72 Chevy C-10 Fuel Sending Unit Problem

[auminer49r]

I am having the most difficult time understanding the way my fuel gauge / sending unit circuit works. I have original the "behind the seat" gas tank set up.
If I may ask a few questions about the circuit...

OK, I just installed my second sending unit and the results havent changed, so I doubt both sending units were problematic.

When full of gas, my sending units both send a reading of 3/8ths.
When empty of gas, my sending units both send a reading of 1/8th.

When I remove the tan wire, the gauge swings to about 2:30 position (past-full). Check!
When I ground out the tan wire, the gauge swings to below empty. That's good. Check!

The gauge demonstrated full range of motion between 100% open circuit and 100% grounded out. Check!

So, I doubt the new fuel sending unit or the fuel gauge is to blame.

The brass floats are soldered air-tight on the sending units. I checked, no leaks.

Where do I go from here?

Thanks, auminer49r.