3-4 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 D321 fluid hold the 3-4 shift valve in the downshifted position. The 3-4 shift valve allows orificed D21Y fluid to enter the fourth band feed circuit. Fourth band feed is then routed to the 1-2 shift valve.