Fuel gauge problem

[53Rick]

I'll try this again. Didn't post last time. I have a '72 GMC 1/2ton. The fuel gauge reads empty at all times. I have replaced the fuel sending unit 3 times in the course of the past 10 years. Nothing changes. I've done the following tests: (1) pulled the sending unit wire at the tank. The gauge swings to Full with the key on. (2) Grounded the sending unit wire to the tank. The gauge goes to Empty. (3) The unit has only the one wire leading to the fuse block. No ground wire. So I have run a ground wire from the sending unit to the frame and also from the fuel tank to the frame. No change and the gauge stays on empty. (4) I've followed the wire from the tank to the fuse block and inspected it carefully for breaks, bare wire, etc. All is fine. Anyone have any suggestions on what the problem is and where I should go next with this? Everything I've read seems to indicate with the above tests, the gauge is good, but I can't believe 3 sending units could be bad. If memory serves correctly, I believe one replaced sending unit registered 1/4 tank when the tank was full for a little while and then quit registering. Any advice would be appreciated. Thanks.