I looked around and found some really threads so I thought I should start a new one.
I just bought a 2006 F-150 that didn't have a working fuel gauge for at least three years. Previous owner (a friend) was quoted $1200.00 to fix. I spent a few days freeing up the bed bolts with heat underneath at the nuts and welding nuts to the TP45 bolts. (I cut off the nuts and ground everything back smooth)
Once I got the bed off I was able to check the sending unit. I removed it, tested it with a multi-meter and found it to be perfect. Since I am a high school auto teacher I had some time when I removed the sender so I bench tested it. We found 150ohm resistance at full and about 10ohm at empty. (don’t quote me on the numbers, I was only looking for scale movement).
I then started tracing the wires for the fuel sender. The two wires I was looking at were the Yellow with White stripe and Black with Orange Stripe at the four wire connector at the sender/pump unit.
I found a broken red wire near a black box about 1.5” X 3/4” X1/4”. This box had five wires;
1) the broken red wire
2) the yellow/white from the tank unit
3) the black/orange from the tank unit
4) the yellow/white to the gauge
5) the black/orange to the gauge

I downloaded a wire schematic and it shows a FLEX FUEL AC/DC converter in the wire system. I studied it and decided to cut out this black box and wire the two corresponding wires together. ie: yellow/white to yellow/white and black/orange to black/orange.
I then cut out the red wire and taped it up.
This cured my issue. I now have a functioning fuel gauge and it cost me $0000.00 dollars.
I immediately told my friend, the previous owner, and rubbed it in. ☺