Transmission Fluid Pressure (TFP) Manual Valve Position Switch
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.
Release fluid also exhausts from the TFP manual valve position switch and the TFP manual valve position switch signals the PCM that the TCC is engaged.