Make sure the gage and sender/tank have good ground connections.
Make sure the gage sender is getting voltage. Test at the back of the cluster and at the wire to the tank. Might have to drop the tank a bit to get to it.
With the power unplugged, the gage should go to empty.
Past that, probably the float in the sending unit is stuck in in the full position.
Do yourself a favor, when you swap out the sender, buy yourself an autometer fuel gauge, wire it up and be done-No more fuel gauge worries ever again. like i did.
Most likely, the sending unit.
I had the same problem & had the sedning unit repalced, problem solved.
I like knowing how much is in the tank, i would hate to run out of gas here in Houston on a freeway.
I ran outta gas in brooklyn, NY. Horrible neighborhood @ about 3am and had to leave my girl in the car, and get out and look for a gas station. then when i found one, dude wouldnt sell me gas because i didnt have the poer container, had to buy one off him for $25 plus the gas, and if i didnt-Had to trek through some housing projects for about 2 more miles to buy gas-
i vowed-that would never ever happen to me again, swapped the sending unit out, and got a better gauge to boot.
Later Ant