mickey deville,
Good troubleshooting. Hope you did those tests with the key ON! If you turned the key on and the gauge read full, then you pulled the sending unit wire clear, and the gauge read full, then you grounded the sending unit wire to a known good ground, and the gauge read full, you have a bum gauge.
If the gauge is good, and you ground the sending unit wire with the key on, the gauge should swing to empty.
This is presuming you did all these tests with a good wire attached directly to the sending unit side of the gauge near the dash.
The part of your post I don't understand is the reference to 12 volts on the F side! There shouldn't be 12 volts anywhere but on the line side of your voltage reducer.
The gauge probably has two terminals..one for your reduced 12/6 volts, and the other for the sending unit wire. There shouldn't be anything higher than 6 volts on this gauge.
If you go back and read some of the above posts, it is pretty straigtforward how to isolate a bad gauge, or bad sending unit.
Stuart
