pump Assembly
Oil Pump Close-up
Oil Pump High Speed
Oil Pump Low Speed
Oil Pump Components Transmission Gear Train
This compact unit features a double planetary gear set. It has a wide ratio
between gears for improved fuel efficiency as well as high performance.
Gear Train Components Front sun gear Front planetary carrier Rear sun gear Rear planetary carrier (front internal gear) Rear internal gear
Operating Principals: Rear Gear Set
The rear sun gear is always powered by the input shaft. The
rear planetary carrier always transmits power to the output shaft.
The one way clutch (O.W.C.) 3-4 prevents the rear internal
gear from turning counterclockwise. Its inner race is the rear
internal gear and its outer race is the forward clutch hub. The
overrunning clutch hub is also connected, by dogs, to the rear
internal gear.
The overrunning clutch provides engine braking during
deceleration except in D-1 and 3-1. The O.W.C. 3-4 is used
in 1st, 2nd, and 3rd gears. The forward clutch is used in all
forward gears. The rear internal gear is controlled by the for-
ward clutch through the O.W.C. 3-4. Additionally, the rear
internal gear is controlled by the overrunning clutch.
The forward clutch connects the rear internal gear to the
front planetary carrier (splined to the for-
ward clutch drum) through the O.W.C. 3-4.
The overr