The clutches and brakes that operate the planetary gear unit are operated by hydraulic pressure. The hydraulic control unit generates and adjusts this hydraulic pressure and switches its passage. The illustration on the left shows the hydraulic circuitry for the A140E model. The hydraulic pressure operates through various hydraulic pressure passages.
If the battery is dead, it is possible to start the engine of manual transaxle vehicles by push-starting it, but with automatic transaxle vehicles, this is not possible. During push-starting, since the oil pump is not operating, the operating hydraulic pressure of the planetary gear unit is not generated. In other words, the power from the tires is not transmitted to the engine. The hydraulic control unit has the following three functions.
1. To generate hydraulic pressure
The oil pump has the function of generating the hydraulic pressure. The oil pump generates the hydraulic pressure required for automatic transaxle operation by driving the torque converter case (engine).
2. To adjust hydraulic pressure
The hydraulic pressure compressed by the oil pump is adjusted with the primary regulator valve. Also, the throttle valve produces the hydraulic pressure appropriate to the engine output.
3. To shift gears (to make theclutches and brakes operate)
When the clutch and brake operations of the planetary gear unit are switched, the gears are shifted. The fluid passage is created in accordance with the shift position by the manual valve. When the vehicle speed rises, signals are sent to the solenoid valves from the Engine & ECT ECU (Electronic Control Unit). The solenoid valves operate each shift valve to shifting the gears. The major components of the hydraulic control unit are the following.