Reverse Shift Solenoid (SS) Valve Assembly (S5)
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.
The normally closed reverse SS valve assembly is energized, allowing fluid pressure in the S5 signal (18) fluid circuit to exhaust through the solenoid.