63 fuel gauge reads full all the time.

[Bearcamp]

As soon as I turn the key on the needle goes from empty to full. Here's what I checked. I had no power to the sending unit and found the wire was unplugged from the fuse box. Connected it and now I have 12 volt at the sending unit. I checked for power at the gauge and I have 12 volt there also. Checked for ground at the sending unit and gauge and I have ground at both. Now what next? I'm thinking that either the gauge or sending unit might be bad. Maybe I'll pull the unit from the tank next.