What year truck? If it's a OBS, here's the diagram - http://userpages.chorus.net/elephant/FuelTankSelector.pdf . Most likely it's NOT in either tank, but a break in the wire between the FSV and the gauge head. Open circuit == infinite resistance == gauge pegs past full.
if you have spare parts on hand, plug in the spares one at a time, so you can avoid opening up the tank. the FSV is possibly a bear to get to though. bottom line, your problem is in one of three areas. one) the sender. two) the FSV and three) the wiring. having a volt/ohm meter will be invaluable for this diagnosis run.
Ok , so I did some testing today. I swapped gauges, no change. If I short the fuel sender plug it makes the gauge go to empty. I ohm tested the sender while it was unplugged and nothing happened on the meter. Bad sender?
With it set to 200 or 2k ohm , can't remember , I touched the two leads coming out of the sender. The meter didn't read anything. I then shorted the 2 pins on the plug side and the gauge went to zero. I checked ohm from fsv to gauge , it read around 6 .
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