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.
Spring force and pressure from the 2-3 shift solenoid (SS) valve hold the 1-2 shift valve in the upshifted position. The fourth band fluid is routed back through the valve and into the fourth band feed circuit. Fourth band feed is then routed to the 3-4 shift valve.