John Clark,
You have measured 12 volts at the sending unit wire (disconnected), so this proves the integrity of the wire. I saw some of this mentioned above, but might clarify it. With the key ON and the wire disconnected from the sending unit, the gauge should read FULL. With the key still ON, ground that same wire to a good ground and the gauge should swing to empty.
If this doesn't happen, your gauge is defective.
You said the gauge worked before the restoration, but that the 12 volt conversion WAS NOT part of that restoration. What was the part of that restoration that could have affected this system? Somehow the actual gauge in the dash has to be grounded. This is usually through its case.
Did any part of the restoration involve any painting or such that might have insulated some component that was not insulated before.
If you do the test outlined above and the gauge doesn't respond, it is the gauge....unless it isn't grounded in the dash assembly (maybe?).
Stuart
