Transmission Control Switch (TCS)
WARNING: This page is about a different car, the 2013 Ford F-550 Super Duty, 2013 Ford F-450 Super Duty, 2013 Ford F-350 Super Duty, and 2013 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The TCS or (O/D OFF switch) is a momentary contact switch located on the end of the selector lever. When pressed, the O/D OFF switch sends a signal to the PCM allowing automatic upshifts that include 5th gear. Overdrive or 6th gear, is disabled during O/D OFF mode. Pressing the O/D OFF button again restores automatic upshifts from 1st through 6th gears or the next ignition cycle.