When you pull out the sender, hold it next to the tank wear it would sit if installed. Move the arm up and down and see where the float is. If the top of the arm's travel is at the top of the tank and the bottom of travel is near the bottom of the tank, then you probably need a new send. Make sure it's oriented correctly with the arm facing the driver side.
If you need a new one, Jon G recommends a really nice one from a seller on ebay from Taiwan
