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Technical Ammeter with 12v alt.

Discussion in 'The Hokey Ass Message Board' started by 00fxd, Aug 20, 2015.

  1. 00fxd
    Joined: Oct 31, 2006
    Posts: 26

    00fxd
    Member
    from BC Canada

    Hi, is there any way to use a stock, in the dash ammeter, in a '48 Chev truck with a 12v internally regulated alternator?
    Thanks,
    Frank
     
  2. Crusty Nut
    Joined: Aug 3, 2005
    Posts: 1,834

    Crusty Nut
    Member

    Ammeter only cares about amps. 6 or 12 volt shouldn't matter. But, a modern alternator will likely put out more amps than the gauge is scaled to measure.
     
  3. You might run a wire between the two terminals at the back of the ammeter to carry some of the current around the ammeter. The original ammeter will now read low but you will get used to it. If you want to know exactly what is happening you can get a friend with electronic meters to check a couple of points to keep in the back of your mind when looking at the original ammeter. This is the way I have it in my '32 roadster.

    Charlie Stephens
     
  4. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
    Member

    You sure about that Charlie?

    Seems that would bypass the ammeter completely?
     

  5. I wouldn't try it. As noted above, most old ammeters were only designed to measure 30 amps, maybe 40 at the most and nearly any modern alternator can produce more than that. Too many amps and you'll risk a fire. 'Bypassing' it will throw accuracy out the window, it may not read at all.

    There is one exception to this; if your ammeter is an 'induction' type and the wire passes through a metal 'loop' on the back of the gauge (no actual connection to the gauge), there's no fire hazard (as long as the wire is big enough for the load) but high current may damage the meter movement.
     
    Last edited: Aug 20, 2015
  6. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    Give it a try. As others pointed out, an ammeter does not care about voltage. And, your 12v alternator will put out 1/2 the amps for the same current so your ammeter will handle an alternator twice as powerful as the old generator.

    The generator was probably 35 amps so you should be good for 70 amps. If it pegs all the time you could try adding a shunt.
     
  7. NO NO NO! Amps are amps, no matter what the voltage. And amps and current are the same thing. For the same load if you double the voltage you halve the amps.
     
    GeezersP15 likes this.
  8. FrozenMerc
    Joined: Sep 4, 2009
    Posts: 3,093

    FrozenMerc
    Member

    Rusty, You have your watts and amps confused. Steve is right on. Most old ammeters are of the shunt variety. They are very low resistance, so as not to cause a significant voltage drop across them. The voltage drop (on the order of 50 to 100 millivolts typically) across the shunt is proportional to the current flowing through it and since its resistance is known, the gauge portion is simply a high resistance voltmeter connected across the shunt that is scaled to directly display the current value. As correctly stated above, the old 6V ammeter won't be scaled to handle the higher amp rating of a 12V generator.
     
    Last edited: Aug 20, 2015
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  9. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    That is more or less what I was driving at. I don't know what electrical loads, or in other words added equipment, the OP has but if the vehicle is basically stock the ammeter should be ok.

    But then it depends what type ammeter he has.
     
  10. Steve,

    I must disagree with your statement "'Bypassing' it will throw accuracy out the window, it may not read at all." My bypass in my '32 roadster cut the reading in half and my lightening fast mind can adjust for that. To the average person the ammeter appears original. Of course I will admit that some people can screw anything up.

    Charlie Stephens
     
  11. 270dodge
    Joined: Feb 11, 2012
    Posts: 742

    270dodge
    Member
    from Ohio

    OK, if I'm reading this correctly the OP wants to retain the originality of the ammeter. So now I'm totally out of the league but could a electrical type guy convert the gauge(change the guts) to read volts while maintaining the original look. Best of all worlds.
     
  12. Actually Summit racing used to offer (free of charge) a diagram how to wire up a modern high output one wire alternator to an older low amperage amp meter. I don't have the diagram ready at my fingertips but I am sure that if someone looked on their site or in an older catalog it could be found.
     
  13. ORIGINAL POST:
    You might run a wire between the two terminals at the back of the ammeter to carry some of the current around the ammeter. The original ammeter will now read low but you will get used to it. If you want to know exactly what is happening you can get a friend with electronic meters to check a couple of points to keep in the back of your mind when looking at the original ammeter. This is the way I have it in my '32 roadster.

    Charlie Stephens

    I may not have been clear. You leave the wires attached to the two posts on the ammeter as well as running the bypass wire across the two posts. I used 10 or 12 gauge wire since the wire will have to carry all of the current in the event of an ammeter failure. Then on the other hand the ammeter may not have two posts and other alternatives will need to be explored.

    Charlie Stephens
     
    Last edited: Aug 20, 2015
  14. Charlie,
    I actually set an older car I had on fire with a 65 amp alt and the Amp meter. We had noticed it seemed pretty bright one night and it felt hot, I blew it off and the next night on the way home I made the last 3 blocks Fireball Roberts style. With my head hung out the window to I could see. I got really bright just before all my wires began to burn. I rolled in the driveway and the missus ran out with my big channel locks and pulled the ground off the battery.

    it was then that I found out that Summit offered a proper wiring diagram for one and it is basically what you are suggesting as I recall.
     
  15. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
    Member


    I wonder if that is due to the specific make up/build of an ammeter?
    In my mind, I just see that bypassing the ammeter entirely
     
  16. Budget36 likes this.
  17. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
    Member

    I get it, the resistance in the wire that Charlie is using, is acting as a shunt for the meter.

    Interesting stuff.

    Thanks
     
  18. 00fxd
    Joined: Oct 31, 2006
    Posts: 26

    00fxd
    Member
    from BC Canada

    I shall try these suggestions,
    thanks men.
    Frank
     
  19. donsz
    Joined: Nov 23, 2010
    Posts: 242

    donsz
    Member

    I have 12 volt system in my 1957 Chevy truck with a 100 amp alternator. The ammeter is stock. It works fine. My guess would be an ammeter that worked in a 6 volt system would be just fine.
    don
     

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