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A friend has a 37 he wants to install ac and needs to convert to negative ground for the compressor. He already has converted to 12 volt. thanks in advance.

If it is a basic AC unit, unless there is a suppression diode at the compressor they are not polarity specific. The diode is not needed for basic relay type control but would be for electronic controls so that is where the issues might be. The evaporator unit might be polarity specific depending on what type controls it has. Electronic, yes but basic on/off and rheostat speed control probably not depending on what type wiring the motor for the blower uses.

To change polarity, reverse the wires on the ammeter for one thing as it will read backwards if you don't.

If he has converted to 12v did he change to an alternator or still stay with a generator. If a generator, that will need to be repolarized but if an alternator it will require changing to one for negative ground because the internal diodes in the alternator will be incorrect. Also, if a generator the regulator might also need to change depending on whether it was a universal type or polarity specific. If an alternator that uses an external regulator then the same caution would be needed.

Without knowing what he did for a 12v coil, can only say the coil leads should also be swapped for best efficiency but if it still has the armored cable and terminal in the base that will be difficult to do. The coil will still work in the original configuration but the spark will be jumping from the colder outer shell electrode to the center electrode and will not be nearly as strong. The weaker spark could misfire with marginal plugs on 6v but at 12v it probably won't be much of an issue. If he went with a modern type coil with the 12v conversion just reverse the wires on the terminals so the negative coil terminal would be going to the distributor.

Am assuming 37s had mechanical temp and oil gauges so no changes there. The gas gauge is another story. I don't know what he did for the voltage supply to the gauge but if a resistor then no change is needed. If a solid state regulator like the Runtz device that will need to change. Radio vibrators are another item to look into if there has been any work done on the radio where they might have installed a modern solid state device.

Thank you Howard. I will pass the information on to the shop doing the installation.