I think you're saying you have 12 volts at the sender terminal of the guage with the sender wire disconnected? If so, that's probably a good reading but doesn't really prove where the trouble is. With the sender wire disconnected from the guage and the key on - what does the guage read? It should swing over to the Full mark. If it doesn't, check your ground at the guage cluster as DADS50 suggests, if that doesn't cause the guage to swing to Full with the sender wire disconnected, you've got a bad guage.
