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Ammeter vs. voltmeter
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Author:  SlantSixDan [ Thu Feb 05, 2004 9:05 am ]
Post subject:  Ammeter vs. voltmeter
There's been a request for a talk on the relative merits of an ammeter vs. a voltmeter. So, I'll throw in my $2.25 to get the ball rolling.

There is a website out there claiming that Mopar ammeters are unreliable, they're a major failure point, etc., and giving instructions for "converting" them (sorta, kinda) to voltmeters. I've read the site and I think the author's offbase. There's nothing "unreliable" or "Failure prone" about the ammeters on our cars. They, like every piece of equipment, have their operating characteristics and their limitations. And, when we drive decades-old cars, we have to remember we're using the machine and all its parts long past the intended service life, so maintenance and refurbishment will be required on parts that are not discussed in the service manual because it was never contemplated the car would be in service long enough for these parts to require attention. One such area is the main electrical circuit (A-1A) where it passes through the firewall and connects to the input and output of the ammeter. Pre-'76 cars run all the car's current through the ammeter, because the ammeter's "shunt" is inside the gauge itself. Post-'76 cars have an ammeter with an external shunt, so only a small fraction of the car's total current is run through the gauge itself. The later, external-shunt type is nicer because it relieves us of the limitations of the original wiring if we do something like install high-demand electrical accessories and/or high-amperage-output alternators. However, none of this makes the pre-'76 ammeters "unreliable".

As far as which device is preferable, some people are of the opinion that a voltmeter is more useful than an ammeter. I disagree. An ammeter tells the driver what is happening with his electrical system in real time. A glance at the ammeter tells you *right now* what's happening *right now*. If you switch something on and charging current goes way up and spikes the ammeter, you get that indication (right now!) that whatever you switched on is drawing a lot of current, perhaps too much. If the ammeter starts pointing to "D" while you're driving, you get that indication (right now!) that the charging system isn't keeping up and you'd better figure out why or the car's going to die. If the ammeter needle wavers and jiggles, you get that indication (right now!) that the charging voltage is uneven and the system may require attention.

A voltmeter, on the other hand, tells you the condition of the system over time rather than "right now". You cannot tell from the typical dashboard voltmeter what your alternator's doing, though it will be happy to tell you "Hey, bud, your alternator quit charging about 75 miles ago". You cannot really tell from the typical dashboard voltmeter the effect of switching on this accessory or that one. You cannot tell from the typical dashboard voltmeter that your alternator's brushes or your voltage regulator are still working but ailing and in need of attention. A voltmeter will tell you whether the system voltage is normal or subnormal, but so will turning on the headlights and seeing if they shine brightly or dimly. So will cranking the engine and listening to the cranking speed. So in my view, an ammeter is much more useful. The most useful combination, of course, is one of each. But if there's only one gauge space, make mine an ammeter, please. And I'm certainly not going to hack an original dashboard to cut in some generic voltmeter that won't look like the rest of the gauges, thank you very much!

But this, as I mentioned in another thread, is why they make different flavors of ice cream. Some like this kind, some like that kind, and some like neither.

Discuss!
Author:  64 Convert [ Thu Feb 05, 2004 9:20 am ]
Post subject: 
I agree that an ammeter is the gauge of choice. Granted, it can't accurately tell the condition of a battery, but neither can a volt meter until it's probably too late. I more often want to know the condition of the alternator than the battery.

The only problem I have with the old ammeters is the fact that when they fail, they can make a real mess of things. That said, among the 30-some early Mopars I've owned, I've never had one fail, though I've seen several where it did happen.

As far as electrical feed mods, the only change I want to make to my ammeter-equipped daily drivers is to bypass the bulkhead connector. If I could find an insulated feed-thru for screw terminals, I'd like to add that to my truck that's now undergoing restification. I know they make them, but I haven't been able to locate one now that I need it.
Author:  SlantSixDan [ Thu Feb 05, 2004 9:40 am ]
Post subject: 
Yep, agreed, it's nice to bypass the screwed-together firewall connection. Easiest and cleanest way is to remove the connector from the bulkhead slot so it's "see through", push a nice 8ga wire clear through from one side to the other, and shoot the gap full of RTV Silicone to serve as a weatherseal and grommet.
Author:  Pierre [ Thu Feb 05, 2004 1:37 pm ]
Post subject: 
A volt meter is as real time as an ammeter, your statments are misleading. I have no problem with you liking your ameter, but I think your statments are misjustified....

If your alternator is dead, your voltmeter will tell you, in real time, right now, that the voltage is 12.6 or below. If it is charging, it will be in the 13's or 14's. If your voltage regulator is messed up, you will see the guage swinging back and forth. If you switch on something, and your ameter spikes, your voltmeter will drop and show a lower voltage. If your wiring is fatigued, and when you turn your headlights on, the voltage at the fuse box drops, an ameter won't indicate that, only a voltmeter placed at the fusebox would.
Author:  64 Convert [ Thu Feb 05, 2004 2:03 pm ]
Post subject: 
Every factory equipment voltmeter I've had in my cars contain nothing more than a green range. The meaning of the needle being in that range was not clearly described in the manual and movement of the needle was so slow and so small, there was no way to tell if it was charging or not.

My '03 Grand Caravan doesn't have a gauge of any kind.
Author:  SlantSixDan [ Thu Feb 05, 2004 2:16 pm ]
Post subject: 
64C: Yes, exactly what I'm talking about. Sure, the picture changes with a really *good* voltmeter like Pierre sounds like he has, calibrated accurately and precisely, but I am referring to factory-type low-precision gauges that convey only qualitative, not quantitative information. "Green zone" vs "yellow zone" vs "red zone" kind of thing. Since that's the limitation of factory-style gauges, the qualitative amps ("charging or discharging the battery?") is a lot more informative than the qualitative volts ("Sorta kinda highish, maybe a little on the low side...I donno, the volts are somewhere between green and yellow?").
Author:  LASwinger70 [ Thu Feb 05, 2004 2:52 pm ]
Post subject: 
Ummm.... the website that you refer to more specifically is anti-bulkhead connector and extra connections on the back of the ammeter.

By doing the bypass you eliminate weak links and possible fir hazards.

Now, you need something to tell you what is going on with the charging system, and a aftermarket voltmeter which is at most $20 bucks will tell you exactly what voltage you have, and it will tell you at the sense point at the main power junction you just created, giving you real time picture of what is going on with system.

Ammeter are a nice idea, but especially if you upgrade your electrical system to a more high current alt set up to run all your modern electronics and have better output at idle, you definitely do not want to have all that current going through a dinky gauge built in to your dash.

I personally have only the factory fuel gauge hooked up in my 70 Swinger, Water, Oil Pressure and Voltmeter are on a 3 gauge aftermarket cluster mounted by my e brake handle under the dash by my leg.

So there.

Tim
Author:  steponmebbbboom [ Thu Feb 05, 2004 7:10 pm ]
Post subject: 
actually the power goes right through the gauge without any real resistance, the gauge is operated by electromagnetic induction. As long as it is sized correctly and the factory ammeter was designed specifically for the car it was put in. I havent seen a voltmeter with accurate increments in a long time, except on analog Simpson test meters.

I have a question though, if you wire the headlights direct to the back of the alternator instead of downstream of the gauge won't that affect its ability to read the power draw of the headlamps?
Author:  Pierre [ Thu Feb 05, 2004 7:17 pm ]
Post subject: 
Yes the older amp gauges have a solid bar of metal that connects the two studs together. I'm sure it can handle 60a fine. I disconnected mine and joined the two wires together because I installed an alternator capable of about 4 times that (wired directly to the battery via welding cable).

Hm, no I think hooking up lights directly to alternator will not affect a stock setup, if you hooked them directly to the battery thats where it would throw the ammeter off.

I built my own voltmeter and its capable down to.. oh, .01 accuracy or so. Still though, in relevance to stock voltmeters, they do their job even if its just a 3 range red/yellow/green type. I'd still rather have a somple voltmeter then any ammeter. You can still diagnose charging system health and use them as you would a quality meter, just your values would be +/- maybe 0.3v instead of +/- 0.001 like with a digital multimeter.
Author:  SlantSixDan [ Thu Feb 05, 2004 8:31 pm ]
Post subject: 
Quote:
if you wire the headlights direct to the back of the alternator instead of downstream of the gauge won't that affect its ability to read the power draw of the headlamps?
If you take your power feed for the headlamps from the alternator's B+ terminal, you will not see the headlamps' draw on the ammeter, but I normally do it this way regardless.
Author:  GuyLR [ Fri Feb 06, 2004 1:23 pm ]
Post subject: 
I've never had an ammeter fail on any of my old A bodies (all pre-'67). The ability to see which direction and how much current is flowing is a great way to see what's going on in the charging system. Just don't try to pull your instrument panel without disconnecting the battery!!! The shower of sparks is somethin' to see.

If you add a voltmeter to get a read on battery state of charge you can pretty much know all you need to know.
Author:  steponmebbbboom [ Fri Feb 06, 2004 6:08 pm ]
Post subject: 
I prefer the ammeter in my car and other vehicles more because if I want to know the voltage, I can always throw my meter on it. It's nice to already have an ammeter wired into the system to tell me the other side of the equation without me having to disconnect the battery and jump the clamp.
Author:  Pierre [ Fri Feb 06, 2004 6:10 pm ]
Post subject: 
Clamp on ammeters...
Author:  steponmebbbboom [ Fri Feb 06, 2004 6:18 pm ]
Post subject: 
Quote:
Clamp on ammeters...
Hey, I'm cheap :lol:
Author:  Pierre [ Fri Feb 06, 2004 6:20 pm ]
Post subject: 
You can get some cheap ones, like the ones you see in battery chargers.
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