Q: 1991 F-350 fuel gauge on 1990 Ford F-350

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My fuel gauge reads over full on front and rear tank. The amount of fuel doesn't matter they read the same. I think the gauge is bad. On occasion when the truck is off the gauge seems to register correctly but changes when it is turned on.
(1) Answer
This sounds like the fuel gauge sending wire is touching ground, because if you ground the gauge wire it should go all the way full. The fact that the gauge reads normal when you turn off the key may mean the gauge is working. I am not ruling out a gauge issue but I would check all of your gauge wiring as well.

here is a web site that has really good wiring diagrams:
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