The 2-3 shift valve assembly consists of the shift valve, its spring, and a throttle pressure regulating plug.
In D range, line pressure is supplied from the manual valve onto a differential area and this assists the spring in holding the valve down against governor pressure.
In this position, the rear clutch apply and front band release circuit is open to exhaust.
With the vehicle in motion in 2nd gear and at a minimum throttle opening, throttle pressure is applied to the top of the regulating plug but under these conditions will not be at a high enough value to move it downwards.
Since the only forces opposing governor pressure are the spring, and the line pressure acting on the differential area land, a 2-3 upshift will take place at a relatively low vehicle speed.
With the vehicle in motion in 2nd gear and at higher throttle openings, higher throttle pressure moves the plug down towards the shift valve and this re-regulates the throttle pressure into a lower value so that it can act on the top land of the shift valve.
This assists the spring to control the shift points relative to accelerator position at lower vehicle speeds.
As soon as governor pressure becomes high enough to overcome the combined opposing forces, the valve upshifts into the top gear position and fluid at line pressure can flow across to apply the rear clutch.
At the same time fluid passes through the servo orifice control valve to the release side of the front band servo and this means the front clutch and the rear clutch are now the only components being applied, providing top gear.
