CK,
Your bench test is perfect, but I think you also need another jumper, or bonding wire, from the gauge case itself to the sender flange. These two items need to be electrically connected to work.
Most chargers are 12 volts. A stock AD fuel gauge is designed for 6 volts. Chev's does sell one they call out as being built for 12 volts. To complicate matters, some folks have run a stock system on 12 volts with no problems while other have seen the gauge immediately go up in smoke when attached to 12 volts.
I think your gauge will work when you run a bonding wire between the sender and the gauge case. Good luck.
Stuart
