It is an actuator that controls the hydraulic pressure for shifting in the automatic transmission according to the output signal (current) of the TCU.
The hydraulic pressure controlled by the solenoid valve is transmitted to the clutch/brake connected to the planetary gear, as a result, the transmission forms a gear ratio that suits the driving conditions. There are PWM, VFS and On-Off types of solenoid valves. In the past (4th speed, 5th speed), PWM type was dominant, but recently (6th speed, 8th speed), VFS, which can perform precise linear control, is mainly used to improve shifting feeling. VFS-type solenoid valves are evolving into high-pressure control/low-leakage specifications to improve fuel efficiency, and various specifications are being developed according to the required performance.