Ammeter vs Voltmeter
Back in the good ol’ days, all cars had ammeters. This type of gauge tells you how much and in which direction the electricity is flowing. When you turn on the lights and accessories, the voltage regulator senses the lower voltage and kicks in the alternator. This sends electricity through the ammeter to the battery and when everything is running smoothly, the ammeter will show a positive (+) reading. If the alternator is not working or unable to keep up, the electricity flows from the battery and shows a negative reading (-).
A voltmeter works differently. Instead of reading electrical flow, it reads voltage. This means the gauge will measure voltage across a ground and a positive wire. New cars use voltmeters, which are preferred by most street rodders. Most alternators typically put out between 13.8 and 14.5 volts while charging.
If you're replacing an old ammeter with a voltmeter, you'll need to wire it a bit differently. You can use the original wire by hooking it to the positive terminal on the voltmeter, but you must also run a wire from the negative terminal to a ground source. If you wire it as it was originally, you will create a dead short. Play it safe, and use all-new 10 gauge wire when installing either a voltmeter or an ammeter.