Automatic Transmission/Transaxle: Description and Operation
GENERAL
The A340E is a 4-speed, Electronic Controlled Transmission (hereafter called ECT) developed for use wit. high-performance engines such as the 7M-GE and 7M-GTE. A lock-up mechanism is built into the torque converter.
The A340E transmission is mainly composed of the torque converter, the overdrive (hereafter called O/D) planetary gear unit, 3-speed planetary gear unit, the hydraulic control system and the electronic control system.
Outline of ECT
The conventional automatic transmission operates by mechanically converting vehicle speed into governor pressure, and throttle opening into throttle pressure, and using these hydraulic pressures to control the operation of the clutches and brakes in the planetary gear unit, thus controlling the timing of up-shift and down-shift of the transmission. This is called the "hydraulic control method."
In the case of the ECT, on the other hand, sensors electronically sense the speed of the vehicle and the throttle opening and send these information to the electronic controlled unit (hereafter called ECU) in the form of electrical signals. The ECU then controls the operation of the clutches and brakes based on these data, thus controlling the timing of the shift points.
SHIFT CONTROL
- Hydraulic Controlled Transmission
Shifting in the fully hydraulic controlled automatic transmission is carried out by the hydraulic control unit as shown.
- ECT
Aside from having an ECU which controls shifting based upon electrical speed and throttle opening signals, the ECT is basically the same as a fully hydraulic controlled automatic transmission. The ECT controls shifting as specified.
PLANETARY GEAR UNIT
The planetary gear unit is composed of three sets of planetary gears, three clutches which transmit power to the planetary gears, and four brakes and three one-way clutches which immobilize the planetary carrier and planetary sun gear.
Power from the engine transmitted the input shaft via the torque converter is then transmitted to the planetary gears by the operation of the clutches. By operation of the brakes and one-way clutches, either the planetary carrier or the planetary sun gear is 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.
HYDRAULIC CONTROL SYSTEM
The hydraulic control system is composed of the oil pump, the valve body, the solenoid valves, and the clutches and brakes, as well as the fluid passages which connect all of these components. Based on the hydraulic pressure created by the oil pump, the hydraulic control system governs the hydraulic pressure acting or the torque converter, clutches and brakes in accordance with the vehicle driving conditions.
There are three solenoid valves on the valve body. These solenoid valves are turned on and off by signals from the ECU to operate the shift valves. These shift valves then switch the fluid passages so that fluid goes to the torque converter and planetary gear units.
(Except for the solenoid valves, the hydraulic control system of the ECT is basically the same as that of the fully hydraulic controlled automatic transmission.)
- LINE PRESSURE
Line pressure is the most basic and important pressure used in the automatic transmission, because it is used to operate all of the clutches and brakes in the transmission.
If the primary regulator valve does not operate correctly, line pressure will be either too high or too low. Line pressure that is too high will lead to shifting shock and consequent engine power loss due to the greater effort required of the oil pump; line pressure that is too low will cause slippage of clutches and brakes, which will, in extreme cases, prevent the vehicle from moving. Therefore, if either of these problems are noted, the line pressure should be measured to see if it is within standard.
- THROTTLE PRESSURE
Throttle pressure is always kept in accordance with the opening angle of the engine throttle valve. This throttle pressure acts on the primary regulator valve and, accordingly, line pressure is regulated in response to the throttle valve opening.
In the fully hydraulic controlled automatic transmission, throttle pressure is used for regulating line pressure and as signal pressure for up-shift and down-shift of the transmission. In the ECT, however, throttle pressure is used only for regulating line pressure. Consequently, improper adjustment of the transmission throttle cable may result in a line pressure that is too high or too low. This, in turn, will lead to shifting shock or clutch and brake slippage.
ELECTRONIC CONTROL SYSTEM
The electronic control system, which controls the shift points and the operation of the lock-up clutch, is composed of the following three parts:
1. Sensors
These sensors sense the vehicle speed, throttle opening and other conditions and send this data to the ECU in the form of electrical signals.
2. ECU
The ECU determines the shift and lock-up timing based upon the signals from sensors, and controls the solenoid valves of the hydraulic control unit accordingly.
3. Actuators
These are three solenoid valves that control hydraulic pressure acting on the hydraulic valves to control shifting and lock-up timing.
FUNCTION OF ECU
- Control of Shift Timing
The ECU has programmed into its memory the optimum shift pattern for each shift lever position (D, 2, L range) and driving mode (Normal or Power).
Based on the appropriate shift pattern, the ECU turns No.1 and No.2 solenoid valves on or off in accordance with the vehicle speed signal from the vehicle speed sensor and the throttle opening signal from the throttle position sensor. In this manner, the ECU operates each shift valve, opening or closing the fluid passages to the clutches and brakes to permit up-shift or down-shift of the transmission.
NOTE: The electronic control system provides shift timing and lock-up control only while the vehicle is traveling forward. In REVERSE, PARK, and NEUTRAL, the transmission is mechanically, not electronically controlled.
- Control of Overdrive
Driving in overdrive is possible if the O/D main switch is on and the shift lever is in the D range. However, when the vehicle is being driven using the cruise control system (CCS), if the actual vehicle speed drops to about 4 km/h (2 mph) below the set speed while the vehicle is running in overdrive, the CCS computer sends a signal to the ECT ECU to release the overdrive and prevent the transmission from shifting back into overdrive until the actual vehicle speed reaches the speed set in the CCS memory.
On this model, if the coolant temperature falls below 60 °C (140 °F), the TCCS ECU sends a signal to the ECT ECU, preventing the transmission from up-shifting into overdrive.
Control of Lock-Up System
The ECT ECU has programmed in its memory a lock-up clutch operation pattern for each driving mode (Normal or Power). Based on this lock-up pattern, the ECU turns lock-up solenoid valve on or off in accordance with the vehicle speed signals received from the vehicle speed sensor and the throttle opening signals from the throttle position sensor.
Depending on whether lock-up solenoid valve is on or off, the lock-up relay valve performs changeover of the fluid passages for the converter pressure acting on the torque converter to engage or disengage6 the lock-up clutch.
(Mandatory Cancellation of Lock-up System)
If any of the following conditions exist, the ECU turns off lock-up solenoid valve to disengage the lock-up clutch.
1. The brake light switch comes on (during braking).
2. The IDL points of the throttle position sensor close (throttle valve fully closed).
3. The vehicle speed drops 4 km/h (2 mph) or more below the set speed while the cruise control system is operating.
4. The coolant temperature falls below 60 °C (140 °F) and vehicle speed is under 60 km/h (37 mph), or 35 °C (95 °F) and vehicle speed is under 40 km/h (25 mph).
The purpose of (1) and (2) above is to prevent the engine from stalling if the rear wheels lock up. The purpose of (3) is to cause the torque converter to operate to obtain torque multiplication. The purpose of (4) is both to improve general driveability, and to speed up transmission warm-up.
Also, while the lock-up system is in operation, the ECU will temporarily turn it off during up-shift or downshift in order to decrease shifting shock.