The tank being grounded and sealed has nothing to do with the way the sender works. The sender itself must be grounded to the frame.
There is a small wire that runs from top of sender down to the variable resistor that the float arm runs on, this wire could be grounded to frame of the sender or the insulated stud of the sender. The fellows in a few post back were explaining this and found it to be their problem.
Variable resistor, full ground signal = empty on fuel gauge. Less ground or none at all = full on gauge
Dead short to ground = empty on gauge and needle never moves. A working system will always move the needle a little even when out of fuel.
Open circuit ( broken wire or no ground ) the gauge will read full or pegged. A full tank of fuel will not peg the gauge.
If you haven't removed the sender from the tank, do so now.
Unhook the small wire and pull it away from the insulated stud,
ground the frame of the sender with jumper wires,
hook the wire from the gauge to the small wire you just unhooked, be sure it's not touching any part of truck or sender,
turn key on
run float arm from full to empty,
Does gauge work? Yes / No
Unhook wire from sender, did anything happen? Yes / No Gauge should go full or pegged. Try tapping on gauge to see if needle is stuck.
