Okay guys, break it up. It's obviously time for a disinterrested third
party to referee this event.
When your electrical system is working properly, either an amp meter or a
volt meter will fluctuate in basically the same direction at the same time
and tell you pretty much the same thing, that all is well, and therefore
the primary effect of these instruments will be to take your eyes off the
road occasionally. In this case the voltmeter wins, because it's easier to
wire and carries less current through the wiring harness, but in reality
they're both pretty useless, which is why most cars have neither of these
instruments installed from the factory. You know the factory would never
want to imply that anything could ever go wrong with the electrical system.
The real value of these instruments comes when something does go wrong with
your electrical system. Here you may have overcharging, undercharging, or
no charging at all.
When overcharging, the amp meter will be reading a few extra amps. But you
may get the same reading if the battery is simply discharged and the dynamo
is working properly, so that indication is not particularly definitive.
However, a volt meter pegged at 20 volts is a pretty good indication that
your generator is about to suffer internal meltdown.
For mild undercharging, the amp meter could show zero, or possibly a few
amps in the red, and the ignition warning light may still not be glowing.
But you could see this same condition if you just turn on the headlights
and all of the accessories at the same time, particularly if you have a
generator with more limited current capacity than an alternator, so that
indication is not particularly definitive either. An amp meter sitting
constantly at 0 tells you practically nothing, but a volt meter sitting
constantly at 12 volts tells you that the battery will surely be discharged
eventually.
For the no charge condition the ignition warning light comes on, so you
already know it's discharging, so what more can these instruments offer?
An amp meter reading -10 tells you it's discharging, but you already knew
that, and that reading won't change until you switch off some accessories
or kill the ignition switch. The volt meter on the other hand offers up
some useful information at this time. With the charging system disabled
and the car running on battery only, your most important concern may be the
state of charge of the battery, especially if you are at the time on an
expressway to nowhere and have no service port handy. 11.5-12.5 volts
means a fully charged battery and some time allowed to get where you're
going without having to panic. 10.5-11.0 volts means about half
discharged, and you had better have plans on where you will be in the next
hour or so. Anything under 9.0 volts means about the same thing as a fuel
gauge pointing at empty, you had better be home, or at a service shop, or
be prepared to walk.
The point of all this is that the amp meter is indicating the rate at which
current is flowing in or out of the battery, but those same readings can be
seen under very different circumstances, some good and some bad. As a
result you may become unnecessarily concerned, or otherwise ignore a real
warning signal, because the thing is not telling you WHY. The volt meter
tells you much the same thing that the amp meter does while the system is
working properly, but when it's in trouble the volt meter is telling you
the state of charge of the battery, which could be a lifesaver. But then
that is why I carry a small DVM in the tool box.
My money says $20 for a DVM, a dime for the volt meter, a nickle for the
amp meter, and two cents for my own advice.
$.02,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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