I don't think the polarity will make much difference in the operation of the fuel gauge, but excess voltage supplied to it just might cause problems. Most gauges are "balancing coil" type, with one winding receiving full battery voltage and going to ground, and the other coil getting battery voltage and completing the circuit through the tank unit with a variable resistance. You really need a variable resistor in the input side, that can be dialed in to supply 6 volts to the fuel gauge while it's in operation. A 1,000-ohm potentiometer works well, with one end of the resistor connected to 12 volts, the other end grounded, and the variable tap connected to the gauge input. Start with the variable resistance maxed out, with almost no voltage supplied to the gauge, and dial it up until you get somewhere between 6 and 7 1/2 volts to the gauge. Lock the potentiometer shaft in that position with a little Loctite or even Elmer's glue, and forget it! (This needs to be done with the gauge hooked up to the tank unit, and the gauge grounded to the dash. It won't work without a good ground to the gauge housing!
Jerry

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