I think that when there is no current to the gauge, the gauge shows "Empty" (such as when the ignition is turned off).
When there is current to the gauge and the gauge is not connected to the sender (and thus not connected to ground), the gauge shows "Full".
These diagnostic notes are from the Shop Manual:
[img]http://ares.cs.siena.edu/~lederman/truck/Documents/Fuel%20Gauge/Fuel%20Gauge%20Diagnostics_files/image002.jpg[/img]
