55 Buick special 46C gas tank sender and gauge issue

Hi, My name is John. My BCA member number is12138. I have 2 Buicks , 1928 54 Master Roadster and 1955 Special Convertible 46C (Grand Nat'l First in AACA in 1991) I have a problem with the gas gauge in my 55 Special convertible. When I turn the ignition to ON the needle tops out beyond Full. When the Ignition is OFF it goes to empty. I purchased a new sending unit and went to the bench and connected the gauge in a used cluster that I had on hand from a parts car and also I purchased a used cluster from an on-line vendor. I have connected both on the bench with 12 volt feed and they do not register the empty to full range when I move the float arm up and down. Should the gauge respond on the bench? I do not want to re install the tank until I know the system is going to function. HELP

John, yes the bench test should work. Hookup would be:

Battery (+) to IGN terminal on gauge
Battery (-) to cluster case and to sender unit case
Jumper wire from sender unit terminal to unmarked gauge terminal

When the gauge pegs out past FULL with power applied, that indicates a broken wire or bad connection somewhere. Maybe it is something easy like the connector in the brown wire in the trunk has come loose.

Attached is the page from the shop manual which gives more detail.

Thank you Todd. My new tank sending unit and gauge in the car are working. For sure my problem was a bad sender. Seems the contacts inside got corroded from sitting or oxidation from ethanol in the gas? I was concerned that my gauge was faulty. I discovered that the sender plug-in on the gauge got disconnected when I removed the defrost hose and nozzle on the drivers side. I was preparing to replace the gauge.I am humbled by my oversight but a visual of the gauge contacts is difficult at my age. I have the dash top plate removed and plan to replace all bulbs and lube the speedometer cable while i can access them from the top side.
John