(1) run a long jumper wire from the sending unit flat circular metal surface all the way directly to the battery negative post to make sure it's grounded well (2) pull the blue sending unit wire and ohm the sending unit stud to the same flat metal on the sending unit face & post what you get. probably need to get the tank close to empty and pull the sender & eyeball the rheostat and ohm it thru it's travel and at the top and bottom of it's travel.