Subaru 4EAT Transmissions

1 Subaru 4 EAT TransmissionsEnd WrenchThe4 Subaru 4 EAT TransmissionsOverviewFeatures and Major ComponentsPower FlowFinal Drive FWD/AWDE lectronic Control SystemHydraulic Control SystemSelf Diagnostic SystemTroubleshooting/Adjustmentshe 4 EAT Transmission is a 4 speed, microprocessor-controlled transmissionthat was first introduced in Subaru models. It is not a 3 speed trans-mission with overdrive. It features a lock-uptorque converter which locks up in all forward gearsexcept 1st. It has been offered in FWD or Full TimeAWD configurations. The 4 EAT transmission has undergone continuousdevelopment and improvement since its unique 4 EAT features have beenadded, changed, or deleted from variousSubaru models over the years.

2 We ll provideyou with a brief overview of these features here. For amore thorough description of specific 4 EAT operatingcharacteristics or features, refer to the applicableSubaru service manual for the year and model Subaruyou are working Subaru models were equipped with a 1-HOLD button. These include: XT and XT6 models 1988-89 L series 1990 LoyaleManual 1st gear is activated when the 1-HOLD button is depressed and the shifter is in manually selected 2 to provide enginebraking. The transmission will upshift through 2nd and 3rd if necessary, in order toprevent the engine from over-revving. The 1-HOLD indicator is displayed on thecombination meter when the button is activated.

3 When the computer overrides the1-HOLD gear selection the display will models have a POWER light located to the left of the tachometer (right onLegacy). It is activated momentarily whenever the vehicle is started. The POWER mode is selected by the computer based upon how quickly the gas pedal isdepressed. This changes the performance characteristics of the transmission. , itdelays upshifts and may downshift if necessary. When selected, the computer turnsthe POWER light ON. An enhanced version of the original 4 EAT was introduced with the 1990 modelyear Legacy. Although similar in design to the original 4 EAT, the shift quadrant is Legacy 4 EAT has a seven position quadrant: P-R-N-D-3-2-1.

4 The 1-HOLD button has been deleted and a manual button was added. Subaru vehiclesequipped with the manual button included: 1990-94 Legacy 1993-94 Impreza 1992-97 SVXWhen 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 transmissionstarts and stays in 2nd gear, but will upshift to 3rd gear at 6500 RPM to preventdamage to the engine. In 1st range manual the transmission stays in 1st gear andalso will upshift to second at 6500 RPM to prevent damage to the , on AWD vehicles, the TCU applies a more aggressive AWD mapwhen the selector is in the 1st position, manual switch ON or OFF.

5 These changes result in improved driveability on low friction road surfaces. The TCU monitors various engine and vehicle inputs, , throttle position andvehicle speed, etc. It also controls the electronic shift solenoids in the to the appropriate model year Subaru service manual, section 6-3, for thelocation of the QuadrantSelector LeverTCU 54 EAT TransmissionsTransfer Clutch Assembly (AWD)The transfer unit consists of a hydraulic multiplate clutch and a hydraulic control system incorporating a duty solenoid. It is housed in the extension case at the rear of the transmission. A caged needle bearing supports the clutch on thereduction drive shaft and a ball bearing supports the clutch in the solenoid C regulates the MPT clutch.

6 It is controlled by the TCU whichdetermines the degree of AWD by altering the duty ratio. As the duty ratio increases,the amount of AWD clutch itself, features friction discs that are designed to slip. This eliminatestorque binding during tight cornering. In order to get power to the front wheels,the reduction gear powers the reduction driven gear which is attached to the drivepinion the rear wheels, power goes from the reduction drive shaft to the MPTclutch hub which is welded to the drive gear. The power is transferred through theMPT clutch where it outputs to the rear drive shaft seal rings direct fluid from the hollow shaft to the lubricationcircuits inside the Control SystemOverviewThe electronic control system consists of various inputs (sensors) and outputs(lights and solenoids) in addition to the Transmission Control Unit (TCU).

7 This is the next generation of Subaru automatic transmission. In addition tobeing smoother and quieter, it is designed to help maximize fuel economy whileproviding monitors the engine and transmission performance conditions, the driversdemands and the vehicle Control UnitThe TCU is a highly sophisticated microprocessor with a self diagnostic longterm memory. It also has a fail safe function which maintains driveability in case of a major electrical component a transmission equipped for AWD the TCU utilizes a program which contin-ually changes the degree of AWD based upon vehicle operating condition(s).The TCU controls shifting and line pressure in addition to thelock-up torque converter and the MPT Inputs Throttle sensor/idle switch Vehicle speed sensor #1 Vehicle speed sensor #2 Tachometer signal Inhibitor switch Cruise control signal ATF temperature sensor Ignition/battery voltage 1-HOLD switch (if so equipped) Forced FWDTCU LocationMPT AssemblyReduction Shaft SealsMPT Power FlowMPT Clutch HubMPT Clutch11 Transmission Gear TrainThis compact unit features a double planetary gear set.

8 It has a wide ratiobetween 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 gearOperating Principals: Rear Gear SetThe rear sun gear is always powered by the input shaft. Therear planetary carrier always transmits power to the output one way clutch ( ) 3-4 prevents the rear internalgear from turning counterclockwise. Its inner race is the rearinternal gear and its outer race is the forward clutch hub. Theoverrunning clutch hub is also connected, by dogs, to the rearinternal overrunning clutch provides engine braking duringdeceleration except in D-1 and 3-1.

9 The 3-4 is used in 1st, 2nd, and 3rd gears. The forward clutch is used in allforward gears. The rear internal gear is controlled by the for-ward clutch through the 3-4. Additionally, the rearinternal gear is controlled by the overrunning forward clutch connects the rear internal gear to thefront planetary carrier (splined to the for-ward clutch drum) through the overrunning clutch is also used to connect the rear internal gear to the for-ward clutch drum and the front planetarycarrier. The 1-2 (Sprag) prevents theforwardclutch drum from rotatingcounterclockwise. The Sprag isapplied when the transmission isoperating in D-1 or Low/Reverse brake issplined to the case.

10 It holds the forward clutch drum in order to keep it from turning when the transmission is in Reverse, 2-1, and 1-HOLD (if so equppped).Operating Principles: Front Gear SetThe high clutch drum (reverse clutch hub) is splined to the input shaft. It supplies power to the reverse clutch and the high clutch. The high clutch hub issplined to the front planetary carrier. When the reverse clutch is applied forReverse gear it powers the front sun gear. When the high clutch is applied in 3rdand 4th gear it powers the front planetary carrier via the high clutch front sun gear is dogged to the reverse clutch drum. The front planetarycarrier is splined internally to the high clutch hub and it is splined externally to the forward clutch WrenchTheFront Sun GearFront Planetary CarrierRear Sun GearRear Planetary CarrierRear Internal GearOverrunning Clutch HubOne Way Clutch 1-2 Overrunning ClutchHub InstalledOne Way Clutch 3-4 Rear Internal GearReverse and High ClutchesFront Gear SetSubaru 4 EAT TransmissionsTransaxle Gear TrainRear Sun/Rear CarrierRear Gear SetForward Clutch DrumLow/Reverse Brake8 Band Servo OperationThe band is applied in 2nd and 4th gears by a two stage servo which is controlled by order to obtain second gear