U2 Shift Valve
WARNING: This page is about a different car, the 2009 Pontiac Torrent and 2009 Chevrolet Equinox. However, it is still accessible from the selected car via links, so may be relevant.
S3 signal (14) fluid is routed to the U2 shift valve and shifts the valve against spring force to the applied position. This allows servo feed (44) fluid to pass into the 4-5 clutch (40) fluid circuit.