1-2 Shift Valve

1-2 shift valve automatically controls shifts between first and second gear by switching shift solenoid B on/off.
In first gear position, with shift solenoid B off, line pressure from passage A acts on the valve to shift, and the passage to second brake is closed. With shift solenoid B on, passage A line pressure is drained, the valve shifts with the spring tension, the passage to second brake is opened, and the position turns to second gear.
In 2 or 1 range, passage B line pressure acts on second coast brake through the intermediate modulator valve, only when the valve is in second position. This enables the engine brake to operate in second gear. In 1 range, passage C pressure acts on reverse brake only when the valve is in first position. This enables the engine brake to operate in first gear.

Fig 1: 1-2 Shift Valve Operation
Courtesy of MAZDA MOTORS CORP.