gas gauge problem

on my 1972 LeMans the gas gauge reads full all the time
there is no voltage going to my sending unit on the tank
should there be and where does it come from
thanks

Someone can correct me if I am wrong, but here goes:
GM fuel gauge sending units in the tank have a variable resistor attached to the float. At empty, they have a resistance of 90 ohms. When full, the resistance approaches zero.

So, first, locate the wire that goes from the sender to the gauge. This wire sends a grounding signal to the gauge, so its not a power wire.

Try grounding this wire to the chassis, with the car ignition on. The gauge should move to empty. If it doesn't, the wire is defective and is either broken or is shorting to ground. Trace the wire and fix it.

If the gauge does move, the sending unit in the tank either has no ground wire or is defective.
You can try running a ground wire to the sender, or if that does not work, pull it out, use an ohm meter and see if the electrical resistance changes as you move the float up and down.

want their a problem with sunoco gas a few months back where they accidently put lead into the gas?