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Why is my Ammeter Always Pointing To Positive (+) Now?

Discussion in 'The Hokey Ass Message Board' started by 4woody, Feb 22, 2013.

  1. 4woody
    Joined: Sep 4, 2002
    Posts: 2,103


    I just completed an engine swap. The old alt was a GM 1-wire, the new one is a Mopar (Powermaster) 1-wire. All electrical systems were fine before the swap with the ammeter showing slight + while driving, & a slight bump of the needle when electric fan or lights came on.

    Now when driving the needle is half way (or more) up the + side, and I don't think the battery is charging properly (the starter barely turns over after a half hour drive).

    Where do I start looking first?
  2. drptop70ss
    Joined: May 31, 2010
    Posts: 1,172

    from NY

    Put a DVM on the battery while it is running, what do you get? "+" and "-" dont mean much to me, I like numbers.
  3. 327Eric
    Joined: May 9, 2008
    Posts: 1,528

    from Diablo Ca.

    What car. The old generators didn't put out near as much amperage as an alternator. The old alternator may have been weak, and your new one may be too much for the gauge. I ran into this on a 58 Chevy Truck I rewired.
  4. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,186


    Did you put the ampmeter between the altermator and the loads?

  5. 4woody
    Joined: Sep 4, 2002
    Posts: 2,103


    The new alt is wired up the same as the old one (that always worked fine) was, and both alts are roughly the same output.

    My original post was how it behaved on the way to work- it was showing ~15amps to the + side. On the way home it seemed to "Fix" itself and was showing 4-5amps max.

    I'm suspicious when things fix themselves, so I'll get a meter on it, but it is a very old gauge, so I'm thinking maybe the movement is sticky or it is just failing.
  6. trollst
    Joined: Jan 27, 2012
    Posts: 1,999


    As you say...the ammeter is old, you might want to think about swapping it out for a voltmeter, ammeters and alternators aren't a good mix. Typically, sometime when the battery gets drained, for whatever reason, you don't want the ammeter sticking full on as the alternator is really pumping the juice through it, you'll have a fire you can't get to. Its not a matter of if, but when.
  7. how long had it been sitting after/during the engine swop? a low battery will cause the charging system to put out more in the recharging process. my guess would be it was recharging the battery and now that it is charged is charging at the normal rate. is the battery old? if it is you may see the system do it again, due to the fact that the battery may be losing it's ability to hold a charge.
  8. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,186


    With the engine off and the lights on does it show discharge?
  9. keyster
    Joined: Dec 27, 2011
    Posts: 26


    Sounds like a weak battery to me.
    Charges all them time and won't crank the engine.
    Joined: Jul 20, 2006
    Posts: 2,422


    Amp gages are not a very accurate way of seeing what the charging system is doing. Do yourself a favor, and install a volt meter. Once you do, you will get a much more accurate reading of what the charging system is doing, plus you won't have 12 volts to a gage in the car, much safer that way
  11. To "prove" the charging system, all you need to do is check voltage at the battery. With the key off, leave the lights on for 15 min or so, then turn them off and use a DVOM to check voltage at the battery. Start the car, rev it up once, then check battery voltage with it running. The "on" reading should be 2-3 volts higher than the "off" reading. If it is, the charging system is working ok and the problem is the gauge.

    If NOT, welcome to electrical troubleshooting 101....
  12. Mr48chev
    Joined: Dec 28, 2007
    Posts: 28,824


    An ammeter works quite similar to a flow meter on a water pipe. It shows how many amps are flowing in which direction but not a lot of the condition of the charging system or the battery. The more amps that are flowing the higher the reading in the direction they are flowing. If they are flowing to the battery you get a reading on the + side and if they are flowing away from the battery you get a reading on the - side of the gauge.

    The first thing is you need to make sure your battery is in good condition and will hold a charge. If it isn't and or doesn't nothing else is going to work right.

    Then check or have the alternator checked for it's actual output. Ideally an alternator should put out right at 14.2 volts when it is working right and charging the battery and keeping up with the accessories or lights you have on. When the battery reaches full charge it may drop back somewhere between 12.5 and 14.2 but that isn't a cause for alarm.

    Also make sure your battery case is clean and dry on top and the sides. Dirty/damp batteries can be drained by power flowing across the dirty/damp tops.

    I'm just curious on one thing. Why does a guy blow 168.00 or more to replace something that seemed to work great and did it's job and could easily be replaced or repaired with a replacement or parts from any parts house? I can see the need if you have a big sound system with a big amp that draws a lot of power but otherwise the high output alternator really isn't needed and just because all of the cool guys have them isn't a good enough reason for the change.

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