Service Manuals, User Guides, Schematic Diagrams or docs for : . Car Manuals Subaru Impreza 2001-2002.2004-2007 Approved Subaru Impreza 2004 WRX STi 08. MECHANISM AND FUNCTION 33. AUTOMATIC TRANSMISSION 11. AWD Transfer System
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W1860BE.book Page 54 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
11.AWD Transfer System
A: MPT MODELS
1. GENERAL
This all-wheel-drive (AWD) transfer system uses an electronically controlled multi-plate type trans-
fer clutch. The clutch is controlled by the TCM through the transfer hydraulic pressure control unit
which consists of a duty-cycle-controlled solenoid valve and is located at the rear of the automatic
transmission section together with the vehicle speed sensor.
The TCM has in its memory a set of duty ratio data, each defining at what ratio the transfer clutch
should transmit the torque for a particular driving condition. Based on the driving condition informa-
tion it receives from the corresponding sensors (vehicle speed, throttle opening, gear range, slip of
wheels, etc.), the TCM selects an appropriate duty ratio from the memory and uses it to control the
solenoid valve. The solenoid valve then regulates the pilot pressure of the transfer control valve
which creates the pressure to the clutch from the line pressure. The clutch is engaged to a degree
determined by the transfer clutch pressure thus created. Through this process, the torque from the
engine is distributed to the rear wheels optimally according to driving conditions.
4AT-54
W1860BE.book Page 55 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
(14)
(13)
(12)
(11)
(10)
(9)
(8)
(7) (21)
(6) (20)
(5)
(6)
(5)
(4)
(3)
(2) (22) (19)
(18)
(1)
(23)
(24)
(25) (17)
(16)
(15)
(26)
AT-00500
(1) Thrust bearing (10) Return spring (19) Plug
(2) Needle bearing (11) Transfer clutch piston (20) Oil seal
(3) Snap ring (12) Rear drive shaft (21) Dust cover
(4) Pressure plate (13) Ball bearing (22) Transfer clutch seal
(5) Drive plate (14) Seal ring (23) Transfer control valve
(6) Driven plate (15) Gasket (24) Transfer valve plate
(7) Pressure plate (16) Transfer clutch pipe (25) Transfer duty solenoid
(8) Snap ring (17) Extension case (26) Inlet filter
(9) Transfer piston seal (18) O-ring
4AT-55
W1860BE.book Page 56 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
2. CONSTRUCTION
Transfer clutch
The transfer clutch drum and rear drive shaft are joined to each other by welding. The rear drive
shaft has drilled oil passages for transfer clutch control and also for lubrication of extension bushing
and ball bearing in it.
(7)
(6)
(5)
(6)
(5)
(4)
(3)
(2)
(1) (14)
(13)
(12)
(11)
(10)
(8) (9) AT-00501
(1) Thrust bearing (6) Driven plate (11) Transfer clutch piston
(2) Needle bearing (7) Pressure plate (12) Rear drive shaft
(3) Snap ring (8) Snap ring (13) Ball bearing
(4) Pressure plate (9) Transfer piston seal (14) Seal ring
(5) Drive plate (10) Return spring
4AT-56
W1860BE.book Page 57 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
Transfer hydraulic pressure control unit
The transfer hydraulic pressure control unit is bolted at the rear end of transmission case through
the transfer valve plate.
The hydraulic pressures used for the transfer hydraulic pressure control unit (line pressure and pilot
pressure) are supplied from the transmission's hydraulic control valve assembly through the pas-
sages formed in the transmission case.
The transfer duty solenoid adjusts the pilot pressure of the transfer clutch valve depending on the
signals from the TCM. The transfer clutch valve in turn modulates the line pressure into the transfer
clutch pressure before it is applied to the clutch piston.
The transfer clutch pressure adjusted in this way engages the clutch to different degrees according
to driving conditions so that the optimum torque is distributed to the rear wheels.
(2) (3)
(1)
X (4)
(5)
(7)
(8)
(6)
(9) AT-00502
(1) Transfer control valve (4) Transfer pressure (7) Line pressure
(2) Transfer clutch pressure (5) Filter (8) Oil pump
(3) Transfer duty solenoid (6) Pilot pressure (9) Transmission hydraulic control
valve assembly
4AT-57
W1860BE.book Page 58 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
B: VTD MODELS
1. GENERAL
Used in the transfer of the VTD models is the SUBARU drive power distribution system which com-
bines a compound planetary gear type center differential installed in the transfer case behind the
transmission and a hydraulically operated multi-plate differential action limiting device (LSD) located
between the output components of the center differential. Differential action limiting control is per-
formed by the TCM according to driving and road surface conditions. This system allows combining
stability provided by the AWD design with good operability.
The input torque is transmitted to the 1st sun gear of the center differential through the intermediate
shaft. From the 1st sun gear, the torque is transmitted through the 1st pinion to the output carrier in
the front wheel output components, and through the 2nd pinion to the 2nd sun gear in the rear wheel
output components.
The center differential performs the differential functions of absorbing the speed difference between
the front and rear wheels and also distributes drive forces to the front and rear wheels at a prede-
termined ratio. In normal conditions (when there is almost no difference in the speed between the
front and rear wheels), the drive force distribution ratio is 45.5 % to the front wheels and 54.5 % to
the rear wheels. The hydraulic multi-plate clutch connected in parallel with the center differential be-
tween the carrier and 2nd sun gear functions as a differential action limiting device (LSD) and also
as a device that controls torque distribution according to driving conditions.
The differential action limiting control is based on the parameters that include the throttle angle, en-
gine speed, vehicle speed, and speed ratio of front and rear wheels. The LSD clutch piston is oper-
ated by the fluid whose pressure is adjusted by the duty solenoid and the transfer control valve in
the transfer case. According to the pressure applied to the piston, the torque distribution ratio chang-
es from the ratio set for the center differential to the direct AWD ratio.
The speed of the front and rear wheels determine the basic signals for the differential action limiting
control. The rear wheel speed is detected by sensor installed above the rear drive shaft and the front
wheel speed is detected by the sensor on the parking gear above the reduction drive shaft gear.
4AT-58
W1860BE.book Page 59 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
(2)
(3)
(1)
X (4)
(5)
(7)
(8)
(6)
(9) AT-00730
(1) Transfer control valve (4) Transfer pressure (7) Line pressure
(2) Transfer clutch pressure (5) Filter (8) Oil pump
(3) Transfer duty solenoid (6) Pilot pressure (9) Transmission hydraulic control
valve assembly
4AT-59
W1860BE.book Page 60 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
2. VARIABLE TORQUE DISTRIBUTION CENTER DIFFERENTIAL
The front-rear torque distribution ratio is basically determined by the gear tooth ratio of center dif-
ferential's compound planetary gears and varied by changing the degree of engagement of the hy-
draulically operated multi-plate clutch that connects the center differential output elements
according to driving conditions and road surface conditions.
1) When the front wheel speed is higher than the rear wheel speed:
Power flow
Input shaft
Intermediate shaft
1st sun gear
Pinion
Carrier LSD 2nd sun gear
Reduction gear Rear drive shaft
Front differential Rear differential AT-00724
AT-00725
4AT-60
W1860BE.book Page 61 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
2) When the rear wheel speed is higher than the front wheel speed:
Power flow
Input shaft
Intermediate shaft
1st sun gear
Pinion
Carrier LSD 2nd sun gear
Reduction gear Rear drive shaft
Front differential Rear differential AT-00726
AT-00727
4AT-61
W1860BE.book Page 62 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
3. CENTER DIFFERENTIAL ASSEMBLY
The center differential is a compound planetary gear set without internally-toothed gears. The input
torque from the automatic transmission is transmitted to the input element of the center differential
(1st sun gear). The front wheel output elements of the center differential are connected to the carrier
and the rear wheel output elements are connected to the 2nd sun gear.
The compound planetary gears uses helical gears for quiet operation and strength. The three pin-
ions are arranged to ensure the best motion balance during operation.
4AT-62
W1860BE.book Page 63 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
(3)
(4)
(10)
(5)
(4) (9)
(3)
(7)
(2) (6)
(8)
(17)
(1)
(16)
(15)
(14)
(13)
(12)
(11)
AT-00728
(1) Ball bearing (7) Snap ring (13) Ball bearing
(2) Reduction drive gear (8) Seal ring (14) Multi-plate clutch hub
(3) Washer (9) Thrust needle bearing (15) Ball bearing
(4) Needle bearing (10) Intermediate shaft (16) Revolution gear
(5) Pinion gear (11) Thrust washer (17) Rear drive shaft shim
(6) Carrier (12) Rear drive shaft
4AT-63
W1860BE.book Page 64 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
4. MULTI-PLATE CLUTCH (LSD)
The transfer's differential action limiting device (LSD) consists of a multi-plate clutch and a transfer
hydraulic pressure control unit incorporating a transfer duty solenoid.
The transfer duty solenoid is an electromagnetic valve which is controlled by the TCM using various
duty ratios stored in its memory as explained in "1. General".
The rear drive shaft has drilled oil passages for lubrication of multi-plate clutch and extension bush-
ing and ball bearing in it.
4AT-64
W1860BE.book Page 65 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
(11) (12)
(10)
(8)
(9)
(8)
(7)
(6)
(5)
(4)
(3) (18)
(17)
(2) (16)
(15)
(14)
(1)
(13)
AT-00729
(1) Thrust washer (7) Driven plate (thicker) (13) Snap ring
(2) Rear drive shaft (8) Drive plate (14) Spring retainer
(3) Ball bearing (9) Driven plate (thinner) (15) Plate
(4) Multi-plate clutch hub (10) Driven plate (thicker) (16) O-ring
(5) Ball bearing (11) Adjust plate (17) Multi-plate clutch piston
(6) Revolution gear (12) Rear drive shaft shim (18) D-ring
4AT-65
W1860BE.book Page 66 Tuesday, January 28, 2003 11:01 PM
AWD TRANSFER SYSTEM
AUTOMATIC TRANSMISSION
5. TRANSFER HYDRAULIC PRESSURE CONTROL UNIT
The transfer hydraulic pressure control unit is bolted at the rear end of transmission case through
the transfer valve plate.
The hydraulic pressures used for the transfer hydraulic pressure control unit (line pressure and pilot
pressure) are supplied from the transmission's hydraulic control valve assembly through the pas-
sages formed in the transmission case.
The transfer duty solenoid adjusts the pilot pressure of the transfer control valve depending on the
signals from the TCM. The transfer control valve in turn modulates the line pressure into the transfer
clutch pressure before it is applied to the clutch piston.
The transfer clutch pressure adjusted in this way engages the clutch to different degrees according
to driving conditions so that the optimum torque is distributed to the rear wheels.
(2)
(3)
(1)
X (4)
(5)
(7)
(8)
(6)
(9) AT-00730
(1) Transfer control valve (4) Transfer pressure (7) Line pressure
(2) Transfer clutch pressure (5) Filter (8) Oil pump
(3) Transfer duty solenoid (6) Pilot pressure (9) Transmission hydraulic control
valve assembly
4AT-66
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