Did you run a new/test ground lead from an attachment bolt on the sender (which is attached to the gas tank) to a good clean ground?

"My fuel gauge only reads full.

If you cannot make your gauge read anything but full then depending on your ohm range you may have one of the following problems:

If you have a 0-30, 0-90 or 10-180 Ohm sender then the sender may not have a proper ground or there is a break in the sender lead wire."


http://www.fifthaveinternetgarage.com/tech_tips_gas_gauge.php