F250 Fuel gauge

the sending unit is bad in one tank. either that or wires to it.

It could also be an open connection at the switch. The sending unit provides the ground for the gauge. The better the ground, the fuller the gauge. A empty gauge indication would be no needle movement due to no ground.

your right tennisshoes. Its susposed to read 145ohms when full and 22.5ohms when empty. An open condition would make it read over full. the signal is shorted to ground somewhere. you could unplug the sender on the affected tank and if it reads full then you know its the sender. if it doesnt change you have a wire shorted to ground.

the switching valve under the truck also switches the gauge circuit. lots of salt and water down there check for corrosion in the elec. connections. it may require a new valve

I just replaced my rear tank and fuel pump on my 89 F250 with 351W. when the tank is full it reads normal till 3/8 of tank left and the gauge will start to rise back up to full.

Christopher On my 1997 7.3 rear tank was reading empty when it was full. Ford wanted $500.00 for new fuel sensor. I took my tank down and removed the sending unit and found my float was full of fuel and had a crack in it. I found a new float on Amazon made of brass like the original and the cost was like $12.00 it just snapped in to the arm

The wire harness only has 2 wires going to the tank and the tank harness has 4 wires coming out