I copied this from the Autometer.com website. I tried to just put a link but
couldn't because of the way they use frames... It seems appropriate with all
the alternator swapping that's going on. This is from their Tech Tip
section:
Caution:
Use ammeter only on an alternator with a maximum output of 60 amps or less.
Higher output alternators must use voltmeter.
Which is better for monitoring a vehicle's electrical system - a Voltmeter
or an Ammeter?
Short answer:
A voltmeter, by far.
Electrical guru Mark Hamilton of M.A.D. Enterprises points out that amperage
is a measure of current flow, so an ammeter is actually a "flow meter"
that's intended to measure current flow to the battery (under normal
conditions) or discharge from the battery (in the case of alternator system
failure). On a typical flow meter, all output must be directed through the
device to obtain an accurate reading. In the ammeter's case, that means all
the alternator output used to recharge the battery must first be routed
through the ammeter under the dash. Which requires a heavy-gauge cable and
presents a possible fire hazard. And the ammeter itself must be able to
handle all this current flow, so it must have a higher current rating than
the alternator's maximum rated output.
All this might be worth the hassle if the ammeter produced reliable
information. But the ammeter can only measure the amount of current output
to the battery for recharging purposes: When the alternator recharges a
"low" battery, the ammeter indicates a high charge rate; with a fully
charged battery the voltage regulator reduces alternator output, and the
ammeter is supposed to indicate a very low charge rate. But how can you
really tell the regulator has reduced alternator output because the battery
is fully charged? Maybe a diode in the alternator rectifier failed, or the
alternator belt slipped after it warmed up, just as if the battery were
fully charged. Or maybe the meter indicates a medium charge rate most of the
time-does the battery want this much or could the voltage regulator be
overcharging the battery?
On the other hand, a voltmeter works like a fuel pressure gauge-but instead
of measuring fluid in psi, the voltmeter measures electrical system pressure
in volts. Just like a fuel pressure gauge, a voltmeter only needs to tap
into a circuit; all the fuel (or electricity) does not have to detour
through the gauge itself. Voltmeter installation is easy, quick, and safe:
It hooks up to a fused, ignition-switched "off/on" source and does not
require any modification of the circuit used to recharge the battery or any
part of the alternator/regulator system. In short, the voltmeter installed
at the dash will be a stand-alone circuit.
The voltmeter directly measures the result of charging-system performance.
With normal alternator/voltage-regulator function, battery voltage is
maintained at 14.0 to 14.5 volts-and this is reported directly by the
voltmeter. In the event of alternator-system failure, voltage will be low
and continue to drop as the battery discharges. In the event of an
"overcharge" condition, the voltmeter will climb above its normal zone. In
summary, there is no chance for misinterpreting a voltmeter's readings as
can happen with an ammeter.
-Mitch
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