But a volt meter tells you the system health in total. 
It will tell you if the charging system is keeping up, what the regulator is doing - undercharging, overcharging, etc. You can tell everything from a volt meter and it's totally SAFE. 
the so-called "idiot light" or alternator fault indicator, or alt light, etc. only tells you of a problem - ot what that problem is, the extent of the problem, or where. A volt meter can tell you how much longer you can drive with a fault, if the car will start next time and more once you learn to read it and understand what everything means. 

I'm considering adding a simple light-based solution to what I do already in having available an LED indicator - green means all is fine electrically, with other lights meaning an over-charge, or undercharge, or a fault such as nothing out the alternator at all. 
That's even better than the ALT light......... 

What are you talking about?  Chrysler used ammeters for decades and ALWAYS inserted them between the battery and the rest of the electrical system.  The intent was to monitor the charging status of the battery, not the entire electrical system.  If the battery was at nominal full charge, the alternator would be supplying all of the electrical load and the ammeter is at zero.  Again, what are you talking about?

LOL - you base using an ammeter vs. volt meter on Chrysler using ammeters for years? They did that for two reasons -

Their charging system didn't have a provision for a light. Their alternator and regulator were the ultimate in simple (not all that efficient, but simple as heck) and they didn't have a place for a light.

That's what they had always done and it was seen as a simple step up from a light - this side means charging, this side means not charging. So they just kept on using ammeters. Cheap, simple, and it was at least a step up from a light - which MOPAR couldn't run from their systems anyway.

The ammeter is for people who don't want to think beyond this side good, that side bad.
But it doesn't tell the whole story. 

First, the volt meter is safer - second, it gives a more information for those who take the time to learn to read 'em right. 

I've also done more re-wires on Chrysler products than any other brand - more wiring fires/burn-ups so unless someone really wants the "cool factor" that an ammeter seems to invoke (ammeters are cool, volt meters are for wimps - is what the general hot-rodder feeling seems to be....) I will always suggest a volt meter over an ammeter.
To be VERY knowledgeable and to really know what's going on, do both............ volt and amp.
But for safety - volt meter, for more information, more accurate and useful information, volt meter. 
For simple good/bad readings, ammeter, not as safe as you could have 60 amps heading into and out of the cabin - MORE if you run a whiz-bang-this is so cool 150 amp alternator!

Uh,  was going to try to be PC, polite, etc. - but frankly it's frustrating when decades of experience and attempting to convey helpful information is slammed, so... PC out the door - I know bloody well where, why and how to wire ammeters and for that matter, entire cars (from scratch I might add) I know how they work, where Chrysler put them, etc. Remember, I wired MOPARS from scratch after wiring fires. 

So far this AM I don't feel like trying to explain 45 years of experience, formal training, courses in electricity and electronics at the HS and college level, doing this for a living for years, etc. to justify my comments.

My thought processes are based on perceptual reasoning - I, myself, am wired differently, so life experiences have far more impact.
To explain years of training and experience on why the volt meter is a better choice technically speaking would take more time and space than I wish to deal with today.........
I stand by my comments - I will finally add that some of it was because some people want an ammeter to "see" the whole system, not just at the battery - between battery and alternator. If you don't explain it clearly they'll do what their shady shade-tree mechanic neighbor has done. 
We have more than just forum members coming here for into.

MOPAR alternators since 1970 couldn't run a light. There was no provision at regulator or alternator for a light. 

People were used to TRACTORs, trucks, and old cars with ammeters and knew this side good, that side bad, but what if it reads charging at all times? Depending on the meter, it should not. Do people know that?

But with a volt meter, they can color the scale - and if it's reading in the red below green that's bad, the red above green, that's bad, green is good......... and as a professional, I can look at a volt meter and tell you a lot............ 
Chrysler had to run a meter, since their simplistic system couldn't run a light, and the ammeter had been what everyone was used to. Also keep in mind even in the 60s and 70s we had people driving who were used to an ammeter....... it was a comfort thing then.

Today it's a "cool thing".... but do people even know what they are looking at? Nope - not in most cases - only that they have these cool gauges so it's a hotter car.

Hammer - that's funny. 
Hmmm, is his name Red Green and is he Canadian, by any chance?  

Well, I am going to be neither PC nor polite.  An ammeter system is cheaper and simpler than a voltmeter system?  In what universe.  Not this one.  As to slamming an attempt to convey helpful information, I didn't criticize you, you are criticizing me.  Do you think you are the only one that has spent 50+ years under the hoods of cars working on electrical systems?  What gives you the right to act like your opinion is the only one that is right?  What gives you the right to dismiss anyone who disagrees with you?  This in my opinion is just another rant of yours where you feel the whole world is ignoring poor little you and the wisdom that just pours out of your mouth.  I've had it.  I am never going to participate in another post where the great God Billd has pronounced the ultimate truth and then criticizes and demeans any conflicting opinion.

if the car is made for one, it's easier to run, since it's there.

to add monitoring, a voltmeter is vastly easier, and billd's totally correct, you can tell as much or more about battery state with a VM.

also, if you upgrade to a large alternator -- for which i never see the point, eg. 60, 80, 100 amp alts in cars designed for 35 -- that amperage will MELT 20, 35 amp wire to/from the battery, if and when that battery gets very dead (leave lights on overnight, etc).  more current (capability) is rarely better, even with a big stereo and added A/C. 

Matt's keys to getting along with people in the classic car world (I often have to rims myself of these):

Seriously. Topics on cars and politics are so much more pleasant if you keep those in mind. 

-------------------------------

I present to you my impersonal opinions and evidence:

Ammeters with a +/0/- scale don't splice into the alternator output wire to measure alternator output current. They splice into the main power feed wire off the battery to measure current either going into or out of the battery from the entire vehicle electrical system. The alternator will keep up with all electrical loads in the vehicle up to the alternator's current rating. The only current going through the ammeter is either going into the battery to charge it (+ reading) or current leaving the battery to power electrical loads the alternator doesn't have sufficient output to handle (- reading). Unless the battery is severely discharged, extreme electrical loads are encountered, or the alternator fails, ammeters will rarely have more than 15A flowing through them. 

Ammeters are hold-overs from the days when cars had third-brush generators that did not regulate themselves. Drivers had to adjust the third brush as needed to provide proper battery charging depending on what electrical loads were switched on and the engine RPM. The easy to read ammeter provided early motorists with a simple "charging or discharging and how fast" indicator for the battery so they could set their generator brush as needed. Ammeters cannot show battery condition or voltage regulator operation however. Because of their potential for carrying high current, ammeters should always be wired with heavy gauge wire and solid connections. 

Voltmeters work fine and provide more information on battery condition and voltage regulator setting than an ammeter can. Unfortunately that makes reading them a little more complex. Failing or defective batteries can cause very strange voltmeter readings as can faulty voltmeter installation. The voltmeter should take its reading as directly off the battery as possible because current flowing through the vehicle's wiring causes IR voltage drop and you don't want the voltmeter to be biased by that. 

Whether you have an ammeter or a voltmeter, a warning light is always a good idea if your charging system has provision for one. A warning light will catch your attention faster than a gauge reading outside of normal would. The light exists to catch your attention with "something is wrong", but the gauge exists to tell you "this is what is wrong". 

The resistor is the shunt.......... thus the name. 
More amperage through a resistance, the greater the voltage drop........ less amperage flow through the shunt or resistor, the less the voltage drop.

I have a Sears Craftsman meter (tach/dwell/volt/ohm/amps) from the 1970s and it operates that way - you put a large resistor in the system and there is a special wire that clips on and measures the voltage drop. That way they can use 18 gauge wires for the leads.

And I think tom or someone brought this up - if you have a so-called "1 wire alternator" these are typically sold to rodders who have the "more is always better" and "more is cool" mind-set - and are usually higher amperage output units than we need or have. 
So............. let's say that 1-wire is a 120 amp unit - you had better measure the length of wire needed, calculate for 120 amps and start putting larger holes and hefty grommets in the firewall. You must plan for the full output to travel through a non-shunt ammeter - into and out of the cabin.

Some will say no, you don't need that as that much won't go from alternator to battery.

Wanna bet?

I had a decent battery charger trashed by a shorted battery. There's also the chances of jump-starting or cranking until the battery is nearly dead and then doing what too many people do "let the alternator charge it" (which is bad idea)

So a shunt type ammeter is a volt meter measuring the voltage drop across a special low-value resistor of extremely tight tolerance (a known value)

A shunt/resistor is placed between battery and alternator, the gauge connects to either side of that.

A typical normal ammeter you need to figure for the full output of the alternator going through it....... just in case things go south. 

Chrysler alternators are A-circuit design, they have two wires. One from ignition to feed the field the other goes to the regulator. The regulator controls the GROUND side for the field. So the field is always hot. Otherwise it has but one wire - the OUTPUT. So there is no driver for a dash light. Chrysler HAD to use a gauge/meter of some sort because their alternator design didn't have a diode trio or isolation diode that allows a light to be used. 
The Delcotron 10si, etc. is also A-circuit field but since it's all internal, and they DO have a diode trio or field diode array inside they can drive a dash light........

The so-called "1-wire" alternators are Delco units that have a third-party regulator in them that senses alternator rotation and "switches on" when the alternator spins up.......... or in some cheapie cases, you have to rev it up to about 1500 to 2000 engine RPM to get it to start charging. 

Anyone with a "1-wire" alternator would be wise to put a volt meter in............ and see if you have the more expensive better design, or some cobbled system that has to rev up high to start charging. 

Where did that come from? The volt meter reads REAL TIME. This means if you are driving along and the meter was reading 13.7 and suddenly drops to 12 it has the exact same meaning as when the ammeter goes from + to -
How can the volt meter tell of past when it's reading SYSTEM VOLTAGE, alternator output voltage, at all times?
If you don't look at it and then there's a problem and you get in the car and the meter reads 11 volts then yeah it is a past problem but to say a volt meter tells you of something that went wrong some time ago is BS, period. I've lived by volt meters for decades- and when you use them instead of ignore them they'll tell you of a problem instantly like an ammeter but with more detail.

I can give a recent real-live example - mine was reading about 14 and I turned the AC on and it dropped way down - still a bit above 12 but not at the 14 it had been. UNLIKE an ammeter that would have likely still showed a bit of a charge, I knew there was a problem before I parked the car.
Bad connection, voltage drop caused by bad connection. It's like an ammeter would not have given me that sort of information. 
If you see the voltage fluctuate, depending on conditions, you can often go right to the culprit.

So explain how a volt meter tells of problems in the past when it's giving you a constant, real-time reading of the alternator regulated voltage?
What if it was reading 14 then suddenly jumps to 15? Isn't that telling me instantly there's a regulator problem to look at? Would an ammeter tell me that or would it just show a greater amperage reading, a higher plus - that may look normal to some? Why is it that people with ammeters can miss an over-charged or dry battery but with a volt meter they know exactly what voltage is proper and when it's too high?

No way a volt meter tells of past issues when it's REAL-TIME. Drivers only need to look - if it's at correct regulated voltage, all is well. If it's lower, it's not keeping up. If it's higher, you have a connection or regulator issue. (or even a BATTERY issue, depending on how smart you are at reading volt meters!)

I've driven cars with volt meters for decades and I get more info from them than I ever have from an ammeter. And I've always been told by the volt meter of a problem AS it happens. 

But you have to glance at them now and then.

I'd still love an explanation of why a volt meter is telling of problems in the past when it's a current, real-time reading of the charging system state.

You can buy a cheap digital meter that sticks in the cigarette lighter,

but it might read 1/2 a volt or more low compared to something wired directly in.

Even the correctly hard wired gauge would likely read low compared to a reading taken directly from the battery.

So you have to know what the voltages should be under operating conditions for that specific gauge & vehicle.

Best way without rewiring an existing gauge would be to see what the gauge tells you in comparison to an actual battery reading.

But if you hop into an unknown vehicle and the gauge reads 11.5v,

it could mean a big problem,

or a gauge connected a long way from the battery through a bunch of crappy connections.

As far as the personal nature of questions, answers and personal responses on a message board like this,

I understand that there are people who can say "good morning" and it sounds like "I hate you".

There are also people who can say "I hate you" and it sounds like "have a great day".

Add in the fact that it can be impossible at times to read intent in written words,

and the fact that most people have their own type of underlying "crazy" that I try very hard to not let any words typed by others affect my mood or mental health.

It can be very easy, or really tough depending on the circumstances,

but its always a good idea to try to insulate yourself from taking stuff on message boards personally.

The only reason to visit sites like this is to improve our hobby & lives (hopefully).

I mostly just try to keep that in mind when I see stuff that makes my eyes roll or my head twitch lol.

Smiles and well wishes to all that have read this far. 

First,  it's 45 years automotive experience, specializing in electric systems. I currently live by testing, repairing, restoring electric systems, especially charging systems.

College trained, as well as factory training from GM, Ford and Chrysler. Further - training by Sun (the test equipment company) as well as others.

So you have a problem - dim headlights, car won't crank well in the morning, that sort of thing.

You drive to a shop and explain you have troubles starting it or the headlights are dim, whatever. A good, well-trained tech comes out with a - get this - VOLT meter. He connects it to a spot close to the battery and has you start the car. He watches the voltage during cranking to see how low it drops, and when the engine starts, he looks at the reading. If it's an AMC, depending on the year, etc. (see the chart I posted showing regulators and voltage settings) and it's a 71 and he sees 14 volts he knows it's charging and the regulator is right where it should be. He'll have you turn on lights, etc. and see what happens. The voltage should stick at or around 14. If it drops - he knows there is a problem either with bad connections or the alternator can't keep up with a load (perhaps bad diode, etc. which he can also diagnose with a volt meter)

If all seems well he'll throw a load on the battery - years ago we'd wheel out the Sun tester, clip the cables on the battery and start loading it while watching the voltage. If it's a 71 AMC without AC the alternator is a 35. He'll load the battery until his meter reads close to rated alternator output - the volt meter should hang around 14. If it does there's nothing wrong with the alternator OR regulator - and he got all that from a volt meter.

If that passes........... there's another test. You've driven the car for an hour, the volt meter reads 14 while it's running, so take it home and park it.

Got out the next morning and turn the key on but don't start it. Wow, the volt meter is just 12 volts.

So what's that mean? The charging system was fine but the battery isn't holding a charge. (or you left the dome light on, etc.)

Again, volt meter diagnosed the trouble.

You are driving and the volt meter reads 14 then you turn on lights and it drops to 13 - turn on the AC and it drops to under 13. It's telling you of a problem here and now.

If it stays steady at 14, you don't have a problem. The charging system works on voltage - it's regulated by voltage. 

The battery represents a load to the system as do lights and other things. More load of course means more amperage draw but with a system regulated to 14 volts it should stay at 14. You keep the voltage constant, vary the load, the amperage will change. Simple ohm's law.
So a volt meter will tell you of the health of the charging system at all times. It won't wait until the battery is dead. Got a problem? first thing anyone here will tell you is to get a volt meter out and connect it across the battery. 
Volt meter in the car will tell you instantly of a problem or a pending problem - AS it happens, not after, because you see it at all times. 
Voltage climbs as you drive - starts out at 14 then slowly or even quickly climbs to 15, then 16 - the volt meter told you that the voltage regulator is toast. 

Voltage drops? it's telling you the charging system isn't keeping up. 
Since the system operates by sensing voltage and regulating at a set, specific voltage, a volt meter will tell you real-time if there's any problem.

As far as WHERE the volt meter is - if it's connected far from the battery and it reads low, that's handy, too as it's telling you that you have faulty connections and voltage drop!! So that's not a problem.. if it reads low, say 13 instead of 14, check with a meter AT the battery and if that's 14 but inside it's 13,, COOL, that handy volt meter in the car just told you that you need to start pulling and fixing bad connections. 

No other meter will give you that information. 

If you suspect a charging system or other electrical problem, a pro like me will first say "get your volt meter out". If you have one in the car - READ the bloody thing and it will tell you if you have a problem or if a problem is STARTING to show up. 

some love their ammeter, that's cool, that's old-school, and people love to see the wide swing of all that POWER. But for a diagnostic tool, an automotive tech trained in electrical systems will use a volt meter or ask you what your volt meter reading is.

I could list a whole list of things that will tell you how to diagnose your systems with a volt meter - I could list what every reading means and how to interpret it. I could list what a volt meter would do in several various scenarios. it's what I do....... and have for decades. 

Not opinion, but factory training and experience. 

If people really want an ammeter - GO SHUNT. I'd not install any other type these days. The early systems, as was said earlier, were lower output so the gauges went up to a whopping ten, twenty, or forty amps but not commonly higher. 
They will tell you only if the charging system is keeping up plus or minus, but not a lot more. I can tell you the condition of your BATTERY with a volt meter, the condition of your wiring, and much more. 

So volt meter or ammeter - your choice based on preference. I know what I'd do......... but if your choice is that you prefer an ammeter based on the opinions here, please consider a modern shunt type and not run that load - and yes, full load, into the cabin. If the alternator fails, you run the whole electric system (except cranking) into the car if it's not a shunt meter.