fuel gauge sender

my fuel gauge used to work when got the car . took out the sending unit ran a wire from the wire that hooks to sending so i can turn key on inside car with sending unit in my hand. works when i ground th side of round cap. but when sending unit in tank hook wire up to ground nothing ive tryed to take a scewdriver and scribe me a new clean piece of metal on sending unit and nutin dont work when i had it in my hand i grounded the side of it and worked fine brass float is fine . has gas when tryed to undo ring gas was comin out just dont want to ground on face of sending unit?