[Jermike]

I just bought a 73 Mustang. The owner told me that the fuel
gauge has not worked for about 15 years. Where should I start
to fix it?

Thanks.

[snowmakerdrew]

Check the ground first, and make sure that the wire on the tank is plugged in. Your float may be stuck to the bottom of the tank, or be sunken if its not the ground of the wire.

[Veronica]

Hello. The sending unit on your gas tank should have a two-wire connector plugged into it. If you unplug that, have the key in the 'On' position, and use a paper clip or something to complete the circuit of one of those wires to the other, the gas gauge should peg at full. If this happens, the problem is either the sending unit itself or the float hanging on the end of the sending unit. In this scenario, it's probably the float. If it's all funky-looking when you remove the sending unit, replace it. The new floats cost next to nothing. 5 or 6 bucks for the brass ones, which is what you want. When you put the new float on, plug the wire connector into the sending unit and hold the sending unit in your hand just like it would be in the tank, except upside-down, so that it reads full with arm all the way down. If the gauge goes to full, problem solved. If not, replace the sending unit.
If, when you connect the two wires together with the paper clip, the gauge does not move, then the problem is either in the gauge itself or in the wiring of the gauge. Hope that helps.