1-2 Shift Valve
WARNING: This page is about a different car, the 2008 GMC Sierra and 2008 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Signal A fluid pressure holds the valve in the downshifted position against spring force. In the First gear position, the valve blocks D4 fluid from entering the 2nd fluid circuit.