I have a 71 F350 and the gas gauge shows half full when empty and when fill it up it goes way past full. I pulled the sending unit from tank and checked resistance and it changed from 10-50 ohms when moving the float lever. checked another sending unit I had and it measured the same and so I put it in and gauge reads the same with it. Both floats were good and the rods were bent the same. I'm thinking it's the gauge itself. Can it be tested?
Those units should be 10-90 ohms if I remember correctly. To test the guage and the wiring going up front, go back to the sending unit and take the wire off, and with the key in run, have someone watch the guage as you ground the sending unit wire, and then take it off ground and let it hang in the air. If the guage goes full scale back and forth when you do this, then it and the wiring are good.
If you decide there is something wrong with the sending unit, I would go ahead and take it apart. I have had a 100% success so far repairing sending units in old trucks as well as newer vehicles. Just carefully bend the 4 little tabs up straight, and you can take them apart. Half the time I have found the little brass wiper inside is cracked or completely broken off. I just solder them back together with my soldering gun.
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