I had a similar problem that when I turned my ignition 'on' my fuel gauge would go to full. There is one wire that went from my autometre gauge to the fuel sending unit and no ground wire. I had to add a ground wire from one of the sending unit bolts and mounted it to body on clean sheet metal. Here is a helpful link I used for troubleshooting.
https://www.fillingstation.com/artic...ugetesting.htm