1-2 Shift Valve (318)
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
With the 1-2, 3-4 SS valve OFF, spring pressure moves the valve allowing D4 pressure to enter the 2nd fluid passage directing 2nd fluid to the Number 2 ball check valve.