my 66 chev p.u. still uses factory gauge problem is it reads past full.useing a test light starting @ back of gauge white + wire shines bright.tan- wire barely lights up test light all the way to sendind unit sender was pulled for a visual inspection it looks fine.all grounding straps were pulled filed clean and re-installed.the omly time i can get the gauge needle to respond is when i use a jumper wire conected to sending unit + stud and grnd it to the - wire just above the fuel tank inside the cab. what gives?