Just replaced my fuel and temp. gauge last week. The fuel gauge has a voltage regulator that puts I think it is 5 volts to each gauge. If the regulator goes neither gauge will work. There are a few websites that offer good troubleshooting tips but I don't have them on hand. Just do a search for Jeep gauges. Good luck.
'85 CJ-7 - 258 bored .030 w/ mid level cam, 4.0 head, Clifford intake and header, Weber 38, TFI w/ MSD 6, Black Diamond 3" lift, Daystar 1/2" lift shackles, 33x12.5 Goodyear Duratracs with stock 3.31 gears.