1-2-3-4 Clutch Regulator Valve
WARNING: This page is about a different car, the 2008 Hummer H2. However, it is still accessible from the selected car via links, so may be relevant.
With the absence of PCS 1234 clutch fluid, due to the default state of the C1234 pressure control solenoid 5, the 1-2-3-4 regulator valve would close from spring force. However, the 1234 clutch default fluid enters behind the 1-2-3-4 regulator shuttle valve and keeps the valve in the open position.