U2 Shift Valve
WARNING: This page is about a different car, the 2003 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
S4 signal (17) fluid is routed through the U2 shift valve into the 345 enable (50) fluid circuit.
Servo feed (44) fluid is routed through the U2 shift valve into the servo apply (45) fluid circuit.