Shift Pressure Relay 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.
S5 signal (18) fluid is exhausted from the shift pressure relay valve and spring force moves the valve to the released position to allow drive or reverse (20) fluid to enter the reverse control (25) fluid circuit. Reverse control (25) fluid pressure is routed through the B4 control valve and on to the B5 control valve.