My fuel gauge stays on full all the time what cause that?

Bad fuel sending unit. It is located on the fuel pump in the gas tank.

It may be the sending unit. . my 2000 buick regal LS has the same problem when the tank gets down lower on fuel the gauge starts to go down but 2/3 to 3/4 tank it stays on full. I replaced the fuel pump and sending unit. but same problem. I returned the whole fuel pump and sending unit and still the same problem. I recall welding the exhaust and forgetting to remove the battery terminal. the gauge was ok till I refueled the car and then the gauge stayed on fuell until the tank gets less then half way and it started coming off the full mark. to this day I haven`t been able to figure it out. lordtrem@yahoo.com