This is an interesting question to me. 55+ passenger cars are three wire gauges. On these gauges you can swap two wires (power and ground) and the gauge will operate in reverse. But the truck gauge is a two wire gauge which, I *believe*, grounds through the body of the gauge and cluster. Swapping wires IIRC lets out the magic smoke. I have an old cluster in the barn and I will try to get out there to do some testing.
http://www.adlersantiqueautos.com/ar...ntcluster.html
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You noted that there was voltage on the sensor wire, there should be no voltage on that wire. It is for a resistance reading only, you might have a wiring issue between the sensor and your fuel gauge.
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Oh, this could be right in some situations. We're talking key on, so there has to be current available from the gauge. The reading on the voltmeter will depend on where the current is allowed to go.
Key on, wire from gauge disconnected = full voltage. Current cannot go to ground, voltmeter reads full potential.
Key on, wire connected to sender, sender not grounded = full voltage. Current still cannot go to ground so voltmeter reads full potential.
Key on, wire connected to sender, sender grounded, float arm all the way up = partial voltage. Most current can get to ground through 30 Ohm resistance.
Key on, wire connected to sender, sender grounded, float arm all the way down = no voltage. All current can get to ground.
The "full potential" number will depend on the type of gauge.