I was recently given a 1978 F250 and most of the gauges did not work. After replacing them all, the fuel gauge was the last one that didn't work. I checked the gauge and the sender and they're both good so I ran a new wire from the sender to the gauge and from the gauge to the ignition switch. It reads about 3/4 and I've put maybe 20 miles on it and it's still the same. It also moves up towards full when I step on the gas pedal. The sender is grounded at the tank, the wire runs up and connects to one post on the gauge, the other post has a wire going straight into a switched 12v lead. Is this the proper way of wiring or is there something in between that I missed/don't know about? I would take gas out but have nothing to do so with and nothing to put it in and I can't afford any more gas for it at the moment so I can't test it that way. Thanks for the help.

 

the gauge would not move much after using less than 2 gallons...can you park it on a hill? try both directions ...the wire plugs into a connection on the top of tank about 4 inches back from front. It may have come loose..probably not the float unless it is tangled in the long filter that is close by. a bad float should read low.

 

The fuel gauge is usually fed from the ign switch. Some used a voltage regulator that was either part of one of the gauges or had a separate part mounted behind the dash.
The test if I remember correctly was to ground the wire from the fuel sender and the gauge went to full (slowly). If it did that the gauge was OK. Did the same thing back at the tank , if the gauge did the same thing we knew the sending unit was faulty.