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Discussion Starter · #1 ·
I'm going to run a 3G alternator that came from a '94 F-150 in my '67 Fairlane. I'm not sure what the amperage rating is but likely 130 amps. If anyone knows how to tell the amperage I'd appreciate knowing.

My questions is compatibility of this 130 (?) amp alternator and an old Stewart Warner 60 amp gage. Is this safe? The car will have A/C FWIW.
 

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In order for an ammeter to work, all the output amperage from the alternator has to run through it....meaning the gauge has to have current capacity to handle the entire amp load drawn by everything electrical in the car, including recharging the battery after a start.

In short, if you've got a 130 amp alternator, you have to have at least a 130 amp ammeter.

Your 30 or 60 amp Stewart-Warner gauge is nothing more than a garage or man cave display piece in the modern world. it's also a major fire hazard if you do try to use it....because you run the output wire of the alternator into the cabin, through the gauge, and back out to the battery. and it's live 12v all the time, plus the fact the load is going to be well above the gauge capacity, it becomes a hot spot due to resistance.

Lot of old car electrical fires that burn the car up were the ammeter as the cause.....the main reason all manufacturers went to volt meters, they are safe.
 

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EricNova is right to a point. The Ammeter in a GM does normally have power to it all the time but it is a shunt circuit and all the amperage does not flow through it. As a matter of fact, not a great deal of current goes through it. But, none the less, it is 12v unprotected and in case of a short the circuit can melt down. I have known guys to install 30amp fuses at both ends of the circuit that would protect it if it grounded out.

In an old Ford, All the amperage did flow through the amp gauge circuit but the amp gauge itself does not connect to the circuit. It just has a loop that goes around the wire and reads the emf (electromotive force) around the wire.

I have a 30amp ammeter in my 53 with a late model alternator along with heat and A/C. I really don't advise it is the best thing to do but I do always know if my alternator is charging. I don't have to wait for it to get down to 10 volts to know something is wrong.

For what it is worth, I did buy a $12 chinese volt gauge the other day with the idea of fitting the meter movement into the original gauge cluster and making it look stock. if I get it to work, I will put a small dot at 12 volts as a reference. Lord only knows when I will get around to it but in the mean time, the 95 amp (?) alternator has not burnt up the 30 amp gauge. Only rarely does it peg but on occasion it does

The good thing about the volt gauge is, all it takes is a wire from any 12 fused source and your done.

John

BTW, If I do get the amp gauge converted to a volt gauge, I will start a thread on it. Maybe it will benefit someone traveling down the same path.
 

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An old style ammeter for 130 Amps would need about #4 heavy battery cable to wire it up...

Voltmeters only draw a tiny amount of power and thin #26 wire is enough...

Fully charged good batteries are about 12.6 - 12.7 volts... engine running voltage about 13.5 - 14.2 volts... starting the engine shouldn't go below 10.5 volts... alternator may show 15-16 volts for a while charging a drained or bad battery...
 

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Discussion Starter · #8 ·
Here's the story guys. I had this set of gauges in my '55 Chevy when I was 18 years old. A loooooooong time ago. I've been into restoring MG's for many years and building this '67 Fairlane is my trip down memory lane. I'm doing it old school and these gauges are going in it. Now, I'm willing to make some concessions and if I have to come up with an old SW gauge that matches the others with a higher amp rating that's ok. Replacing it with a period correct voltmeter is ok too. Either way, I'll make sure it's safely done and it sounds like #4 wire, fused on both ends is the only safe way to run even a 130 amp meter. I'll see what kind of luck I have finding vintage looking meters that match but a voltmeter sounds like the safest bet.
 

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EricNova is right to a point. The Ammeter in a GM does normally have power to it all the time but it is a shunt circuit and all the amperage does not flow through it. As a matter of fact, not a great deal of current goes through it.
In layman's terms it's a -/+ voltmeter that measures the voltage drop across a shunt wire between alternator and battery. (Shunt meaning it bypasses the meter. I can't remember if it's resistance wire, or just a normal 10 or 12 gauge wire that has xx ohms/foot resistance. Of course the most current those systems ever saw was around 40 amps.

The voltage measurement points were on each side of the core support, and the wires to the meter were fused right at the connection points. Kind of a cool way to keep all the charge current from running through an ammeter.
 

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By the way, a good fire extinguisher and an ammeter go hand in hand. Ask my how I know!
:):):)

I remember wring in a full current ammeter in my nearly new 70 Nova. Never had a problem with it, but it was probably an accident waiting to happen. Of course with no A/C, pwr locks, pwr windows, or electronics other than a stock radio, I doubt total current draw through those wires ever exceeded 30 amps.
 

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The GM ammeters with a shunt design are using the voltage drop between two known points in the wiring to make an estimate of the current flow. For example, if voltage right off the alternator is 13.8 and voltage next to the fuse panel is 13.75, the gauge may estimate that as 20 amps. The big catch in this design is that as wiring ages it may have more resistance and cause more voltage drop, which makes this amperage “estimate “ very inaccurate.

Bruce
 

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The GM ammeters with a shunt design are using the voltage drop between two known points in the wiring to make an estimate of the current flow. For example, if voltage right off the alternator is 13.8 and voltage next to the fuse panel is 13.75, the gauge may estimate that as 20 amps. The big catch in this design is that as wiring ages it may have more resistance and cause more voltage drop, which makes this amperage “estimate “ very inaccurate.

Bruce
Exactly. That is a perfect explanation.

But, GM ammeters usually aren't calibrated anyway. we just really want to know if the system is charging.

John
 

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Discussion Starter · #16 ·
So, bottom line, what I'm hearing is that to be safe will require running #4 wire, with multiple fuses, to and from a different, higher rated ammeter. I've found a 100-0-100 meter but that still requires the wire and fuses. It makes me wonder why anybody runs one. I've found a SW voltage meter that closely resembles the vintage ones. I think I'll go that way. Thanks guy.
 

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So, bottom line, what I'm hearing is that to be safe will require running #4 wire, with multiple fuses, to and from a different, higher rated ammeter. I've found a 100-0-100 meter but that still requires the wire and fuses. It makes me wonder why anybody runs one. I've found a SW voltage meter that closely resembles the vintage ones. I think I'll go that way. Thanks guy.
Sounds good. Obviously, you have discovered why most people won't run one. I just wanted to make it clear it can be done safely.

Good luck with it. I have bought a spare amp gauge for the 53 to see what it will take to convert it to a volt gauge.

John
 

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Discussion Starter · #18 ·
John,

You're teasing me now. Is there a way to use an amp gauge to read voltage? If there is I'd gladly modify the amp gauge I have.
 

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John,

You're teasing me now. Is there a way to use an amp gauge to read voltage? If there is I'd gladly modify the amp gauge I have.
All electrical gauges read voltage... in an ammeter they read the voltage drop across a resistance load and the face is marked in amps... I don't know off hand what voltage it takes to max out your gauge... you could take it apart and wire a small voltage direct to the gauge and see what it reads... or read the voltage across the internal load of the ammeter when it's working reading amps... I'd start with a few millivolts to avoid burning out the meter... need a VOM... or start with reading millions of ohms on the gauge... see what it takes to move the needle...
 

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Exactly. That is a perfect explanation.

But, GM ammeters usually aren't calibrated anyway. we just really want to know if the system is charging.

John
Exactly! But the gauge will always be correct (pointer straight up) at no current flow though the shunt.
 
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