Originally submitted by T:
A voltmeter requires only the thinnest two wires connected to it to work because the current it draws is only thousandths of amps (called milliamps). Because it only needs thin wires a voltmeter can be easily connected anywhere without interfering with the electrical system.
Once a few simple facts are known, reading a voltmeter is child's play.
A normally functioning car will show the alternator's regulated output voltage (the voltage stamped on the alternator) any time the engine is spinning above 1,000 rpm.
If the engine is idling, the 14 volt output will only be present if the engine is running with the headlights, wipers and Hi-Fi turned off.
After a cold start and with the wipers, headlights internal fans and other loads present the voltmeter will likely read under 12 volts and stay there until the engine rpm increase above 1,000 rpm and the battery is recharged.
A voltmeter tells you that the required system voltage is present or absent when it is expected to be and gives you strong indications of an alterantor that is falling in output over a period of time, or has just dropped dead. A voltmeter will indicate a broken fan belt minutes before the temperature gauge gets around to displaying it.