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Have you made a decision on whether your system will have a common ground?
Not entirely.. AC and the 47 has kind of been on the back burner lately and right now it is 12v positive ground. There is nothing really polarity sensitive in my unit. I bought and modified a very low key basic AC to fit my prewar replica evaporator case. If the compressor clutch had a built in back emf diode which would be needed with an electronically controlled unit that would be an issue (as would an entire electronic AC) but the one I have does not have a diode.. If I make the switch to an alternator then it becomes a question of whether a 12v positive ground alternator is readily available. Haven't really researched what is available in depth yet but if it requires something custom then will probably do the neg ground for AC.
Common grounds are not an issue as long as the two power supplies and the items they feed are kept separate and there are no feedback paths. The big issue for us comes about when you have something with switched polarity that can accidentally touch ground. Something like a phone charger plugged into a separately powered 12v negative ground outlet with an exposed tip that could fall across and touch a positively grounded item. Typically the metal shell of the usb plug or charger tip end is at the same ground potential as the power outlet. That would result in a series connection between the two power supplies so in our Packards, an 18v direct short.
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