Needed some input on a possible solution to my sending unit issues in my 74 F250. The main (rear) tank gas gauge seems to work when the tank is over half full, then goes to empty, even thought the tank is not. I have the bed off of the truck right now, and I have pulled the sending unit out to test. When I have the float up all the way, the gauge reads 3/4 full for about 5 seconds, then seems to start dropping towards 1/2 full.

I have power at the switch on the dash, power where the wiring harness connects in the engine compartment, power where the sending unit plugs into the harness, and a good ground. So, does this mean my sending unit is bad? Just want to make sure before I drop $70 on a new one.

Thanks in advance for the advise

 

Your unit probably is bad. They all seem to be. Buying a new Sending unit, or finding a good one in the Wrecking yards is the only other elternative.

I talked this problem with a Psyics professor. He'd made a counter that counted down, based on a fuel flow meter, and that's about the only safe alternative you have. There's no voltage potential inside the fuel tank.

In my Boat is a fuel sending unit made up of a float which twists a shaft as the float rises and falls.
That shaft had a magnet at the top, and the magnet in turn carries part of a Rheostat around with it as it rotates. Thus sending a vaiable reading to the Fuel guage.
With that system, no voltage is present inside the fuel tank.

In all my fords, 11 of them, I had at least one, and on most of them both fuel sending units go bad. Once I found a truck in a wrecking yard that had two good sending units.

I made up for it by using the odometer as a reference, and switched tanks at whatever milage the capacity of that tank dictates.

I think your fuel sending unit is getting it's Voltage from the Voltage regulator in the Dash board. If you put +12v from the battery (to test) you may burn the rheostat as you are trying to trouble shoot it. Catch a wire from the back of the Fuel guage or the Tank selector switch for the test.