Another fuel gauge question

[ScottH]

So I'm working on a 63 c10 attempting to get it back on the road. Having several wiring issues but this issue is with the fuel gauge. I recently installed a tank & sending unit I picked up used. The sending unit looked brand new as well as the tank.

What is happening is with the tank empty & I turn the ignition on, the fuel gauge goes to just under a half full & stays there. If I disconnect the power wire at the sending unit, the gauge goes to full. If I ground the power wire it goes to just under half full.

The tank & sending unit are installed & I had some issues with installing the sending unit so would prefer not to take the unit out of the tank unless absolutely necessary. Also I tested the unit with a multi-meter & it reads 0 as tank is empty.

Looking for any info or suggestions that can help me figure out where I should do next to figure out the issue. Any info would be most appreciated.