Transmission Fluid Pressure (TFP) Manual Valve Position Switch Assembly
WARNING: This page is about a different car, the 2007 GMC Yukon XL, 2007 GMC Yukon, 2007 Chevrolet Tahoe, 2007 Chevrolet Suburban, and 2007 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
D4 fluid pressure is routed to the TFP manual valve position switch and closes the normally open D4 fluid pressure switch. This signals the PCM that the transmission is operating in Overdrive range.