my 86 fuel guage shows its full even when the plug is pulled at the tank or at the harness under the dash .also my antenna relay makes a humming noise when ignition is turned of (it works but does not go up or down)
If you have the G.M. manual it will help you in this regard. You will be looking for the common ground wire that comes out of the fuel sender unit. It should be very close to the tank screwed to the rear frame rails.
You may have a bad relay on the antenna.
It could also simply be a bad sending unit. The good news is that you can replace the sending unit from the top. You have to remove the fuel door and the rubber/plastic insert around the gas cap. You will then have access to pull the top part of the tank off and extract the sending unit. With the unit out of the car and the wires attached, you should be able to move it up and down and watch the gauge move. There are better ways to test it actually using a volt meter and watching the voltage change.