how do i test a bay boat fuel gauge after i have removed it from the boat.?

Mechanical resistance type senders are always a pain on a boat, unlike a car a boat is always rocking and moving so the sensor does about ten times the movements and wears out that much quicker. I come from the position when a dip stick is the only trusted way of checking a tank. There is to my mind problems in hiding fuel under a floor, and restricting access to it and the connections and senders - but you must do with what you have. If the only way you can access the sender is to cut holes I guess you have to, if you are nervous about that (and rightly so) it may be worth getting a dealer or boat yard to do that part of it.

Testing the gauge can be done with an old (working) sender from a wreckers. Testing a sender involves measuring the resistance across the terminals as the float is moved to see if the resistance wire is worn through - the resistance should change as the sweeper contact moves. Many problems on boats involve the terminals corroding so sometimes you get lucky and that's all you need to fix - still need access though, so good luck.

Sound like the sender wires are reversed. There is a ground wire and a "sender wire' What I'd do is pull the sending unit and hook up the wires. Move the float up and down and see if the Gage works. There could be 2 other causes. if the float arm hits the side of the tank and can not drop as fuel is used, you will not get any readings. Secondly if the float arm is too short, or too long, it might take a while for the level to cause the gauge to move. Recheck the installation instructions ant trouble shoot as needed. One final comment, most marine fuel gauges don't work as well as an auto gauge. So don't rely on it.

Check the restince at the conector it should change