System Description
A241H:
A241H (For All-Trac/4WD Station Wagon)
The A241H automatic transaxle is a 4-speed automatic transaxle developed for full-time 4WD use. Its Construction is that of the favorably received A240L automatic transaxle mounted in the front wheel drive Corolla, combined with a transfer equipped with a center differential. In addition, a center differential control clutch, which operates in response to driving conditions, has been incorporated.
The center differential control clutch controls excessive differences in the rotation rates of the front and rear wheels to provide the most suitable driving conditions.
NOTE: Special handling methods are necessary for full-time 4WD vehicles during inspection and maintenance. Instructions concerning these handling methods are given in Service Precautions. Service Precautions
Torque Converter, Planetary Gear Unit and Hydraulic Control System
TORQUE CONVERTER
Construction
The torque converter is composed of the pump impeller which is rotated by the engine, the turbine runner and lock-up clutch which are fixed to the transmission input shaft, and the stator which is attached to the stator shaft via the one-way clutch. The torque converter is filled with ATF.
Operation
The pump impeller is rotated by the engine, which causes a flow in the ATF inside the torque converter.
The flow of ATF caused by the pump impeller strikes the turbine runner, providing a force to rotate the turbine runner, and transmits torque to the input shaft.
The flow of ATF which has hit the turbine runner rebounds and tries to flow in the direction opposite to the direction of rotation of the pump impeller, but the stator returns the flow to the original direction of rotation. So the ATF becomes a force which supports the pump impeller and increases torque.
HINT: Although the stator is immobilized by the one-way clutch, should the one-way clutch become defective the stator will be rotated by the flow of ATF, the flow of ATF will not be reversed, torque will not be increased and the problem of inadequate acceleration will occur.
The lock-up clutch is pushed against the front cover by fluid pressure so that the engine revolutions are directly transmitted to the input shaft without the medium of the ATF.
PLANETARY GEAR UNIT
Construction
The planetary gear unit is composed of planetary gears, clutches which transmit power to the planetary gears, and brakes and one-way clutches which immobilize the planetary carrier and planetary sungear.
Operation
Power from the engine transmitted to the input shaft via the torque converter is then transmitted to the planetary gears by the operation of the clutch.
By operation of the brake and one-way clutch, either the planetary or the planetary sun gear are immobilized, altering the speed of revolution of the planetary gear unit.
Shift change is carried out by altering the combination of clutch and brake operation.
Each clutch and brake operates by hydraulic pressure; gear position is decided according to the throttle opening angle and vehicle speed, and shift change automatically occurs.
A240L, A241H:
The conditions of operation for each gear position are shown on the illustrations.
Differential and Transfer (A241H)
FRONT DIFFERENTIAL
The front differential is incorporated in the ring gear mounting case. The right edge of the front differential case is integrated with center differential left side gear. The center differential control clutch is joined to the outside of the front differential case.
CENTER DIFFERENTIAL
The center differential has the following functions:
- Power from the transmission is delivered equally to the front wheel drive shafts and the rear wheel drive shafts.
- The rotation rate difference of the drive shafts of the front and rear wheels during turning is absorbed.
Operation of the center differential eliminates the tight corner breaking phenomenon which occurred with part-time 4WD and makes possible full-time 4WD operation.
TRANSFER
The right case, which is integrated with the center differential right side gear, rotates and power is transmitted to the driven gear via the transfer ring gear after the direction of power transmission is changed by 90°.
CENTER DIFFERENTIAL CONTROL CLUTCH
- The center differential control clutch operates the piston by hydraulic pressure so that the clutch plates and clutch discs are pressed together. The friction resistance thus created transmits the torque and operation of the center differential is controlled.
- Because the clutch plates and clutch discs are interlocked with the spline of the ring gear mounting case and front differential case respectively, both cases try to rotate as a unit in response to the friction force from the contact of the clutch plates and discs. In the way, operation of the center differential is restrained and the power to control the difference between the rotation of the front and rear shafts produced.
- The hydraulic pressure operating on the piston is adjusted according to the throttle valve opening angle and the shift position.
Hydraulic Pressure Control System
The hydraulic pressure operating on the center differential control clutch is adjusted by the No.2 pressure regulator valve to a pressure corresponding to the throttle valve opening angle and the shift position. By operating the solenoid valve the clutch control valve opens and closes the fluid passages to the piston, and regulates the operation or non-operation of the center differential control clutch.
CLUTCH CONTROL VALVE AND SOLENOID VALVE
The clutch control valve is a valve which decides if the center differential control clutch will be operated or not operated.
- When the solenoid valve is ON, the hydraulic pressure acting on the upper part of the clutch valve is drained, the clutch control valve is moved by spring tension, the fluid passage to the center differential control clutch opens and the control clutch operates.
- When the solenoid valve is OFF, hydraulic pressure acts on the upper part of clutch control valve, the valve is moved by the hydraulic pressure and the fluid passage to the control clutch closes.
NO.2 PRESSURE REGULATOR VALVE
This adjusts the hydraulic pressure operating on the center differential control clutch according to the throttle opening angle and shift position in order to provide clutch operation pressure suited to the driving conditions.
- Operation in D, 2 range.
Throttle modulator pressure acts on the lower part of the No.2 pressure regulator valve and the line pressure is adjusted to a pressure corresponding to the throttle valve opening angle. Accordingly, the center differential control clutch operates at a pressure corresponding to the throttle valve opening angle.
- Operation in L range.
When the shift lever is shifted into L range, the line pressure acts between plungers (A) and (B), and plunger (A) and No.2 pressure regulator valve are pushed up. Thus pressure adjustment does not occur at No.2 pressure regulator valve and the line pressure acts directly on the center differential clutch to provide strong operation pressure.
- Operation in R range
When the shift lever is shifted into R range, the line pressure acts on the lower part of plunger (B); plunger (A) and (B) and No.2 pressure regulator valve are pushed up. Thus pressure adjustment does not occur at No.2 pressure regulator valve and the line pressure acts directly on the center differential clutch to provide strong operation pressure.
POWERTRAIN
- Power from the transmission is transmitted from the ring gear to the center differential. Also, because the pinion shaft is connected to the inside of the center differential, the power is transmitted from the pinion shaft to the pinion gear and then to the side gear on both sides.
- Power transmitted to the left side gear of the center differential is then transmitted to the front differential case and further transmitted from the pinion shaft pinion gear both side gears. The drive shaft is interlocked with the side gear so power is transmitted from the drive shaft to the front wheels.
- Power transmitted to the right side gear of the center differential is transmitted from the transfer drive gear to the transfer driven gear, changes direction and is then transmitted from the propeller shaft to the rear differential.
- On the other hand, when hydraulic pressure acts on the center differential control clutch, power is also transmitted from the ring gear mounting case to the front differential case.
CONSTRUCTION
The overdrive cut system can be broadly divided into three sections, the sensors, the ECU and the actuator.
OPERATION
1. O/D Main Switch in ON Position
When the water temperature is 55 °C (131 °F) or below, the O/D solenoid is turned ON and the overdrive is cut.
If the water temperature is 55 °C (131 °F) or more and the following conditions are both fulfilled, the O/D solenoid is turned ON and the overdrive is cut:
- Vehicle speed less than or equal to 125 km/h (78 mph)
- Throttle valve opening angle is 70% or more.
2. O/D Main Switch in OFF Position
The O/D solenoid remains ON and the overdrive is cut.