The Stovebolt Site Forums The Shop Area The Electrical Bay Ampmeter to Voltmeter

Ok I got a new panel with gauges for my 1961 C-60 but i never have liked the ampmeter or the tell tell lights. So my plam is to convert the orginal ampmeter in this panel to a voltmeter. I bought a voltmeter for like a 73 Chevy. Hope to try this weekend. If anyone has any advise please feel free. I will keep you posted.

jra88,

Be sure to remember that the ammeter (Bill ) is a device that goes in series with the load, and the volt meter is connected between a voltage source and ground.

You will have to reconnect the wires that were attached to the ammeter.

Stuart

This will bring out a bunch of dissenting opinions but here is my opinion anyway.
A voltmeter will tell you what you already know. If you're driving down the road and you look at the voltmeter, it will say 14 volts. You already knew that. The heater is working, the radio is playing and the engine is running.
Tomorrow you get in your truck and "click" or maybe nothing. You look at the voltmeter, but you already knew it wasn't going to say 12 volts.
Every day you drive down the road and look at an ammeter, it says "0" or slightly above. Life is good. Tomorrow you look at it and it says 5 or maybe 10 amps. You don't check it out today. The next day you put a voltmeter on it and it says 14 volts. You find the receipt and it says this Diehard is only six years old. What the heck? Tomorrow you stop and get a new battery. Life is good again.
A long story, short. An ammeter will give you a warning, if you watch it.

Richard,

I subscribe to your theory. The ammeter actually shows any "work" being done. I have read articles that say, if you know how to interpret it, the voltmeter can show you electrical health quite effectively, you just have to learn how to read it. It is tapped into the same "system", it is just reading a different value.."pressure" rather than "flow".

Maybe the voltmeter can watch the charging circuit better than the ammeter, which looks at the system load. :confused:

Stuart

Hi Stuart,

As Richard pointed out previously, a voltmeter is really hard pressed to tell you anything about the charging system operation other than it is (sort of) working.

On the other hand am ammeter is a 'realtime' instrument that tells you if the charging system is operating by showing which direction current is flowing while the vehicle is operating;

From the alternator/generator to the vehicle (+)(Good)

or, From the battery to the vehicle (-)(Bad).

I vaguely remember reading that the real reason the OEMs started using voltmeters was to save the expense of the larger copper wires required for the ammeter as all the current for the vehicle (other than the starting current) passes through them.

I will take am ammeter over a voltmeter anytime, as I couldn't care less what the voltage is, but I really care to know if the vehical is being poewred by charging system or the battery.

Just my $.02, course I'm a dinosaur and kind of set in my ways.

Ted

Hmmmmmm interesting. I like a voltmeter better. Here is why i do? First a volt meter will tell you the condition of the battery. running at 14... charged and alt or gen working... running a little below 14. something is pulling alot of load. (Im talking a good voltmeter). with engine off ya got 12 or slightly above...battery has charge, or wherever it is... if its down to 8 ya know its bad or low or dead cell whatever. So in my opinion a voltmeter will tell you both if its charging, (wouldnt have 14 if not) and the state of the battery. Ampmeter wont tell you about the battery,,only if its charging..??? Kinda funny how everyone thinks different huh..??? interesting...Course could do like i do sometimes is add a voltmeter with the ampmeter and use both...

fatweed,

I think you explained what I didn't understand about the voltmeter. It is merely a battery/charging monitor. Maybe that's why Detroit found the idea of a voltmeter more attractive than a ammeter...if John Q Public knew how much current his Dolby triple bypass digital amp was drawing, he would commit suicide.

Stuart

well that and maybe it cheaper...only one wire, and doesnt have to be hi gauge wire or in series in the charging circuit.... you know detroit...lets foolem with this cheap stuff and as long as its shiny they will buy it....LOL....

LOL...ya ever notice when ya see them bump bump stero cars how the lights dim every time that bass goes whomp..??? LOL....

The REAL reason that GM started using voltmeters was because the design (yes, even the shunt resistor design) was that they had a tendency to literally burn up as they got older - setting fire to the wiring harness and the cab.

The ammeter design has to do an in-series type of circuit. This type of circuit demands that you use a low resistance in order to work properly. The problem with low resistance loads is that they devour power at an astounding rate. I can prove it mathematically.

P = VI, or Power = Voltage * Current (I).

and V = IR, or Voltage = Current (I) * Resistance.

but also I = V/R.

If you back substitute into the equations:

P = V (V/R) = V (squared)/R

Now lets say that the power resistor shorts out a little bit, and the resistance drops. The alternator is going to try to keep that voltage at 14 volts or thereabouts - it will keep pumping out that power as long as it is able, even in a short circuit condition.

Initially, the power dissapated across that resistor is low, because the resistance is - lets say - 100 ohms

P = V (squared)/R = 14*14/100 = 1.96 watts.

Now the resistor shorts out, for whatever reason. The resistance is now 1 ohm.

P = V (squared)/R = 14*14/1 = 196 watts

That resistor isn't going to last long disappating nearly 200 watts.

This is what in fact happened to a number of GM designs before the 70's when they started putting fusable links in their circuits and started using voltmeters instead of ammeters. They were basically designing out a fire hazard in their wiring diagrams.

Voltmeters read a circuit in parallel, so they use the highest resistance possible in order to get a good reading. When the gauge fails, it tends to fail in the open circuit position, rather than a short circuit condition. This pretty much eliminates the fire hazard in their early designs.

Whether GM admits it or not (and I'm sure they won't admit liability to a fire hazard) this is why they changed the circuit design.

I don't like ammeters due to the fact that they require all the electrical load to pass through them so they need to have big heavy wires,and any bad connections will lead to burning.
I find voltmeters very usefull if they are accurate,14v=healthy alternator/battery charging,less than 12v-battery depleting.

I like the volt meter better because it is a diagnostic tool I understand. Amp meters tell you if the system is charging or not. I want to know more.

Watching the voltage as you start the vehicle gives you a good indication of the starting side of the system too. Voltage drops too low you have a problem.

Once running, if it says 14 volts, you are charging. Much more you've got a problem. Much less you are not charging.

If the motor is not running, you should see 12 volts, less you are in trouble, either discharged or battery is failing or maybe a short or load of some sort.

Volt meter helps me a lot more than an amp meter. But that's just me.

Don't forget that, with the Ampmeter, when it burns out everything is lost, no start, no nothing!

As much as you need to really know, the voltmeter does that! with less heat/load involved!

By the way chevy was still putting ammeters in their truck in the 70's, I have a 71....

If your using an 80 amp or above alternator then you must use a voltmeter. I'm not too sure why but all the experts agree.

I have installed several aftermarket ammeters in various cars and trucks I have owned. These were usually the Stewart Warner brand gauges with the standard two post arrangement. Power in on one side and all electrical loads on the other. If you wanted the gauge to monitor the device, it had to be hooked in series or it wouldn't affect the gauge. This meant, as pointed out many times above, that the poor old gauge had to carry the entire current load for the system.

I have various meters at work for checking current draw. They use a inductive pickup coil around the wire that senses current flow and displays it on the meter in amps.

Were any of the ammeters installed in the later model rigs of an inductive type...ones that didn't have to be wired in series and carry the full load...safer from a failure standpoint!

I personally have never had a ammeter fail, catch fire or short out, and don't believe I know anyone that has.


Stuart

The GM trucks late 60,s early 70,s used the harness itsself for a shunt. They had 2 small black wires with inline fuses fastened at 2 different places in the main feed wires. The ammeter registerred off this. I prefer a voltmeter. Once you learn to interpret it & have an idea of volts & amps, it will tell you more. I have seen many breakdowns from burned out ammeters & damaged wires but never a fire.

You don't have to agree with me about ammeters. It's a free country, and you can use whatever you want in your truck. All I can do is to inform you of the potential safety hazards of this design.

From a systems design standpoint, they are a single point of failure - another reason they aren't good. There have been various designs like the inductive, the fused or the shunt ammeters - but they still have a basic problem that all the current has to go through the big wire feeding whatever ammeter design you have.

If there is somehow a short in this wire - anywhere along the wire, the truck dies - no matter what ammeter design it has. A short will usually melt the large feed wire going from the alternator to the ammeter. If you want to find an example of an ammeter-type design failure, all you usually have to do is look for a melted feed wire in a junkyard vehicle.

And yes, I have had several ammeter-design fires that I have had to deal with. Since I usually deal with 60-66 trucks, this is the ones I have dealt with the most. Usually it just melts the wiring harness, nothing else managed to catch fire. I burned my hands pretty good trying to disconnect the underdash wiring harness before it completely self destructed. The plastic melted to my hands, which took a while to heal.

Anyway, I won't hook up ammeters in any of my trucks anymore. I ran the output of the alternator to the battery, then I set up a fused feed wire to the rest of the truck using a fuse from a later 73-87 truck (the one on the firewall), and ran it directly from the battery.

GM stopped using ammeters because 90% of the people that plunked down their money for a new car didn't know squat about how a car worked. It was easier to put in an idiot light, than explain that "0" on an ammeter was a good thing. Quite frankly, I think an idiot light is just as usefull as an in-dash voltmeter.
A (in-dash)voltmeter will not tell you the condition of the battery. It will tell you what the voltage regulator is doing. Any variation in voltage is going to be more temperature variations than anything else. Any of you stovebolters here know that you adjust a voltage regulator with a thermometer and a voltmeter. You can get a jump start if you have a bad battery and that voltmeter will say 14 volts all day and all the way home. Get a jump on a car with an ammeter and that meter will read high and never go down all the way.
A (in-dash)voltmeter will not tell you the condition of the battery, other than when it's dead. Let's see. Put key in slot, turn key, nothing, I wonder if my battery is dead, I better look at the voltmeter so that I know for sure.
If you are driving down the road and say a fan belt breaks. A voltmeter, a ammeter and an idiot light will all give you an idea of what that sound was.
Only an ammeter can give you an idea of what the battery condition is before it leaves you sitting.

When these trucks were built, the symbol for Voltage was "E". If a shunt wire in a ammeter is a low resistance load then what is a straight piece of copper wire? Just thought I'd ask.
The majority of ammeter problems are at the spade connecters in the firewall bulkhead plugs. It has nothing to do with the resistance going down. On a bad connection the resistance goes up. If the resistance goes up, the voltage drop goes up and the heat goes up. SMOKE.

Given the relationhsip between voltage and current to say that one tells you nothing and that only the other is useful seems to miss the point.

If you wait to look at your voltmeter until your battery is dead its not going to tell you anything. If it reads 14 while the motor is running your charging system is working. If it reads 12 while the motor is running you have a problem.

If it drops below 8 or 9 while starting you have a problem.

If you think a volt meter is useless that's your problem.

Just like the oil pressure guage and temperature guage, waiting until you have a problem to look at the guage defeats the purpose.

Learning the correct way to use the tools might make you better prepared than 90% of GM's customers, but ignoring those tools until you have trouble sure doesn't.

In a day where it takes 100 amps plus to support all the accessories on many vehicles using an amp guage to show what is happening means some real big wire running from the alternator to the guage.

I think that is a problem. But it can be your problem if you prefer.

Guess what folks. If that real big wire powering everything shorts it gets hot whether it runs to the ammeter or to the junction block so that part of this argument is moot. Whether you have an ammeter or a voltmeter, your wiring is smoked.

I like my ammeter, but my generator only puts out about 17 amps. One problem with a non-inductive ammeter is that you need two large wires going through the firewall that carry the full current of the system into the passenger compartment and are subject to fraying or shorting under the dash. The cold cranking amps from a modern battery can crisp things up really fast in that event. All the wires that enter the passenger compartment with a voltmeter rather than an ammeter can be fused.