Here is a link to troubleshoot the gauge if you have a multimeter and a little electrical know how, or a buddy with electrical know how. http://www.tflx.com/pdf/marfue%7E1.pdf
The quick check, make sure that the sending unit base is grounded to the battery, clean connections, and the case of the gauge is grounded. 12volts at the "I" terminal. If the case ground is hooked to the S terminal it should read full.