Gas Gauge
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Senior User
Joined: Jan 2011
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From: Dothan, Alabama
Gas Gauge
My '78 Ford f150 gas gauge doesn't work. I first checked the fuses. Well where the gas gauge fuse is there is a prong missing from one end to complete the circuit. How do I replace it?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
There should not be a fuse solely for the fuel gauge. There is a fuse for the fuel tank solenoid, but if you have a single tank, then it will be blank and missing a prong. This is most likely what you are seeing. Power to the fuel gauge is grouped with one of the other fuses. The wiring diagram I'm looking at is not clear enough to decipher which one, but it's one that's hot in accessory. To be safe, just check each fuse.
The most common reason that the gas gauge doesn't work is because either (1) the float within the sending unit fills with gas and sinks to the bottom of the tank, permanently reading empty, or (2) the frame crossmember to which the sending unit grounds at the back loses continuity with true ground.
To find out which, turn the key to accessory and crawl under the truck. Remove the wire going to the sending unit, and ground it (use the same point where the sending unit grounds, if possible). If the fuel gauge pegs to full, then all of the associated wiring and grounding is good, and the sending unit must be replaced. If the gauge does not move, then the frame rail has lost ground. Often times you can restore ground by running a jumper wire from where the sending unit grounds to sheetmetal on the bed (if your tail lights still work with no odd problems, then the ground at the bed sheetmetal should be in good shape). Note that the sending unit could still be bad, so if you've lost ground, you'll still need to test the sending unit once you've restored ground. If you fix the ground issue but still cannot get the needle to peg by grounding the power to the sending unit, then there could be an issue upstream of the sending unit (such as faulty wiring, or a bad instrument cluster voltage regulator).
Just as a side note, since you asked - the prongs in the fuse panel come in from the back and are very difficult to remove. They're a special Ford-type terminal that are crimped and then snapped into place.
The most common reason that the gas gauge doesn't work is because either (1) the float within the sending unit fills with gas and sinks to the bottom of the tank, permanently reading empty, or (2) the frame crossmember to which the sending unit grounds at the back loses continuity with true ground.
To find out which, turn the key to accessory and crawl under the truck. Remove the wire going to the sending unit, and ground it (use the same point where the sending unit grounds, if possible). If the fuel gauge pegs to full, then all of the associated wiring and grounding is good, and the sending unit must be replaced. If the gauge does not move, then the frame rail has lost ground. Often times you can restore ground by running a jumper wire from where the sending unit grounds to sheetmetal on the bed (if your tail lights still work with no odd problems, then the ground at the bed sheetmetal should be in good shape). Note that the sending unit could still be bad, so if you've lost ground, you'll still need to test the sending unit once you've restored ground. If you fix the ground issue but still cannot get the needle to peg by grounding the power to the sending unit, then there could be an issue upstream of the sending unit (such as faulty wiring, or a bad instrument cluster voltage regulator).
Just as a side note, since you asked - the prongs in the fuse panel come in from the back and are very difficult to remove. They're a special Ford-type terminal that are crimped and then snapped into place.
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