Can't get my 1994-4.9l 6 cylinder F 150 to switch to front tank. I move the switch too front tank, and it reads empty when full, and keeps using rear tank. Why ?

Check that the switch is actually working. At the switch, you should have battery voltage on the R (Red) wire when selecting the Front tank and on the BR/W (Brown with White trace) when Rear is selected. The share a ground. If you aren't getting power, replace the switch.