The Stovebolt Site Forums The Shop Area The Electrical Bay ammeter wiring

I have a rebuilt gauge set with an ammeter,but I have a 12 volt system and an alternator
Is there a way to use the original ammeter ??

I'm no expert but I do have some personal experience. The voltage isn't as important as the amperage. I put a 60 amp alternator on my '38 while leaving it 6v. I had a couple of "experts" tell me it wouldn't hurt my ammeter so I left it alone. As these old vehicles are on occasion I experienced an occasional hard start when hot which then caused the alternator to put far more than the 30 amps the ammeter was designed to take through it while recharging the battery. I noticed that it stopped displaying correctly after a while. I ended up putting a vintage voltmeter under the dash because I can't trust the ammeter.

Still wondering how to wire and how it will read??

Wire it the same as when you had the generator, of course you have to lose the external regulator. The output of your alternator goes directly to your amp meter. Remember the amp meter is there only to measure the charge/discharge rate of your battery, nothing else. As noted above, an alternator is capable of produceing much more current than a generator thus you can/will peg the needle in a discharged state. One can do several things to solve the problem. One is to use a shunt in parallel with the meter. Getting the meter to read correctly is an issue left to an electronics guy. The othe way, again noted above, is to replace the amp meter with a volt meter. If you convert to a volt meter the wiring changes somewhat.

OK,
I found some other threads on wiring the old style ammeter to alternator wiring.
I guess I'll have to find out how to shunt the amperage going in.

Glad you figured out how to wire the amp meter. Most guys just want the meter to read 'charging' when the system is charging and of course the opposite. They don't pay much attention to how many amps are going where ever.
Hotrodrod Lincoln has said a piece of hose clamp hooked up in parallel will shunt enough current around the meter so it will work. If you are as picky as I be prepared for a lot of trial and error to get it to read exactly how much current is being produced or used.
If you have spare cash, there are company's that will convert your amp meter into a volt meter. It will look the same as before except that it's a volt meter now.

There used to be a number of companies that would do that. Check Hemmings Motor News.

I would like it converted, but as long as it reads so I know the alternator is charging ,I'll be happy

I have an alternator with the stock ammeter, no problems, works as it should. My guess is that since the ammeter measures current, the voltage shouldn't matter (assuming the ammeter is designed to handle the expected current). Actually the 12 volt system probably produces less current than a 6 volt one anyway. And the alternator is only going to produce the current your system is designed to consume. I have a 100 amp alternator, I never show more than 30 amps on initial start-up (starter draw). After that it deflects back to mid-point as it completes charging.
don

A 12 volt system is not the point. The old generators don't produce the current of your 100 amp alternator. Put 100 amps through your 30 amp meter and most likely you will send it to the old car grave yard.
The ohm's law formula of P(watts)=I(current) X E(volts) where for a constant P and you double E (6 to 12) then I will be cut in half. It doesn't work in this case. Boy, my head is starting to hurt.
Also, sending 100 amps through a wire which should carry no more than 30 and you will let the smoke out if you don't burn your ride up.

You want your head to hurt? Add to that, the 80 amp alternator doesnt push 80 amps through the meter normally. Its much less because the battery has to accept the current. However, if you use that 80 amps for things like AC Inverters, etc. You need to put a SHUNT in the circuit. If you don't you WILL burn up your meter, wires that are not able to carry that much, and any electronic device in the circuit.

So, to prevent any surges that could completely destroy your system, let alone a fire that would burn down at least your truck if not your house, put a shunt in the circuit.

Here is the problem, and I have been actively reaching out to every electronic engineer that will listen....

First, do we know the exact amperage rating of the stock meter? I have been telling people its 30 to 50 amps so I get replies back that make sense and can be translated mathematically to the real number since I do not know.

Second, once we KNOW the amp rating of the meter, we need to calculate the number of ohms (This will be milliohm range), that is needed to properly SHUNT the ammeter circuit based on the alternator's amp rating and the battery.

Third, I am really into test equipment and have most of what I need, but I do NOT have a way to measure milliohms. Anything less than an ohm is only one digit and that isnt even close to accurate enough. Anyone having information that could help us with mathematical and electronic heavy lifting, please let us know!

Here is an article that explains it pretty well, but I have questions and the engineers there do not want us to just know a shunt value and place to buy it, they want us to purchase their products.. which is their job.

https://www.bluesea.com/articles/552

The point is: all things being equal; 12 volt devices typically require half the current of a 6 volt devices to operate. If a 12 volt device and a six volt device both need to a certain wattage to work, the 12 volt device would require half the current to make the same power (P=IxE). All ammeters already have shunts (to protect the galvanometer itself). Hence, six volt wired systems typically have heavier gauge wire than 12. My truck has all the normal accessories including fog lights, radio, CB, etc., no problems and the ammeter works as it always has.
don

Don, the meter movement is protected up to the amperage rating of the meter. No more than that. The problem we have when making suggestions here is that if you are wrong and someone followed your advice, they are homeless and carless because their truck burned down in the garage. We want to err on the side of caution when giving advice to others.

Though its true that a 6 volt power source (battery) drives twice the current as a 12 volt, many factors are in play when it comes to power output. Circuit resistance, design of the devices, and best practices come into play. The problem with suggesting that 6 volt wiring is stouter and can handle twice the current is that its 50 year old stouter wiring! Maybe hanging on the terminal by two strands! Since most of the wiring on most of the trucks I have seen is completely shot, I wouldn't rely for a minute on wiring that old unless it has been thoroughly checked. I am sure you do that, but others do not. That IS their problem, but in this case it can also be the problem of their wife and children.

In the very least if you have more than a 50 amp alternator, do what HRL suggests and cut a piece of radiator hose clamp to go in between the poles on the ammeter. It was a great suggestion because it at least gives the extra amperage an alternate path (which is what a shunt is).

If you want to understand how to scale a shunt for an ammeter this might be helpful: http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/ammeter-design/
Warning: Some math required(not a lot, but some) Refer to "parallel" circuits if need be. Hint: Parallel circuit, volts is common, current is different.
To be fair, the warning about fire from old wire is a good and honest attempt to keep us safe. If you need a 100 amp charging system. Then you need to protect to that level. Good wire, proper gauge(wire size), proper fuse(and/or circuit breaker), and safe routing along with protective coverings. All of that stuff matters. Would you use cheap, small, funky wire in your house? Of course not. Don't do it in your truck.

Thanks again Steve. What I want to come up with is a shunt for most of the common alternator sizes. So I am interested in 30, 40, 50, 60, 80, 100 mostly. The problem is, our math calculations would mean more if we knew what the ammeter is rated for! The only clue I have is that GMC during those years used a gauge that reads 50 - 0 - 50. But since GMC made things beefier than Chevy, maybe ours is a 30? Pretty hard to calculate when you don't know.

According to the manual: http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1208.htm
About the middle of the page. Max charging current is 40 Amps. So my "opinion" is 50 Amps for the gauge. Same as the GMC but different color. That would be the "internal" shunt rating. The meter movement is as fragile as any other.

starting to sound like the safest bet is to leave ammeter alone and use a voltmeter somewhere

My 1951 Chevrolet Parts Book yields the following information;

Group 2.275 lists Generators for 1948-51 ALL with ratings of 25, 40, 50 & 55 amps.

Group 2.500 lists Voltage and Current Control Regulators for 1948-51 ALL, with ratings 40, 50 or 55 amp.

Group 2.515 Ammeter only lists one meter for 49-51 COMM and UTILITY.

So the meter must be rated to carry the highest Generator/Regulator output of at least 55 amps.

Mike B

I saw that too Mike. It left me with more questions though. The gauge could still be rated at 50 amps since they over engineer stuff like this by about 10%. Its unlikely its a 55 amp gauge because that would be an oddball. Even with this information, I think we are honed down to a level we can work with. 50 or 60, the difference in shunts is so miniscule that only laboratories have the test equipment to tell the difference.

Check this out...
http://www.summitracing.com/parts/sww-413717

If you read the paragraph it says "Are you still using those 6 V gauges, but are converting your vehicle to a 12 V system? If so, don't fall victim to a burnt-out gauge. Available in several amp ratings, these Stewart Warner ammeter shunts will ensure that your 6 V gauges are not affected by your 12 V conversion."

Although it plays to people that think voltage has anything to do with it, but its a good play because its a necessary item to properly upgrade your system to a higher output charging system.

Problem now is, the geniuses at Summit do not have a listing for the "available in several amp ratings" part so I am battling with them right now trying to get a list of what amp ratings are available.

Good
Freakin'
Grief!

Those dash ammeters are about as accurate as using a piece of log chain nailed to a fence post as a wind sock at the Kansas farm I just visited. If the chain is standing straight out- - - -"It's windy today!". Put a horseshoe-shaped strip of stainless steel across the back of the ammeter to carry most of the current, and be done with it! It protects the gauge, and there's enough bleed current through the meter to make the needle deflect. Anything more sophisticated is a big waste of time and effort!
Jerry

And do you think that enlightening this fact will discourage him?
You are playing into his wheel house.
Carl

42 dollars plus shipping? All I ever used the "Ammeter" for was indication of charge/discharge. To the right of center, good. To the left of center, bad. How many Amps? Far right, Lots. Far left? No good can come from this. At least HRL gave you stainless steel. It won't rust. On my truck that would make part #2. The badge on the hood and the shunt on the ammeter. About as good as it will get.

Talked to a few experts today on Ammeters and Shunts. The way they explained it to me was the Ammeter Shunt is a protection circuit that allows for excess current to bypass the meter so that it doesn't burn out the meter. (knew that already).

Since the ohm values are so incredibly low, the Stewart Warner engineers decided to make a shunt that is acceptable for UP TO 100 amps, and another one UP TO 150 amps. (this I did not know). The Engineer went on to say that it would be impossible to match a specific gauge to a specific alternators output because this protection device has such generic qualities. We are talking about metallurgy and combining metals to obtain very very low ohm values. This makes matching impractical, as well as unnecessary.

To make sure you do not burn up your truck with a 100 amp Alternator or less, get this one:

http://www.summitracing.com/parts/sww-407616

For a 150 amp Alternator or less, get this one:

http://www.summitracing.com/parts/sww-413717

I would say if you have a 60 amp alternator or something less than 80, do what Hot Rod Lincoln suggested a long time ago. (sorry Jerry but I am a little slow).
Get a radiator clamp and shape it to look like the ohms symbol, drill a hole on each end and drop this across the two screws on the Ammeter. Be sure to observe any grounding issues and ensure its not touching anything.

Not so fast! I should know better than to look at a picture at the Summit Site and imagine that shunt being almost the same size as what could go across the terminals of our gauges. When I get it, it is 8 inches long and 2 inches wide and is very heavy. It could double as a giant fuse to power an entire building! (or neighborhood)

It does not change the fact that a larger alternator has the potential of burning your truck to the ground should that meters limit get exceeded. So, more research, more testing, etc. is on the list! Please take this issue seriously. At this point it may require changing out the meter internals for a larger rated gauge.

http://devestechnet.com/Home/Native12VUpgrade

thank you, I will probably do this when the time comes with the new wiring harness ans shunt as described

Ah, there is nothing better than someone finding the one true religion.

Except when they come across someone else who had found a different one true religion.

Some of us do not like to read preaching, proselytizing, and name-calling (and condemning others to eternal ignorance and or to eternal damnation).

Just post what you do, how you do it, and how well it works.

I was not referring to one specific person.

Amen.

SteveH: $40 plus shipping is cheap! If you think of it this way.. you purchased a 100 amp alternator. (why I cant help you) If 50 amps of that 100 isnt shunted each and every time that much amperage occurs.. what if only 40 amps are shunted? The meter is toast. Since that 10 amps makes a huge difference, I would want to make sure the conductivity rating of that shunt is the proper value to protect the movement. If stainless steel radiator clamps have been tested to meet or exceed that requirement, I am all for it.

Dang Deve! Start back up on whatever bad habit which you have apparently been trying to quit.
I get cranky whenever I pointlessly attempt to quit tobacco products.
Just saying (as they say).
Carl