Re: Voltmeter or ammeter??
Actually an ammeter will tell you more, but it's much more
complicated to hook up than a voltmeter.
The ammeter goes between the battery on one side, with the
system load and the alternator on the other side.
The starting circuit does not go through the ammeter.
That way when everything's working correctly, you see how
many amps the battery is getting. The battery only gets
what the alternator has left over after it's provided power
to everything else.
Properly connected - After it runs a few minutes after starting
it should show about 0 - the battery's fully charged - if the
battery is fine.
The alternator is feeding all the load, so you don't see it
on the ammeter.
Then to interpret it, a (-) reading shows you are discharging
the battery, and a (+) reading shows how much it's charging.
When using a voltmeter and an alternator fails, the voltage
drops slowly, by the time it's low enough to notice it's too
late, you are about to start walking.
The ammeter will show it instantly, if you look.
And, a sulfated battery (dying) will show lots of voltage so
you think it's fine. An ammeter will show a constant heavy
charge going to it - indicating something's wrong.
The cheapie ammeters from China aren't worth having, neither
are the temp gauges or oil pressure gauges.
Would you trust your engine to cheapie junk?
Stewart Warner or other quality gauges are expensive, but
still much less than a new engine.