Subaru 4EAT Transmissions - The Subaru SVX World Network
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1. Duty Solenoid B Subaru LEAT Transmissions Shift Modes Shift Solenoid 3 Shift solenoid 3 is located on the upper valve body It is used to control down shifts It quickly releases the 3 Release pressure during low speed heavy load situations in order to provide smooth 3 2 downshifts It operates the overrunning clutch in order to provide engine braking during deceleration It is also used to cancel the overrunning clutch momentarily during light throttle 3 2 downshifts or closed throttle 2 1 downshifts This reduces the shift shock Duty Solenoid A Duty Solenoid A is located on the upper valve body It regulates line pressure at three levels e Basic Altered with load vehicle speed and range signal e Shifting Lowers line pressure in between shifts to minimize shift shock e Start up With low ATF temperature or a low tach signal cranking speed it sets line pressure to a minimum Dropping Resistor The dropping resistor is wired in parallel with duty solenoid A and is used to regulate line pres sure It is located on the right front shock tower behind the MPI fuel system dropping resistor It takes over line pressure control during the OFF portion of the duty cycle for the duty solenoid In other words the duty solenoid is never fully OFF 3 E Dropping Resistor Duty Solenoid B Duty Solenoid B is located on the lower valve body next to the A2. D 1 D 3 3 3 Operating Mode The input shaft powers the rear sun gear clockwise When the forward clutch is applied the O W C 3 4 outer race forward clutch hub is attached to the front carrier The high clutch is also applied and the front planetary carrier is therefore powered In this case the planetary gear set is locked up and the output ratio is 1 1 During the coast mode the O W C 3 4 releases the rear internal gear and there is no engine braking Power Flow D 2 Power Flow D 3 D 4 Operating Mode The input shaft powers the rear sun gear clockwise When the forward clutch is applied it doesn t affect the power flow It free wheels through the O W C 3 4 When the high clutch is applied the front planetary carrier is powered clockwise Since the band is applied the front sun gear is held stationary The output is clockwise through the front internal gear rear planetary carrier and the output ratio is 0 694 1 Engine braking occurs during the coast and deceleration modes Neither Power Flow D 4 O W C is used Oil Pump Cover Seals Subaru LEAT Transmissions 2 1 and 1 HOLD Operating Modes The power flow is the same as in the other 1st gears except as follows The rear internal gear is held by the overrunning clutch instead of by the O W C 3 4 In addition the forward clutch drum is held by the Low Reverse brake instead of the O W C 1 2 Engine braking occurs during coast and
3. If the speed is less than approximately 40 MPH a time lag of up to 3 seconds will occur before resuming the normal shift pattern 2nd Gear Indicator The 2nd Gear Indicator is located with the shift position indicators It is controlled by the TCU Other quadrant indicators are controlled by the inhibitor switch The 2nd gear indicator is affected by the 1 HOLD switch input The TCU grounds the 2nd gear indicator light with the 1 HOLD switch OFF and the shift lever in 2 except Legacy 1 HOLD Indicator Light if so equipped The 1 HOLD indicator light L Loyal and XT only is located near the combination meter shift position indicator It is activated by the TCU whenever the 1 HOLD button is depressed with the shift lever in 2 It changes the display and cancels the 2 indicator The vehicle stays in 1 HOLD unless the TCU determines a potential engine over rev condition at which point the transmission will then upshift NOTE WHEN 1 HOLD IS ACTIVATED ALL OTHER SHIFTING INPUTS ARE IGNORED Beginning in the 1990 model year Legacy the 1 HOLD button and light have been deleted and a MANUAL switch and light added When Pole neuen EEG the transmission is in 3rd range MANUAL switch ON it locks out 1st gear and shifts 2 3 When in 2nd range MANUAL the transmission locks out 1st gear and is held in 2nd gear until 6500 RPM when it will upshift to prevent engine damage When in 1st range MANUAL t
4. a VARUR Transaxle Gear Train Rear Gear Set FORWAAD CLUTCH GALI G i i CTEAARLMAIAS CLUToH Te Low Reverse Brake Overrunning Clutch Hub Installed Subaru LEAT Transmissions 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 i Front Sun 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 Ist 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 planeta
5. body Next loosen the 3 retaining bolts and rotate the inhibitor to adjust Finally reinstall the cable and reconfirm the cable adjustment Inhibitor Switch Adjustment Stall Test The stall test checks the operating condition of the AT clutches the torque converter and the engine Perform these checks in D 3 2 and Reverse Stall Test Results Higher than normal RPM indicates one or more of the following e Slippage of the forward clutch e O W C not holding e Low Reverse brake slipping e Overall low line pressure Lower than normal RPM indicates one or more of the following e Incorrect throttle adjustment e Poor engine operation e Torque converter stator slippage SUBARLI Time Lag Test The time lag test checks the operation of the forward clutch the reverse clutch the low Reverse brake O W C 3 4 and O W C 1 2 Perform this test at operating temperature Idle the engine with the A C OFF Confirm the proper idle speed in N and correct if necessary Then shift into D and measure the time seconds to full engagement It should take less than 1 2 seconds Then shift into R and measure the time It should take less than 1 5 seconds Time Lag Results If the time takes longer from N to D it may indicate one or more of the following e Low line pressure e Worn forward clutch e O W C problem If the time takes longer from N to R it ma
6. description of the user mode see the appropriate troubleshooting section of the service manual wN NOTE THE SYSTEM WILL FAIL SAFE FOR LIMP Shift Quadrant HOME WHEN A COMPONENT FAILS THE LIGHT WILL NOT LLUMINATE UNTIL THE IGNITION IS SWITCHED OFF AND THEN SWITCHED TO ON AGAIN J ek For specific information on the self diagnostic dealer modes see the appropriate troubleshooting section of the service manual The Select Monitor requires the specific 4EAT cartridge to per form the analysis This form of analysis is the preferred troubleshooting self diagnostic method Code Interpretation The Select Monitor identifies current problems past problems through long term memory and indicates actual circuit and component performance It also allows data to be observed under actual driving conditions which is of great assistance in the diagnosis of intermittent problems The Select Monitor has four major modes Function modes 15 These modes indicate sensor and component performance data and are displayed on the LED screen Example Mode F 01 F 01 12 6 volts Battery voltage Select M onitor A modes 3 These modes indicate the operation of 13 different switch inputs which are displayed by the red numbered LEDs LED ON switch ON LED OFF switch OFF Functions of the red LED s are identified by letter code on the upper screen B modes 2 These
7. two modes display the current TCU self diagnostic codes and the past TCU self diagnostic codes long term memory NOTE WHEN USING SELECT MONITOR IN THE B MODES IT IS NOT NECESSARY TO PERFORM THE NORMAL SELF DIAGNOSTIC ACCESS PROCEDURES C mode 1 This clears the trouble codes that are stored in long term memory NOTE REFER TO THE APPROPRIATE SERVICE MANUAL SECTION 3 2 AND REVIEW SELECT MONITOR USAGE AND TROUBLESHOOTING PROCEDURES Subaru 4EAT Transmissions Troubleshooting Adjustments Preliminary Inspection Check the following Fluid level ATF leaks Road Test e Check proper shift points e Engine performance Correctly adjusted throttle sensor Gearshift cable adjustment Correct stall test results Inhibitor switch connections Correct pressure test results Gearshift Cable Adjustment Place the transmission in neutral with the engine OFF Then loosen the locking nut and the adjusting nut Push the shift lever arm rearward and tighten the adjusting nut until it contacts the connecting block Finally secure the cable with the locking nut and double check the operation After adjusting the gearshift cable verify the correct inhibitor switch position Remove the cable from the inhibitor switch in order to perform the adjustment The switch must be in neutral Then insert the special tool 499267300 Stopper Pin through the 2 levers of the mies switch into the depression in the switch
8. HOLD switch if so equipped e Forced FWD yi 1 i LL A a VARUR Throttle Sensor iE PL Combination M eter Inhibitor Switch Subaru EAT Transmissions Legacy TCU Inputs The Legacy fuel system ECM beginning with model year 1990 sends new inputs to the TCU for line pressure control It networks the MPF1 ECU RPM and altitude compensation inputs This provides additional line pressure control for high altitude compensation to reduce shift shock ABS system inputs turn OFF the over running clutch when ABS is active and fixes the duty ratio of the MPT clutch mostly FWD e ECM RPM e Altitude compensation e ABS system Throttle Sensor The throttle sensor idle switch is basically electrical throttle pressure The load signal effects shifting line pressure and lock up The closed throttle input effects the lock up release mode as well as smooth downshifting into 2nd gear It also causes a reduction in line pressure Vehicle Speed Sensor 1 Vehicle speed sensor 1 is mounted to the transmission and is basically electrical governor pressure It is used to detect vehicle speed and it effects shift points lock up and line pressure In FWD transmissions the speed sensor reads parking gear rotation at the front output shaft In AWD transmissions it senses the transfer clutch drum rotation at the rear output shaft Vehicle Speed Sensor 2 Vehicle speed sensor 2 is built into the c
9. Subaru LEAT Transmissions Overview Features and Major Components Power Flow Final Drive FWD AWD Electronic Control System Hydraulic Control System Self Diagnostic System Troubleshooting Adjustments SUBARU 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 F o except 1st It has been offered in FWD or Full Time E 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 We ll 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 e 1987 5 91 XT and XT6 models e 1988 89 L series e 1990 Loyale Shift Quadrant 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 t
10. TF temperature sensor It operates the lock up clutch in 3 modes ON OFF and a gradual ON OFF control of the lock up clutch during gear shifting in order to reduce shift shock Duty Solenoid C aa Duty Solenoid C is located in the extension Duty Solenoid C housing It is also controlled by the TCU It varies the degree of AWD Power Indicator Light The POWER Indicator Light is located in the combination meter It is activated momentarily whenever the vehicle is started as part of the self check system It turns ON when the TCU selects the POWER mode with the vehicle in the D range It blinks a self diagnostic code whenever the TCU is programmed for self diagnosis The TCU monitors the throttle angle opening speed i e Combination Meter how quickly the gas pedal is depressed When a predeter mined rate is exceeded the POWER mode is activated NOTE THERE ARE A NUMBER OF PREDETERMINED RATES BASED ON THE VEHICLE SPEED VS THROTTLE ANGLE RELA TIONSHIP THESE DETERMINE EASE OF ACCESS TO POWER MODE AS A GENERAL RULE IT IS EASIER TO ACTIVATE THE POWER MODE AT LOWER SPEEDS FROM A LIGHT THROTTLE THAN IT IS AT HIGHER SPEEDS FROM A LIGHT THROTTLE SUBARU The POWER mode increases the upshift and downshift points It is deactivated by the vehicle speed and the throttle angle i e if the speed is equal to or greater than approximately 40 MPH with a light throttle deactivation is immediate
11. ated when a circuit is activated This ensures smooth component application The accumulator resistance will vary in direct proportion to the line pressure Accumulators normally operate at a fixed rate in other automatic transmissions Therefore as the transmission pressures rise the accumulator cannot further compensate due to the constant value of the spring Pressure shocks are thus transferred to the components In the 4EAT Accumulators however the line pressure is applied to the back of the accumulator piston Therefore the resistance to pressure is proportionally increased hydraulically This keeps the pressure shock under control allowing smoother component application An additional accumulator is located in the lower section of the valve body next to the manual valve It absorbs line pressure pulses created by the sudden changes in line pressure NOTE THE 4EAT IS A HIGHLY RELIABLE TRANSMISSION SHOULD AN ELECTRICAL COMPONENT MALFUNCTION HOWEVER IT WILL ALWAYS FAIL SAFE FOR LIMP HOME Fail Safe Components And Failure Results If a speed sensor fails the remaining sensor signal will be used In case of throttle sensor failure the idle contacts will signal the throttle opening Line pressure will go to maximum at open throttle and it will go to minimum at closed throttle Although the inhibitor switch may fail the manual valve will still be in the correct position for all selected ranges In P and N howeve
12. deceleration due to tres conditions described above The overrunning clutch is ON and MeS the Low Reverse brake is ON Power Flow 1 Hold POWER FLOW 1 HOLB i b WERRAUNMING CLUTCH Reverse Operating Mode When the reverse clutch is applied the front sun gear is powered The Low Reverse brake is also applied which holds the front planetary carrier stationary Output power is through the rear carrier at the ratio of 2 272 1 Power Flow Reverse Gear Train Components Front sun gear Front planetary carrier Rear sun gear Rear planetary carrier front internal gear Rear internal gear Final Drive FWD AWD Differential Overview The hypoid gear set is mounted in the aluminum torque converter case It is supported by taper roller bearings and features removable stub axle shafts The pinion is mounted through the oil pump housing Oil Pump Housing Features The housing is made of cast iron for greater rigidity Double taper roller bearings are used to support the pinion This allows for the thermal expansion of two dissimilar metals aluminum and cast iron These bearings also improve the durability and reliability of the unit The pinion is preloaded by a locknut which allows for easy serviceability Pinion depth is set by shims which are located between the bearing flange and the oil pump housing A double lip oil seal separates transmission fluid from the hypoid gear oil This ereatly improves th
13. e fluid system reliability Differential Carrier Features The ring gear is mounted on the right side of the carrier This design adds to its compactness and makes it easier to service The backlash is easily adjustable through the carrier bearing retainers Final drive ratios have gone through several changes in the years since the 4EAT was originally introduced on 1987 5 models For accurate final drive ratio information refer to Differential Carrier the applicable Subaru service manual MPT Assembly MPT Clutch MPT Clutch Hub SUBARUS Reduction Shaft Seals MPT Power Flow Transfer 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 the reduction drive shaft and a ball bearing supports the clutch in the case Duty solenoid C regulates the MPT clutch It is controlled by the TCU which determines the degree of AWD by altering the duty ratio As the duty ratio increases the amount of AWD decreases The clutch itself features friction discs that are designed to slip This eliminates torque 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 drive pinion shaft For the rear wheels power goes from the reduction d
14. electronic AWD engagement system MUST be disengaged by installing the spare fuse 15A of the fuse box into the FWD connector located under the hood Failure to do so could result in movement of vehicle Refer to owner s manual a Forced FWD Switch TCU Outputs There are two types of outputs solenoid controls and light controls The solenoids control shifting line pressure lock up and AWD The light controls indicate operating conditions to the driver They indicate the POWER mode manually selected 1st or 2nd gear or hot ATF AWD only On the 1990 model year and later Legacy models the light controls indicate hot ATF FWD and AWD gears 3 2 1 MANUAL mode and POWER mode Shift solenoids 1 and 2 are located on the upper valve body The TCU induces ON OFF conditions which regulate the shifting of the forward gears When a shift solenoid is ON it passes pilot pressure to shift valve A and or shift valve B The valve s will then shift feeding the appropriate controlling member circuits high clutch band etc NOTE PILOT PRESSURE IS NOTHING MORE THAN A PRESSURE HELD AT A CONSTANT VALUE When a shift solenoid is OFF the affected shift valve will move to its static position due to spring pressure The appropriate controlling member circuit will then be fed high clutch band etc Shift Solenoids land 2 WAG Lol OWC Bo 1 eat Hine EST Shift Solenoid 3
15. he 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 e 1990 94 Legacy e 1993 94 Impreza e 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 cha
16. he transmission is locked in 1st gear until 6500 RPM when it will upshift to prevent engine damage NOTE TCU WILL ALWAYS OVERRIDE THE SELECTOR POSITION AT 6500 RPM ATF Temperature Warning Lamp An ATF Temperature Warning Lamp is provided for L Series and XT AWD only and for the Legacy FWD and AWD The lamp is located on the combination meter It is activated by the TCU to indicate overheated ATF fluid The TCU will shift the transmission as if in the POWER mode to help reduce the ATF temperature Hydraulic Control System Valve Body The valves and solenoids control the lubrication circuits the lock up torque converter shifting etc The valve body is divided into two major sections upper and lower The valve body works in conjunction with the TCU It is designed to provide smooth shift control and component longevity It also reduce unnecessary high pressure in certain instances As an example line pressure is lowered between shifts The valve body features shift step control This means that gear members are momentarily applied between shifts it allows them to be brought up to speed which reduces shock Lower Valve Body Accumulator Operation Subaru 4EAT Transmissions Accumulators There are four accumulators mounted in the transmission case e 4 Apply 4A e 2 Apply 2A e 3 Release 3R e Neutral Drive ND They are designed to lessen shift shock by absorbing the sudden pressure change gener
17. is more sophisticated than the KDLH system and less objectionable for the consumer ATF Temperature Sensor When the ATF is hot AWD only the TCU shifts the transmission as if in the POWER mode This pushes the shift points higher which allows the engine to run faster The oil pump then circulates ATF through the oil cooler more quickly so as not to overheat the engine coolant System Voltage The TCU also monitors system voltage in order to correctly interpret the inputs and alter the control of the outputs For example the system is designed for 12 volt operation When running however most vehicles have other than 12 volts available The 1 HOLD switch is located aft of the shift quadrant When activated it creates a forced 1st gear NOTE THE TCU WILL SHIFT 2ND TO 3RD IF NECESSARY IN ORDER TO PREVENT THE ENGINE FROM OVER REVVING Forced FWD Switch The FWD switch changes the driving mode from AWD to FWD and should be used for diagnostic purposes only Do not drive the vehicle in this mode The FWD switch is located on the left front shock tower The Legacy FWD switch is located on the right strut tower It is activated by inserting the spare fuse into the under hood connector The FWD light on the combination meter verifies that the vehicle is in FWD Shift Quadrant Maintenance Precautions Before jacking up one or two wheels for maintenance with the engine running or before running the vehicle on a chassis dynomometer the
18. m the 2A accumulator pushes the 1 2 piston UPWARD which tightens the band For third gear the servo is in the 3 Release mode In this case hydraulic pressure from the 3R accumulator aided by the return spring pushes the 1 2 piston DOWNWARD in order to release the band For fourth gear 4 Apply mode hydraulic pressure from the 4A accumulator pushes the 3 4 piston UPWARD in order to apply the band Band and Servo Servo Operation later 4EAT models Power Flow D 1 or 3 1 Operating Mode The input shaft powers the rear sun gear clockwise When the Forward clutch is applied the O W C 3 4 outer race forward clutch hub is attached to the front carrier With the O W C 3 4 operating the rear internal gear can only go clockwise With the O W C 1 2 operating the forward clutch drum can only go clockwise Output power is through the rear carrier and the output ratio is 2 85 1 During coast mode the O W C 3 4 is released and there is no engine braking D 2 3 2 Or 2 2 Operating Mode Again the input shaft powers the rear sun gear clockwise When the forward clutch is applied the O W C 3 4 outer race forward clutch hub is attached to the front carrier The band is also applied which holds the front sun gear stationary Output power is transmitted from the rear carrier front inter nal gear where the output ratio is 1 545 1 During the coast mode the O W C 3 4 is released and there is no engine braking Power Flow
19. nges 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 l he 4EAT Transmission is a 4 speed microprocessor controlled transmission Selector Lever MPT Assembly MPT Clutch MPT Clutch Hub SUBARUS Reduction Shaft Seals MPT Power Flow Transfer 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 the reduction drive shaft and a ball bearing supports the clutch in the case Duty solenoid C regulates the MPT clutch It is controlled by the TCU which determines the degree of AWD by altering the duty ratio As the duty ratio increases the amount of AWD decreases The clutch itself features friction discs that are designed to slip This eliminates torque 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 drive pinion shaft For the rear wheels power goes from the reduction drive shaft to the MPT clutch hub which is welded to the drive gea
20. ombination meter In FWD units it is used as a back up for speed sensor 1 In AWD units it is used as the front output shaft speed sensor TCU The TCU compares the speed signal from the front output shaft with the signal from the rear output shaft sensor 1 The speed differential helps the TCU determine the degree of AWD along with other inputs Tachometer The tachometer signal effects the shift points at kickdown The TCU uses the signal to prevent the engine from over revving Inhibitor Switch The inhibitor switch is located on the right side of the transmission It is operated by the gear shift lever which controls the linkage that operates the manual valve It signals the TCU of the selected gear range which will then control the shift points and adjusts the line pressure accordingly NOTE THE TCU WILL OVERRIDE THE INHIBITOR SWITCH IF NECESSARY IN ORDER TO PREVENT THE ENGINE FROM OVER REVVING Cruise Control Signal The cruise control signal tells the TCU of cruise control activation This allows for a wider operating range in 4th gear unless a large speed differential exists from the set speed in which case the transmission may downshift This improves fuel economy The ATF temperature sensor is located on the lower valve body next to duty solenoid B Cruise Control SUBARLI When the ATF is cold the TCU won t allow an upshift into 4th gear The object is to warm the engine quickly for lower emissions It
21. r The power is transferred through the MPT clutch where it outputs to the rear drive shaft Reduction shaft seal rings direct fluid from the hollow shaft to the lubrication circuits inside the transmission Electronic Control System Overview The electronic control system consists of various inputs sensors and outputs lights and solenoids in addition to the Transmission Control Unit TCU This is the next generation of Subaru automatic transmission In addition to being smoother and quieter it is designed to help maximize fuel economy while providing performance It monitors the engine and transmission performance conditions the drivers demands and the vehicle speed Transmission Control Unit The TCU is a highly sophisticated microprocessor with a self diagnostic long term memory It also has a fail safe function which maintains driveability in case of a major electrical component failure In 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 the lock up torque converter and the MPT clutch TCU Inputs e Throttle sensor idle switch e Vehicle speed sensor 1 e Vehicle speed sensor 2 E A e Tachometer signal i e Inhibitor switch e Cruise control signal e ATF temperature sensor e Ignition battery voltage e 1 HOLD switch if so equipped e Forced FWD yi 1 i LL A
22. r it may effect start up therefore there is a potential for a no start condition In Reverse the TCU is passive Therefore an inhibitor switch failure has no effect If multiple signals are seen in the forward ranges the inhibitor switch is ignored and there is no fourth gear If the 1 HOLD switch is defective the system operates in the same manner as an inhibitor switch failure in the forward ranges If the MANUAL switch is defective Legacy only the transmission will shift normally in D position It will operate the same as when there is an inhibitor switch failure when the transmission is in selector position 3 2 1 If shift solenoids 1 or 2 malfunction the TCU deactivates the other This results in either 3rd gear or Reverse when selected If duty solenoid A fails line pressure goes to maximum If duty solenoid B fails the torque converter lock up will not occur If shift solenoid 3 malfunctions the overrunning clutch is always ON and there will be engine braking during deceleration If duty solenoid C should fail the AWD control will be set to maximum and the rear wheels will always be powered SUBARL Self Diagnostic System The 4EAT self diagnostic system has three modes a user mode and two dealer modes In the first instance the user is notified through the POWER light when a malfunction occurs The failure is communicated after the next igni tion OFF ON cycle For a more detailed
23. rive shaft to the MPT clutch hub which is welded to the drive gear The power is transferred through the MPT clutch where it outputs to the rear drive shaft Reduction shaft seal rings direct fluid from the hollow shaft to the lubrication circuits inside the transmission Electronic Control System Overview The electronic control system consists of various inputs sensors and outputs lights and solenoids in addition to the Transmission Control Unit TCU This is the next generation of Subaru automatic transmission In addition to being smoother and quieter it is designed to help maximize fuel economy while providing performance It monitors the engine and transmission performance conditions the drivers demands and the vehicle speed Transmission Control Unit The TCU is a highly sophisticated microprocessor with a self diagnostic long term memory It also has a fail safe function which maintains driveability in case of a major electrical component failure In 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 the lock up torque converter and the MPT clutch TCU Inputs e Throttle sensor idle switch e Vehicle speed sensor 1 e Vehicle speed sensor 2 E A e Tachometer signal i e Inhibitor switch e Cruise control signal e ATF temperature sensor e Ignition battery voltage e 1
24. ry carrier splined to the for ward clutch drum through the O W C 3 4 The overrunning clutch is also used to connect the rear internal gear to the for ward clutch drum and the front planetary carrier The O W C 1 2 Sprag prevents the forward clutch drum from rotating counterclockwise The Sprag is applied when the transmission is operating in D 1 or 3 1 The Low Reverse brake is splined to the case 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 One Way Clutch 1 2 Rear Internal Gear Operating Principles Front Gear Set The 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 is splined to the front planetary carrier When the reverse clutch is applied for Reverse gear it powers the front sun gear When the high clutch is applied in 3rd and 4th gear it powers the front planetary carrier via the high clutch hub The front sun gear is dogged to the reverse clutch drum The front planetary carrier is splined internally to the high clutch hub and it is splined externally to the forward clutch drum SUBARLI Band Servo Operation The band is applied in 2nd and 4th gears by a two stage servo which is controlled by accumulators In order to obtain second gear the servo is in the 2 Apply mode Hydraulic pressure fro
25. y indicate one or more of the following e Low line pressure e Worn reverse clutch e Worn low rev brake MPT Test Port Pressure Test This is a preliminary test which should be conducted prior to any disassembly Perform a pressure test when all the circuits show evidence of slippage or when the circuits show negative results from the time lag test This test should also be performed if there is excessive shift shock delayed shifting or if the vehicle is immobile Perform this test by connecting the pressure gauge to the oil pump outlet test port in order to determine the overall line pressure Should a particular component be suspected perform pressure tests at its unique test port Refer to the Service Manual Sec 3 2 for other ports Check for minimum and maximum values at each port On Car Service Adjustments The following can be performed on the vehicle e Checking changing fluids e Band adjustment e Valve body servicing e Shift linkage adjustment replacement e Inhibitor switch adjustment replacement e Harness repair replacement e Transfer clutch assembly servicing replacement e Speed sensors replacement Fluids inhibitor Switch Band Wire Harness Valve Body MPT Assembly Linkage Speed Sensor 19
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