Lawrence,
Well, the stupid answer is that a voltmeter measures volts, and a ampmeter
measures amps! ;>)
Seriously, the voltmeter will measure the voltage potential across the
electrical system, from positive to negative, while the ampmeter will measure
the amount of current flowing through the electrical system.
What this means for a voltmeter equipped car is that you should be seeing in
excess of 13 volts when the car is running. The alternator (or generator)
should produce somewhere between 13-16 volts in order to keep the battery
charged. If, for some reason, you only get a reading of 12 volts or less,
something in your charging system has packed up and gone south. A voltmeter is
always hooked up from positive to negative.
On an ampmeter equipped car, the meter is measuring the amount of current flow
that is being drawn from the charging system. Now, just to make it more
confusing, the meters in cars usually read positive and negative, with zero in
the middle. The idea here is to measure how much current the various electrical
thingys in the car are drawing vs. how much current the charging system is
producing. If all is well, the meter should be reading zero or slightly
positive (the extra current should be flowing into your battery to charge it).
If the ampmeter is reading negative, the car is drawing more current than the
charging system is producing. You're headed for a dead battery. Ampmeters are
always put in series from the charging system to the rest of the car.
I've also seen the term ammeter, which may be the correct term for the device I
described. The difference would be that the ammeter measures differential
current, whereas the ampmeter measures total current draw through the system.
Or I could just be full of *&$#, which is probably more than likely. Maybe the
automotive engineers just left out the 'p'.
Waiting on error correction from the rest of the gang.
David Rupert
BSEE GMI Engineering & Mgt. Institute '89
75701.242@compuserve.com
1967 TR4A (rigid axle)
1980 TR7 convertible
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