Voltage measurements with the alternator running fluctuate

Question

I own a 1994 Mazda Protege. On Sundays, I measure the voltage at the battery terminals, and register it. First with the car turned off, and then with the car idling.
The voltage with the car turned off is usually between 12.76 V to 12.8 V. With the car idling and the alternator running, it usually goes up to 14.46 V to 14.5 V.  But yesterday something weird happened: the battery is ok, but when I start the car, and measure the voltage at the battery terminals, it does go up to 14.5 V, but it drops suddenly for about half a second to very low voltages, ranging from 10 V to as low as 5 V! Then it goes up again to over 14 V, holds there for 6 to 15 seconds, and then dips again. The multimeter is fine, because I tested it on my other car, and there is no problem, and I tested it on another person's car just to be sure and it also worked well. So the problem must be on the Mazda's side. My questions are the following:

What can cause these fluctuations in voltage?
This second question is just out of "scientific" curiosity: even if the alternator was dead, shouldn't there be a reading of at least 12 V? How is it electrically possible for the voltage to drop so low, if I'm measuring the voltage at the battery terminals and the battery is ok, meaning it's charged and has a voltage over 12 V?

Thanks in advance for your help!

John Lord · Answer

There are three parts to the charging system on newer cars.
1)  The alternator (built-in regulator)
2)  The wiring
3)  The battery

We can eliminate three immediately.  the car starts on its own and the battery is holding voltage.
The alternator is a little more tricky.  You are most likely to see problems in the alternator when it's cold and idling, but unfortunately that applies to the wiring as well.  The fact that you see voltage dropping BELOW the battery voltage makes me think that it's not the alternator.  This leaves the wiring.
Here's what I deduce:  I believe you have a loose connection that firms up as the engine heats up.  I am making assumptions here but my reasoning is sound.  Assumption 1 is that when you were checking voltage, you were putting your meter on the wires and not directly on the battery.

However, i have a diagnostic for you to do that will narrow it down.
 Turn on your headlights while it is doing this weird voltage jiggling.  If the lights stay a constand brightness while the voltage wobbles, the alternator is definitely not the culprit, and it must be the wiring, as when a car is running, it gets all of its power normally from the alternator, so it would behave normally.  Conversely, if you see the lights wavering along with the voltage, then the alternator has catapaulted itself into prime-suspect and it's most likely the voltage regulator.  The regulator may be replacable.  It depends on the alternator.

Voltage measurements with the alternator running fluctuate

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