In order of diagnosing. 1st 1968 FB conv. New tank, sender, fuel lines, harness. With fuel tank in place, fuel in tank up to 10 gallons guage reads E. Removed fuel, dropped tank with ground intact, sender still in tank manually moved float guage works. removed sender placed in large container of gas float works and guage works. Put tank back in place, same ground etc. Guage reads E. Can anyone help me, I'm out of ideas ?? Chris

I would start with all of the electrical connections. Have someone watch the gauge while you wiggle or twist each one. Good luck

mine reads full all the time. does that sound like a ground problem?

Sounds like it to me, my 68 indicated the correct quantity of fuel until I turned on the lights, then it pegged past full. I took apart the rear wiring harness in the trunk and re-ground the tail, brake and backup lights, then a new ground for the fuel sending unit and now things work as they should.
Although the float and arm of your fuel sending unit could be physically stuck in the full position resulting in a full reading.
Al.

Check every inch of the tan wire as well, to be sure that you do not have a void in the insulation that is grounding out when installed. The lead starts in the trunk latch support and runs through the trunk floor then beneath the trunk.

Sir, from what I have been told, is you should run a ground wire from the battery negative to the end of your sending unit harness. The guage should read "E". If not some say your guage is bad ??

I suggest going to the trunk and pulling the wire to the sending unit and grounding it out. This will test the wiring and the gauge very quickly.