1-2 Shift Valve
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
High 1-2 solenoid fluid pressure holds the 1-2 shift valve in the downshifted position. The valve allows second clutch fluid from the second clutch piston and from the 1-2 accumulator piston to pass through the valve and to exhaust.