Re: Ammeter problem?


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Posted by David Sherman [72.47.9.228] on Wednesday, June 30, 2010 at 15:25:48 :

In Reply to: Ammeter problem? posted by Galen [164.214.1.54] on Wednesday, June 30, 2010 at 14:44:42 :

Power equals volts times amps. If your 12 volt alternator generates the same power as your 6 volt generator did, it will do it at half the amps. That's the main reason why ammeters on 12 volt conversions don't show much deflection -- they were designed for the higher currents involved in a 6 volt system. The only cure is to replace it with a more sensitive ammeter. The problem there is that most modern aftermarket ammeters are designed for new vehicles with alternators that are like Hoover Dam compared to the 25 amp units on old trucks, which again means very little deflection. The general rule of thumb for designing meters and gauges in any system of any kind is that you want the normal operating condition to be around half of full scale so you can see that something is happening, but there's also room to show an overload if that happens. With an ammeter on a truck, however, the maximum output current is set by the regulator and most of the time it's loafing along at a small fraction of that. I like to see the ammeter needle go close to all the way over after starting after an episode of hard cranking. I also like to see some noticeable deflection to the left when I turn the lights on.

Probably part of the reason voltmeters are more popular these days is that they are independent of the current capacity of the charging system. The main reason, however, is that they don't require running heavy wiring behind the dash board. A voltmeter will tell you some things that an ammeter won't, but an ammeter will also tell you some things that a voltmeter won't. They each have their place. If you don't feel like finding a more sensitive ammeter, replacing it with a voltmeter would probably be practical.

As far as wiring goes, you basically want the armature (armature) wire from the alternator/generator to connect to the main power block, and then have one wire from that go to one side of the ammeter. The other side of the ammeter goes to the ungrounded terminal of the battery. There may be fuses and switches involved, but that's the basic arrangement. By doing it that way, the ammeter will indicate the net current going into or out of the battery. Any time you hook something straight to the battery terminal or to the fat post on the starter, it's bypassing the ammeter and so the ammeter won't tell you if whatever you have hooked up there is sucking or blowing electrons into or out of the battery. Only two things should be connected to the ungrounded terminal of the battery -- the starter motor and the ammeter. Everything else should go to the other side of the ammeter, possibly via some complicated arrangement if you want, but it still should end up there.

If you connected your new alternator to the battery on the battery side of the ammeter, then the ammeter can't see the current that it's putting out. Likewise if you connected your headlights, say, to the battery side of the ammeter, the meter won't see the current they're drawing.



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