Voltmeter vs ammeter
Originally submitted by T:
An ammeter reads 0 when the electrical system is working perfectly and also when the ammeter is busted.
Since an ammeter can really only report that the battery is discharging, recharging or having the same voltage on each side of it (balanced situation) it can make for misleading results.
Unfortunately a balanced situation can occur when the battery is half charged and the alternator is only making half the system voltage making the 0 reading even more worthless.
The installation of an ammeter, whether it uses a shunt or not, places a resitance in the electrical system. Even though the resistance is small, in a perfect world it is undesirable.
A voltmeter reports the car's system which will ideally be 14 volts at all times. If the alternator fails, the voltmeter will drop fast. If the alternator/regulator go overvolt, you will soon see the voltmeter's needle hit a danger area on the gauge.
If the alternator brushes wear out, the voltmeter will slowly read fewer and fewer volts.
Because a voltmeter only places a tiny load on the electrical system it can be placed quite a distance from the battery or alternator and still give an accurate reading.
An alternator in current limit will drop its output voltage. This can be confirmed by temporarily reducing the load on it. With the load reduced, the system voltage should reappear.