The Stovebolt Site Forums The Shop Area The Electrical Bay Ammeter or Voltmeter ???

I am in the process of rewiring my 1952/53 Chevy truck and I bought a "EZ wire" harness and I have been talking with the vendor and was told don't try to run the original voltmeter gauge as it would catch fire if I did with a 12 volt 80 amp alternator (thank you Jim Carter truck parts again).

So I went to the local NAPA store and bought a aftermarket ammeter then I got it home and it only ran up to 60 amps...so today I went and got a volt meter from the same store....

So what are you guys doing for this gauge??...I know I want to monitor the electrical system of the truck as it runs down the roadway...

Any insight would be great.

MikeC

I think the original 1952/53 gauge was an ammeter, not a voltmeter.

http://www.classicparts.com/1949-53-Battery-Gauge-w_-White-Needle/productinfo/24-701B/#.VVY5zRdHf48

Maybe Jerry or someone else will comment on using a "shunt" with an 80 amp 12v alternator.

I have run a 60 amp 12 alternator with no problems.

On my 1956 I bought an accessory (2-1/2" I think) Volt Gauge for about $25. I took it apart and installed the new gauge in my original cluster. I even modified the face plate to fit so the hash marks and needle are still calibrated...looks good to me and wired right in with my new wiring harness. You might be able to do the same with your gauge cluster.

Mike B

Use both. I put a shunt behind the ammeter to avoid overloading it. A piece of a stainless steel hose clamp in a "U" shape between the ammeter terminals works well. The real question is WHY anybody would want to use an 80 or 100 amp alternator in the first place? P.T. Barnum must be in the electrical parts business these days!

To properly monitor what's going on in the charging system it would also be beneficial to have an onboard hydrometer to monitor the specific gravity of the battery acid, in addition to knowing the charging voltage and amperage, and enough experience at repairing charging systems to correctly interpret those three readings.
Jerry

Some people like voltmeters over ammeter because they don't like that big wire coming and going in the passenger compartment. I think you need to look at the size of the wire going to the ammeter Is it big enough to handle the current. I believe most aftermarket wiring harnesse makers ASSUME you are using a voltmeter.
Jerry, an onboard hydrometer? I don't think I've every seen an onboard one. I have used the glass cylinder type many times. I would think you would have to have multiple sensors (one for each cell). Then rather that getting a value of specific gravity from each cell, have a warning pop up on the dash unit when the specific gravity falls below a set point and indicating which cell is failing. I know there have been DNF's in NASCAR due to battery failure plus the fact that the batteries (2) sit inside the passenger compartment. I had a battery explode on me years ago in a UH-1 Huey. It was not pretty. Got my [censored] chewed out for using one of those big fire bottles on wheels they park on the 'hard stand' instead of that baby one beside the pilot's feet. You may be on to something.

I'm being a little facetious- - - -there's no such thing as an onboard hydrometer, at least not on cars/trucks. The submarine electrician who used to hang out here and try to impress everybody with his knowledge might have had something like that when he was a swabbie.

The point is, neither a voltmeter nor an ammeter paints the whole picture of what's happening with a battery or a charging system. "Normal" voltage in a "12V" system can be anywhere from 9 volts on a hard start to 14.5 Volts or more against a fully-charged battery. "Normal" amps can be from -300 to +100 or more, depending on what's happening in the rest of the circuit and the size of the alternator.

The only way to tell if a battery is discharged, half charged, or fully charged is to check the specific gravity of the acid in each cell- - - -something 90-something percent of the people reading this post will never do.
Jerry

I guess that I am currently in that 90-somthing percentile.
I have done it before and ruined my jeans. Then for a while a lot of batteries could not be opened to be checked.
Now I don't do it for economic reasons.
I have become such an expert at working on these trucks, that I have had to increase the rate which I am now forced to charge myself for my services. It is now more economical to just buy a new battery.
Carl
'Daddy Rich'

I'd bet 95+% will never do - and never miss doing.

My old trucks' batteries are sealed, Absorbed Glass Mat (AGM) batteries. Even if there is a way to measure specific gravity, who cares.

Actually, there is a method of determining the level of charge of a sealed-top battery by monitoring the open circuit voltage at the terminals. Since the battery is the final voltage control, even more so that the "voltage regulator", it is pretty important to know whether it's up to par. A slightly shorted battery with one cell unable to come to full charge can create all sorts of charging system gremlins, and that situation can be extremely hard to diagnose. I've seen plenty of examples of multiple generator/alternator/regulator changes being done before a vehicle got to my shop, when a simple hydrometer test of the battery was all that was necessary to find the problem- - - -one bad cell in the battery!
Jerry

Now that is some good real-world information right there Jerry!
Carl

Excellent tech tip Mike and Gus, clear instructions and lots of pictures...Heck even a jpeg. Pretty involved but looks professional. Hard to believe someone isn't making a replacement cluster as many folks are working on these old trucks.

So which do I wanna use? I am not looking for all that work to do that fancy gauge swap you guys did...I just want to not burn this thing to the ground when I put power to it...

Also was the EZ wire guy telling me a story or not with the wiring?

MikeC

There is a reason the guy at EZ Wire is selling parts instead of fixing trucks- - - -his ignorance is showing bigtime. Even if the original ammeter (not voltmeter) pegs momentarily after startup, it's not going to destroy the gauge or start any fires. Using an 80 amp alternator is a foolish exercise in overkill on charging capacity, but it's not that critical. Run a #8 wire from the back of the alternator to one side of the ammeter, and another #8 wire from the ammeter to the battery cable connector at the starter. Tap all the electrical loads like ignition switch, light switch, etc. off the side of the ammeter with the alternator wire on it. The ammeter will show "charge" and "discharge" normally, just like it did 50+ years ago.

If you're worried about the ammeter pegging out right after startup, cut off a piece of stainless steel radiator hose clamp about 3" long, bend it into a shape resembling the "ohms" symbol on a wiring diagram, and drill holes in the ends so it can attach between the terminals of the ammeter. This shunt will carry the majority of the charging current, and the gauge will deflect a little to indicate charge or discharge. Problem solved!
Jerry

So get the hydrometer before the headlight fluid replacement kit? Just joking, I have the battery hydrometer in the drawer along with the tach and dwell meter and the vacuum gauge. Most kids look in there and go,"what this stuff for?"

HRL really clarified the ammeter/shunt system, so I read a bunch of stuff and now I am confused again! (I know, normal op for me.)

The research I did says.. 80 amp alternator, 80 amp ammeter. So the 30-0-30 is a 30 amp, the 60-0-60 is a 60 amp, and I have an 80 amp alternator. Why do I have an 80 amp Alternator? Because for some reason, they are making them bigger these days for cheaper. I will be running an inverter and power some semi-portable electrical tools, or AC accessories, computers, stereo equipment, etc. I didnt even think about the ammeter issue.

So how do you make the original ammeter (THINK its 30-0-30) with such a big honkin alternator? If I use HRL's stainless steel radiator hose clamp idea and 8 gauge wire, is that sufficient? If so, I am putting heat shrink tubing over the clamp! Maybe a few thicknesses of it.

I am wondering how a person can calculate what ohm shunt is needed for each size alternator, but can't find that anywhere. Also, 80 amps is an odd number for ammeters, so I cant find an 80 shunt, just a 60 or 100.

Unless you are a purist, since the horse has left the barn on your choice of alternator, one option would be to drop the ammeter altogether, peace of mind is worth something and go the voltmeter route. In the interim, get your voltmeter and do the swapout with the original cluster as a winter project. I've never been good with electricity, but it helps me to think about it like a fluid, water for example. Amps = volume or flow of electrons, volts = pressure, resistance = restriction, relay is a valve, shunt = bypass, etc. So you have this big ole pump (alternator), putting out maybe 3 or 4 times the "volume" or amps your system was designed for, and you need to restrict/bypass the flow down to where your gauge can read it. Seems like too much work to me, instead let's just measure "pressure" go with a voltmeter, and move on.

I guess Allen, what gives me pause on converting to volts is, you can measure an almost dead battery and since you are not measuring the flow volume, it reads 12v. As far as I know, without knowing how much power the battery has in its ability to PUSH its voltage, you are really measuring something that makes you feel good, but isnt really telling you very much.

If you measure the "pushing power" (current) of the battery, you are able to ascertain if your alternator is pushing and all of the pushing is equal, thus the center zero in an ammeter. I agree its MUCH easier to just measure volts, but I have a few batteries I have been meaning to get rid of, and they are toast, yet measure 12 volts.

I think this is why they chose an ammeter over a voltmeter in the first place. Since I am on a mission to get meaningful information about all this, want to take this issue further. Hopefully someone here knows, if not, I have a few other sources. I will report back here in any case. I also do not like that much current in the cab, but if you ever have a battery powered clock in your home or business and it stops due to the batteries are DEAD, they still measure 1.5 volts. Not sure how measuring voltage is meaningful. No?

Ok, this may be worth repeating... I have a friend who is an electronic engineer genius that I always ask when I have questions like this. I believe him for the most part whatever he says. (he HAS taken advantage of my gullibility before) This isnt one of those times.. here goes..

Concerning how to properly shunt an ammeter:

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If you want to alter an ammeter to read a "Higher Current" ....... You'll have to lower the shunt resistor value. Current shunts are very low value resistors that are rugged and have to pass high currents at low voltages... The dude that says "use a stainless steel clamp" is correct. It doesn't make any difference the shape...it's the minute resistance value, and the ability of heat dissipation. The tread holes in the stainless clamp strap increases the surface area of the acting "resistor" and will better handle the current generated heat. If you have a "digital ohm meter" that can measure tenths of an ohm or "lower"...you can get a ballpark value of your 30 A meter's shunt by measuring the resistance between the two posts.... If you put another value of resistance (equal to the value you measure) across the posts it will in theory make the 30 A meter into a 60 A meter. Ni-Chrome wire is another way of making a LOW resistance shunt... You can use trial and error, and tin snips, too.
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But it doesn't stop there... THIS is what he said about the feasibility and logic on what meter to use...

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A simple way to describe an ammeter is to call it "a voltmeter measuring the voltage drop across a small fixed value resistor"! On my tractor restorations, I didn't install an ammeter, but purchased a "voltmeter" and used that to monitor the charge voltage coming out of the alternator. It actually gives you more information than an ammeter....... It gives you the state of battery charge before you "start", and tells you if the alternator or generator is putting out the proper charge voltage. Many times a generator or alternator has a defective voltage regulator that either over charges or under charges the battery. An ammeter only tells you the charge current......you have no idea what the voltage is coming out of the alternator or generator. An example....you could have a shorted cell in your battery. The ammeter will show your generator is "pumping out" 20 amps...... "You'll think"....man my electrical system is working great! Problem is: your
battery, with a shorted cell, is actually a 10 volt battery instead of a 12 volt, and the charging system is trying to overcome the short!! With a voltmeter, you would "see" the lower voltage and know the battery has a problem. Charge voltage is typically 14.5 volts. If your battery is fully charged, and you do not have any "load" on except for the ignition system.....you'll not pull much current, and the ammeter will be slightly above "0". With a voltmeter, you'll "know" everything is good because you'll see 14.5 volts with the engine running, and about 12.8V with it off and the "key on".

An 80 amp alternator is for a large "big rig" truck with "out the [censored]" clearance lights and loaded with bells and whistles...... Most pickup trucks and cars that have "heated seats", rear window defrosters, A/C, power seats and windows....etc......have 65 amp alternators. High output alternators are NOT for charging batteries. They power the high current demand options.

I recommend the voltmeter route because of the expense of the heavy gauge wire required from the alternator to the ammeter in the dash and back to the battery. Plus it's an electrical fire hazard! I put an ammeter in my '59 Chevy when I was in High School...... The wires shorted and about burned it up...
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This is not what I wanted to hear, but I know why I was thinking wrong. The battery is accepting the charge from the alternator. You aren't reading battery voltage alone. So, now I get to modify my gauge set. Woohoo!

Cool discussion, I plan on running both when I upgrade to a 12v alt from a 6v gen.

On my 49 the brake light switch wiring can come loose from the switch. They are barrel type connectors that slide on and pinch to two posts on the switch, most of you know this. In any case, the connections sometimes work loose and I loose my brake lights.
The ammeter tells me when this occurs when sitting at idle at a stop. It will show a slight neg deflection with the brakes applied. As I let off the brakes the ammeter will either deflect back to "0" or show a slight pos deflection. I use this to determine if my brake lights are functioning properly.
It's also good when driving at night with the lights on. In this case I know when to bump the idle up enough to either zero out the ammeter or increase rpm more to show a charge when idling at a long stop.

I like a voltmeter as it tells me the basic condition, i.e. charge on the battery with the key ON but no other load applied. Then once started it'll tell me the basic condition of my charging system by indicating how much voltage over the key on voltage reading the charging system is putting out.

Both can and are very useful indicators, especially for those who like indicators

Dave

Too many errors to waste time listing them. Nobody's listening anyway. Untold millions of cars and trucks ran ammeters for 70 years or so. I'm surprised all of us didn't get burned to death in those firetraps! Didn't Forrest Gump's Mama have something to say about this?
Jerry

What has changed is the cheap. Alternators are so cheap and they sell them at such high amperage, your stock 1950 gauge would need some serious shunting. People buy them because inverters work so well with them. 80 amps is entry level when dealing with inverters.

I DID like what he said about how to shunt them properly. With knowing the original resistance, if you have a meter that can read that low, its easy to put the proper value of shunt material across the terminals for the particular amperage alternator. Done correctly, with the proper gauge wire and the proper value shunt, there is virtually no worry about fire, but its something to think about.

I may go with two like you said Dave, not sure yet. The voltmeter is so easy to hook up just about anywhere. Just gotta find a place to put it. And the Tach too!

You could put two discrete terminals in the downfacing part of the dash (underneath). The kind of terminals used for test points. One black, the other red. Nobody would see them and you could just carry around a multimeter in those drawers under the seat pedestal. Whenever you suspect a problem, you can hook up the multimeter.

An original-equipment stovebolt ammeter is about as accurate as driving finishing nails with an 8-lb sledgehammer, so why (with one notable exception) would anyone care if the shunt is totally accurate? Put any sort of shunt in there to keep the meter needle from going full scale in either direction, and get on with being anal-retentive about a thousand other things! If $5.00 worth of heavy wire is going to keep anyone here from eating regularly, why are they tinkering with old trucks in the first place?
Jerry

Ammeter, shows at best charge/discharge. A Volt meter will tell you more, if needed. What did you add to your truck that needs 80 amps? Modern car/truck thinking leans to electric powered stuff like windows, steering, and Tech goodies(includes about 100 sensors, computer, GPS, entertainment and who knows what). Oh don't forget AC and big heater motors, and electric doors to move air around the inside. We don't use much of that stuff, do we? The question, is a good one. Ammeter, or Volt meter. I like both, but then my truck has a radio and a heater. I do have an Alternator, it came with the truck. External regulator and all. It works but, darn, it is ugly. Just looks wrong next to the engine. But that is me. An under dash volt meter makes sense, even an under hood one would make sense. But I can pop the top off the battery cover and put one on the battery if I need. With good wires. Proper sized wire. Even a big amp alternator will be fine. Add a shunt if you need peace of mind. The line between a "Daily driver", Restoration, Hot Rod/Custom, are fuzzy on a good day. Does a daily driver need to be 12 Volt? No. Is a 12 Volt a Restoration? No. Is a 235 a Hot Rod? No. Is my truck fun? Yes. Enjoy yours.

I always appreciate what the older guys have done to get things running. I spent my 40's around people in their 70's and 50's around people in their 80's. They drop by the shop from time to time and we have conversations that are so valuable. I will miss them. They like stopping by because they know I will have a few questions they can argue among themselves about. Its fun!

The problem comes up when those kinds of guys.. ingenious farmer fixers with great common sense and the best work ethic ever come into contact with present day. We have learned so much more about electronics, technology in general. They like to say "Phooey!"

The reason we are very concerned about things like the shunt on an ammeter and the exact resistance needed is because Alternators are getting cheaper for more amps. More Amps means we can run an inverter, etc. Since 40 amps was the standard for a long time, if you put in a shunt for the stock 30 amp ammeter, you could stuff a piece of coathanger in there and it would work fine. The coathanger would get a little warm maybe. So do that with TWICE the amperage and watch that coathanger start to GLOW. But do a few calculations and you can make that shunt be just right on temperature. A proper shunt is all about heat dissipation and the management of it. The closer to the correct shunt you get, the cooler the gauge stays and the most accurate as is possible too.

Inverters are cool. You can run a small fridge...I think I will do that.. in the bed I will have what looks like a toolbox with half of it refrigerated. Cant be without my uh.. milk. The other half tools. When you go to a salvage yard, how many times did you wish you had your Makita grinder with cutoff wheels with you? The possibilities are endless. 80 amps may not be enough.

I'm learning alot in all of this back and forth. Thanks everyone!

Too many misconceptions to waste time correcting! The old saying about being quiet VS opening one's mouth comes to mind! The very idea that an alternator, no matter what the maximum amperage happens to be, will put its full output through a shunt to operate accessories displays a level of willful ignorance that is off the charts.
Jerry

I don't understand why you like to put words in peoples mouths when you don't know something. When you tell people they are fulla crap that endears you to them how? And I know you don't care, but other people who listen might start to wonder when the first thing out of your mouth is stuff like "Too many errors to waste time listing them" or "Too many misconceptions to waste time correcting" about other peoples posts. Then you interpret things like you have something wrong with you... the ammeter takes the full brunt of the current having nothing to do with accessories. You know this, but you argue for what purpose? Because something new was introduced by someone else!

If you are such a genius that you can never be wrong, how about rather than sniping at everyone, explain where my electronic engineer who has done this his whole life is wrong? He is not infallible, but you do this all the time to everyone.

If I make a statement on this forum of the technical nature, it has been vetted. I have emailed people in the know and they have responded. Period. So please if you don't know something, or are just now coming to the realization that with bigger alternators being so prevalent, we need matched shunts, you do not have to respond. It's okay if others have information to share that you don't know.

And you wonder why I take my bat and ball and go home rather
than stand nose to nose with the referee!!!

dg

Amen Denny !!!!

I went to NAPA and bought an ammeter then the next day I went and bought a voltmeter...

I will hook up one of them under the dash and pray when I put a battery into the battery box it wont burn down...I will be doing my testing of this system outside...I already lived through one garage fire I don't wanna go through another one...you guys blow my mind every time...my head is still spinning in circles....wish I listen to my dad when he was around...he was an electrician for 50 years...me...well...I didn't get that gene....

MikeC

There is an old Tex Smith "How to do Electrical Systems" book that has an excellent chapter on this subject. Beg, buy, or steal a copy. It answers all these questions with diagrams and good logic.

Mar-Ktech