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Ammeter vs Voltmeter

Discussion in 'The Hokey Ass Message Board' started by Joe/Ct, Jun 23, 2010.

  1. Joe/Ct
    Joined: Dec 15, 2009
    Posts: 23


    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?
  2. smiffy6four
    Joined: Apr 12, 2010
    Posts: 333


    I'd go with a volt meter. Simply wire it between any positive wire that's hot when the key is on and a ground.
  3. They both have their uses. Ammeter tells you whether you're charging or discharging, voltmeter tells you how much voltage your battery is putting out. Of course, if your voltmeter reads low, obviously you're not charging, so the ammeter becomes pointless.
  4. 49ratfink
    Joined: Feb 8, 2004
    Posts: 18,084

    from California

    whoever told you that was nuts. had an ammeter for 10 years in my OT truck. no problems ever.

  5. Diavolo
    Joined: Apr 1, 2009
    Posts: 815


    I'd use a voltmeter. Just because your charging system worked the way it was supposed to with an ammeter, doesn't mean anything. +1 about the previous comment about voltmeters and ammeters. Since you are running 12V, you need 14.5V to charge, no matter what. Voltmeter will tell you that.

    Additionally, an ammeter measures current flow. To do this, it must measure all current flowing thru the electrical system. Which means you have to basically run the fat wire from the back of the alternator thru the ammeter. A voltmeter only measures the voltage and doesn't require anywhere near as much amperage to function. It is a standalone circuit. Therefore, the voltmeter is a much safer choice.
  6. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    All you need to do is go to the parts store and get the highest capacity fuseible link and install it in the heavy alternator wire where it connects to the system. Probably the starter motor battery lug. I have had an alternator short out internally. The fusible link will blow immediately protecting the wires and the gauge.

    I prefer an ammeter. A volt meter can't tell you if you left the lights on.:D I always look at the ammeter before getting out to make sure I didn't leave anything on.

    I am talking about an average hotrod and not an air conditioned cruise controlled streetrod with 3 electric fans.
  7. HOT40ROD
    Joined: Jun 16, 2006
    Posts: 961

    from Easton, Pa

    I like volt meters . If it above 12.5 volts it charging. Ammeter are ok. But if there not wired right you can fry your wiring. And I always run a master kill switch.
  8. TagMan
    Joined: Dec 12, 2002
    Posts: 6,156


    The phenolic insulator on the ammeter in my '37 split and shorted out. Burned the insulation off in about 1/10 of a second. Luckily there wasn't anything flamable near it or I would have lost the whole car.

    I like voltmeters, myself..........................................
  9. 39 All Ford
    Joined: Sep 15, 2008
    Posts: 1,531

    39 All Ford
    from Benton AR

    IMO amp meters can be safe if the current used and produced is relatively low, and also (imo) the "shunted" style amp meters are safer than the "thru" type (ala stock 39)

    A fused circuit would make either style safe enough, but I don't think the stock 39 one is really made for a 60+ amp alternator.
  10. bigdog
    Joined: Oct 30, 2002
    Posts: 666


    There have been numerous cases of cars burning up because the ammeter failed and shorted out. As said above, the entire electrical load for the car runs thrugh the ammeter. Voltmeter is a much safer choice.
  11. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    Not if you had a fusible link in the line.

    I know what you mean though. I had an alternator break in rush hour traffic at about 25 MPH. It killed the engine losing my P/S and PB. The truck cab filled with smoke and I burnt the shit out of my palm as I reached up under the dash and yanked the burning wire breaking the circuit. The ammeter was a victim of the short inside the alternator. The smoke would not have been inside the cab with a volt meter but the wire would still melt all the insulation all the way to the battery and the truck might have burned down because I couldn't reach it from my seat to break the circuit.

    I put a fusible link in all of my charging systems. cheap insurance.
  12. Special Ed
    Joined: Nov 1, 2007
    Posts: 6,895

    Special Ed

    There is a reason the automakers no longer put ammeters in their vehicles...
  13. Ice man
    Joined: Mar 12, 2008
    Posts: 984

    Ice man

    What Tagman said is very true with an amp meter. I has just installed a volt meter in my 29 Pk Up. They now make one that fits in the original dash with no cutting, and it shows just what I need to know. Iceman
  14. RopeSeals???
    Joined: Jul 2, 2007
    Posts: 444


    I like annunciator (idiot) lights
  15. Black_Sheep
    Joined: May 22, 2010
    Posts: 1,267


    Voltmeter all the way!

    The ammeter in my Plymouth failed internally resulting in a dead car on the shoulder of the highway. For that reason alone, I feel that a voltmeter would be more reliable.

    The concern about fire is valid too. High current flow and old wiring are not very compatible...
  16. Joe,

    When the person said you should change to a volt meter did they say why?

    Charlie Stephens
  17. Larry T
    Joined: Nov 24, 2004
    Posts: 7,667

    Larry T

    I worked in an auto electric shop in the 60's. I can still remember rewiring under the dash of several cars that shorted in the ammeter circuit. It takes out a lot more than one circuit if they're in a loom. It wasn't a joy to do back then and I sure wouldn't want to stand on my head to do one now.
    Larry T
  18. arkiehotrods
    Joined: Mar 9, 2006
    Posts: 6,477


    A letter from Skip Readio of Ayer, MA, in the August, 2010 Street Rodder magazine, pg 16:
    "Regarding the sidebar in the latest issue of Street Rodder about ammeters versus voltmeters, Stewart Warner makes 100-0-100 ammeters and has done so for years. I use them frequently...By the way, I sent Ceridono a note as well regarding the ammeter/voltmeter section. Stewart Warner has carried 100-0-100 ammeters for years. I use them frequently when working with Powermaster alternators. They're identical to any deluxe instrument except that they have 1/4 inch studs instead of the No. 10 studs. There's already an eight-gauge wire powering the fuse box so the argument about the 'fat wire running through the cabin' doesn't hold up, either, as the other wire on the ammeter (to the battery) doesn't have any current on it 99.99 percent of the time. If you go to the Stewart Warner website you can see how the ammeter chart was formatted ("

    The editor of Street Rodder responded, "Skip, good to hear from you. For readers of SR, Skip is a longtime hot rodder and quite the wiring expert. He also has a rare distinction, having been to more NSRA Nats than most of us can count in his chopped and channeled '34 Ford two-door sedan."
  19. Paul2748
    Joined: Jan 8, 2003
    Posts: 2,182


    Voltmeter, hands down.
  20. Joe/Ct
    Joined: Dec 15, 2009
    Posts: 23



    They said that it was dangerous in that the alternator can put out over 60 amps and I only have an aftermarket ammeter for +/- 30 amps.

  21. claymore
    Joined: Feb 21, 2009
    Posts: 896


    Another vote for voltmeter much safer and very easy to install. And it has all the info you need to monitor your electrical system.
  22. I like my ammeters. If it's wired correctly there is nothing unsafe about an ammeter and I like to know exactly what my charging system is doing.

    Skip is on the HAMB. Hope he chimes in.
  23. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    I'd just like to remind the voltmeter fans that without a fuseable link or a big fuse, a short in the main line to the battery or inside the alternator can still burn your car down even if you are running a voltmeter.

    It's a matter of preference. Being a skeptic, I think the factories went to voltmeters to save a few pennies on the size of the wire required rather than a safety issue.
  24. NVScouter
    Joined: Apr 29, 2010
    Posts: 57


    Well putting double current through your 30amp meter is dangerous. But volt meters are good for cars and a clamp on meter for set up/diagnostics. But I dont see a differnce between real use as both will tell you what you want to know using Ohms law.

    If you have to put a new one in then a volt meter will be easier and cheaper IMHO.
  25. NVScouter
    Joined: Apr 29, 2010
    Posts: 57


    Since both are a link from power to ground how is it safer? I think the install/wiring are the real important part. If a gauge fails and creates an "open" condition its safe if it "shorts" then it can create heat build up and start a fire.
  26. 1950ChevySuburban
    Joined: Dec 20, 2006
    Posts: 6,188

    Member Emeritus
    from Tucson AZ

    Ammeters are NOT a link to ground. They interrupt power. (inline circuit).
  27. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,659

    Atwater Mike

    So that sews it up, eh? Tell that to 3 Pantera owners that got stranded from burned out ammeters. (no shunt in system)
    Voltmeter is safe, as it is NOT in series with main power. Ammeter is safe IF installed with parallel shunt.
  28. NVScouter
    Joined: Apr 29, 2010
    Posts: 57


    This makes sense actualy since your talking heavy wiring and inline to main power VS a voltmeter thats 18g wire would probobly act like a fuseable link and only melt a bit before going into open.
  29. flathead fred
    Joined: Jul 18, 2006
    Posts: 298

    flathead fred

    I agree wholeheartedly about the voltmeter. But I must disagree completely about fusible links, having a wire that is designed to burn under heavy load makes no sense to me and is a hazard. Just use an actual fuse rated correctly for the load and wire size / ampacity.

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