I found with mine that the sender terminal (non-ground) was making contact with the plate of the sender, effectively grounding it. This the result of a very small grommet that's supposed to isolate it, and possibly me tightening it too much. If this is your problem, you can tell it easily by loosening a little and moving the wire on the sender terminal. I replaced the grommet on mine, and then was very careful to make sure the stud was centered in the hole when I tightened it. Good luck.
