It may be the gauge, not the sending unit
You seem to be able to point people in the right direction, so here goes. I have a 1984 Ford Ranger with a 2.8-liter V-6. The truck is a short wheelbase with a 5-speed manual transmission and four wheel drive.
The fuel pump is mechanical and it’s mounted on the engine. The fuel tank is a mid-mounted 17-gallon unit. Apparently this combination uses a unique sending unit. I have searched the Internet extensively, and cannot find a new
one. The Bronco unit is readily available, but is too long for the Ranger tank. I had the sending unit “rebuilt” once but it didn’t work accurately for very long. I have checked the rheostat and the contact for cracks or breaks and can find none. The float is brass and is not taking on fuel. Every time I have checked the sending unit off the vehicle it seems to be sending the right signal to the gauge. When it is installed, the gauge reads near or over full until the level in the tank drops to about half, then it drops to there and stays there until the tank level is near empty. I really would like to find a new one, but failing that do you have any suggestions for a rebuilder or what I may have missed?
The fact that the sending unit works correctly when you test it off the truck but not when it’s installed in the truck likely indicates that the problem is not with the sending unit. Have you considered the possibility that the gauge is malfunctioning? Have you looked at the fuel gauge wiring? If you wire the sending unit directly to the gauge does the gauge read correctly as you move the sending unit through its range of motion? That would obviously tell you that there’s something wrong with the truck’s wiring.
If independent testing of the sending unit (for example, with a meter) indicates that it’s working properly but when it’s connected directly to the truck’s gauge the gauge reading is incorrect then there’s something wrong with the gauge.