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Old 06-23-2010, 01:05 PM   #1
Joe/Ct
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Post Ammeter vs Voltmeter

I have a '39 Ford coupe that has been converted to a 12v, negative ground system. I also have an alternator, instead of a generator. I currently have an ammeter for display and have been told that, with the alternator, I should change to a voltmeter. Comments/recommendations? Wiring diagram?
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Old 06-23-2010, 01:26 PM   #2
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Default Re: Ammeter vs Voltmeter

If you keep the alternator with polarity change, you have to reverse the direction of the wire going into the ammeter loop. These old Ford ammeters are safe as they use an induction principle so will not short out if an overcurrent condition is met.

If you want a voltmeter to measure the battery, hook the wires directly to the battery for the most accurate measurement. Otherwise run the negative to a convenient ground and the positive to a convenient switched power source.
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Old 06-23-2010, 01:29 PM   #3
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Default Re: Ammeter vs Voltmeter

ive been using an ampmeter on my 57 stude after changing from gen to alt. for 3 years and it works fine
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Old 06-23-2010, 04:59 PM   #4
George/Maine
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Default Re: Ammeter vs Voltmeter

I have the same setup on mine.Its a one wire and when you get the rpms up it shows voltage increase .If you are over changing, with a amp meter you can,t tell.With a voltage meter I think its of more use.
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Old 06-23-2010, 05:36 PM   #5
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Default Re: Ammeter vs Voltmeter

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Originally Posted by George/Maine View Post
I have the same setup on mine.Its a one wire and when you get the rpms up it shows voltage increase .If you are over changing, with a amp meter you can,t tell.With a voltage meter I think its of more use.
im using the 3 wire, they are easy to wire, basically all you need is a switched hot wire going to the alt, and the 3 wire doesnt need to be wound up to about 1000 rpm to make it work

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Old 06-23-2010, 09:22 PM   #6
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Default Re: Ammeter vs Voltmeter

Go with a voltmeter. Shows what the alternator is doing as far as ooutput.

No manufacturer uses amp gauges anymore, there must be a reason, besides having the car go up in smoke.

Simple wiring. Connect hot to any 12V source that turns off when the key goes off (Ignition switch -accessory), and a ground.
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Old 06-23-2010, 09:33 PM   #7
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Default Re: Ammeter vs Voltmeter

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Go with a voltmeter. Shows what the alternator is doing as far as ooutput.

No manufacturer uses amp gauges anymore, there must be a reason, besides having the car go up in smoke.

Simple wiring. Connect hot to any 12V source that turns off when the key goes off (Ignition switch -accessory), and a ground.
The reaso0n that manufacturers don't use an ammeter any more is cost. A voltmeter is cheap and very easy to install requiring only a switched B+ connection and a solid ground. The bad part is that the voltmeter shows only system voltage and not a charge rate. If the voltmeter reads 13.9V is it system voltage or is it using only the battery? I have heard all the horror stories of someone running an ammeter and it burned up destroying the car and such. I've been in the business 48 years and NEVER saw an ammeter do anything of the sort. When they quit, they fail to an open circuit and no current flows. BUT, the ammeter must be matched to the charging ability of the system. A 30 amp unit is useless on a system with a 100 amp alternator and will cook itself if overloaded. High reading ammeters are available but NOT cheap and that's why OEMS use voltmeters. I'm currently using ammeters on my '32 roadster, '68 El Camino, and my next ride toy. All are matched to system output and will last after I'm pushing up daisies.
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Old 06-24-2010, 09:20 AM   #8
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Default Re: Ammeter vs Voltmeter

I think if your voltmeter is reading 13.9 volts, it has to be charging, since batteries only put out 12 volts at best. My only other thought is that an ammeter requires all the generated current to run through the dash to get to the gauge. With an alternator, this will be at least 60 amps or more. This might increase the potential for a catastrophic short. It happened to me once years ago in my old Model A. Voltmeters are easy to fuse for protection.
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Old 06-24-2010, 10:03 AM   #9
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Default Re: Ammeter vs Voltmeter

I agree with Deucemac,amp gauge tells you what your charging system is doing or not doing. Major reason for melt down is, using too small of wire ,poor connections,bad ground. JMO.
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Old 06-24-2010, 01:17 PM   #10
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Default Re: Ammeter vs Voltmeter

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Originally Posted by cbillelder View Post
I think if your voltmeter is reading 13.9 volts, it has to be charging, since batteries only put out 12 volts at best. My only other thought is that an ammeter requires all the generated current to run through the dash to get to the gauge. With an alternator, this will be at least 60 amps or more. This might increase the potential for a catastrophic short. It happened to me once years ago in my old Model A. Voltmeters are easy to fuse for protection.
Actually a decent 12 volt batterywill have around 13-13.2 volts in it.

Also, the only time 60 amps would flow continuously through he charging system, is if it were required.
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Old 06-24-2010, 01:32 PM   #11
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Default Re: Ammeter vs Voltmeter

Some aircraft still run an amp meter but they have to either run a shunt or a shunt wire to control resitance on one side of the meter. With a 70 amp system you have to turn on all the lights, landing light, etc. just to ge the meter to move a tiny fraction but it will show a discharge of about a needle width when it ain't chargin.

We also put in an accessory volt meter to make sure the battery voltage is high enough to start the engine. We mostly use 24-volt batteries but still have some 12-volters. We set the regulators to give near 14-volts on a 12-volt system and 28-volts on a 24-volt system. If the volt meter doesn't read 14 or 28 volts or at least battery voltage, the charging system isn't working. If your charging system isn't working, voltage will steadily drop till it won't run anymore. That's another reason aircraft still use magnetos. You can still run if all else quits.
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Old 07-02-2010, 09:45 PM   #12
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Default Re: Ammeter vs Voltmeter

I just went through this exercise with my '59 Jaguar saloon. I recently added airconditioning. This required an electric cooling fan for the radiator, as well as a secondary small auxiliary heater radiator (with fan) to keep the old beast cool under the hood as well as in the cabin.

Problem was, running at night... headlights, A/C compressor, dual blower motors, electric fan, and aux radiator fan... the new 60 amp alternator was pumping some 35-40 amps, and all of it trying to go through a 50 year old 30 amp-max ammeter. The gauge got so hot I could hardly touch it, and melted a bit of insulation off the incoming wires.

My solution: re-wire the primary ammeter supply wires to a larger gauge, and totally eliminate the ammeter from the circuit. The car came stock with an "ignition" (idiot) light for gross reporting of a catastrophic charging failure.

I didn't want to muck up the look of that beautiful wood dash with a NAPA voltmeter hanging underneath in a plastic housing, so I found a very small, thin, digital voltmeter display on eBay (LED), and fabricated a polished aluminum housing, accommodating a momentary push-to-light button so I wouldn't have that display on all the time at night.

It works great, and gives me the opportunity to monitor system performance under all conditions. The voltmeter requires only 18-20 gauge wire, so no issue passing current. I'm definitely sold on the value of voltmeter information vs ammeter.
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Old 07-03-2010, 07:53 PM   #13
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Default Re: Ammeter vs Voltmeter

A voltmeter will only show what the alternator/generator is putting out(subject to the regulator). If either is not putting out, it will show no more that 12 volts (battery). Batteries did not typically hold more than 12 - `12.5 volts so a voltmeter, if reading something like 12 - 12.5, will show you that the charging system is not working correctly. Generators/alternators should put out 13.7 to 14.3 volts when charging correctly.

Haven't seen a good rationale yet for an amp meter over a volt meter.
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Old 07-04-2010, 09:46 AM   #14
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Default Re: Ammeter vs Voltmeter

The way Henry put the ammeter into the wiring circuits, it did not measure the output of the generator but the current to/from the battery for everything but the starter. In this way the ammeter shows whether the battery is being charged or discharged and how much.

In this fashion if the generator/alternator is putting out 60 amps to the rest of the appliances, the ammeter will not measure this. It would only measure this if the gen/alt failed and you tried to power the appliances off the battery. In which case you have a problem and it is obvious very quickly, whereas a voltmeter will only show a slow decline as the battery is drained. Time to power down all non essential appliances.
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Old 07-07-2010, 12:01 PM   #15
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Default Re: Ammeter vs Voltmeter

I had a similar problem in putting a high output alternator on my XK150 Jag coupe to maintain original appearance. I used a "shunt" to continue to use the original ampmeter. I am not a electrical engineer but you can go online for "rules"/equations to calculate the correct size "shunt wire" (essentially a jumper wire of correct gauge that transfers current around the terminals of the instrument). I have not had problems with my set up but the shunt I installed was a little too large of gauge so the ameter does not give accurate readings. My 39 deluxe has a voltmeter (which has been trouble)--I wish it was an ammeter (which was standard equipment on 39 standard fords).
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