Is there an issue with this or is it not a simple yes or no question? I'd like to use a 30's era amperes gauge however, I'm running 12v negative ground. Can someone explain the theory to me?
Switch the amp gauge wires when switching polarity. Even the smallest output 12v alternators are 37 amps, so it will bury the needle after initial startups I would think. It will probably not hurt it, as the amps will go down real quick in a few seconds. I assume your gauge is 20 or 30 amps?
you can run a 6v amp gauge on 12v no problem. Voltage is inversely proportional to amperage so to put it simply if you have 2 Headlight bulbs that are the same brightness but one is 12v and one is 6v, the 12v one will use half as much amperage.IE 6v bulb using 15amps the 12v one would use 7.5amps. SO what usually happens is your gauge pointer will move not nearly as much. Also sometimes you have to switch the wires opposite to get it to read the right direction.
Cantstop is correct. One additional thing though. In the directions for the wiring kit I bought for the car I'm building it said that if you are running a modern alternator which puts out a lot more amps than that gauge can handle then you must switch out to a voltmeter. If you are just running a 12 volt generator system like me, your amperage gauge should work just fine.
I went from 6v positive ground to 12v neg ground. When I did it I thought for sure I had to switch the wires around on the gauge. Nope, now my gauge reads backwards... No numbers on mine, but it hasn't pegged yet. It rides constantly at the 1/2 way mark(not straight up and down, half way between that and max) Edit: I'm running a GM 3-wire
You can also use a wiring shunt when the amperage is too high for the amp gage you want to use. A wiring shunt is basically a electrical pathway around the amp gage. For example, if you connected the wiring to the amp gage as you normally would and the 30 amp gage was reading 25 amps at idle you could reduce the reading with a shunt. In this example a 10 gage wire connected across both terminals on the amp gage would reduce the amp gage reading by half. The reduction in the reading is because part of the amperage is going through the gage and part of the amperage is going through the 10 gage wire. An 8 gage wire would reduce the reading even more while a 12 gage wire would reduce the reading less. It all has to do with "resistance".
Sounds good. I get it. I have not purchased my gauges yet. I'm just doing the research before I buy something that looks cool but doesn't work for me. Related question, are the speedway voltage reducers a good choice for say, an old water temp gauge?
The current draw has nothing to do with the capacity of the alternator. A 125 Amp alternator will only supply what the car asks for. It depends only on the load you place on the alternator, which doesn't change unless you put in a 1000 watt stereo and a set of electric pumps for your air bags or hydraulics. Cantstop is correct (and well said). If you raise the voltage, you halve the amount of amps the car needs to perform the same tasks, like lights, etc.
Only explanation for that is they were backward to start with, or you thought you switched them and you actually got them back on just like they came off. Voltage and polarity follow OHM's law, you just don't get a case where it works backwards.
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