Where's the Ammeter Shunt?
Moderator
Joined: Jan 2001
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From: Virginia
One thing that bothers me in the discussion about which shunt value to use is that the resistance values must be fairly precise (.XXX Ohms) for the gauge to be effective. However if you consider how easy it is for the resistance to change in the average harness, the gauge could be rendered useless fairly easily.
If you think about it, the "idiot light" or "gen" light is really the better solution. It will give you a indication either "I am ok and charging" or "I am not ok". It also will tell you if a belt flies off which can make other bad things happen. The later trucks with the 3g use the idiot light and a voltmeter.
Like I said "voltmeter" or "ammeter" are just labels. You need to do work to move the needle on any meter, even your digital meter requires power to work from the circuit it's measuring. And to do work, you need current AND voltage.
Post Fiend
Joined: Jul 2004
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From: Northern California
The difference between voltmeters and ammeters are basicly how they are wired up, either in series or in parallel, and by their movement. Ammeters zero out in the middle of the movement, where voltmeters zero at one of it's ends.
Voltmeter design : Dc Metering Circuits
I will agree, both are very similar, but not identical and they really should be treated as two seperate things.
Voltmeter design : Dc Metering Circuits
I will agree, both are very similar, but not identical and they really should be treated as two seperate things.
Senior User
Joined: Oct 2012
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I think I may be on to something that would allow these ammeters to function more reliably and accurately. I found a circuit called a Unity Gain Inverter which may allow us to basically reference an ideal charging voltage on it's input and vary the potential on two outputs based on the input voltage. I'll try and put something together on a breadboard in the next few days. If it works, it wouldn't matter what current flows in the charging system because the gauge would be looking at voltage, which as it's often said is a more reliable indicator anyway.
Senior User
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Yes, but it will operate just as originally conceived, and it will be an original gauge. The objective is to have the gauge read a discharge condition when the voltage drops below 13.7-14.2V, or a charge condition above that range. This is way beyond me at present, but it should be possible.
pedant
Joined: Jan 2008
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From: EXTREME southwest CT
Yes, but it will operate just as originally conceived, and it will be an original gauge. The objective is to have the gauge read a discharge condition when the voltage drops below 13.7-14.2V, or a charge condition above that range. This is way beyond me at present, but it should be possible.
14.5V is about the upper range before you risk warping the plates or boiling the battery.
Ideally a good voltage regulator would vary the charging current and voltage, tapering off as the battery reached a fully charged state.
I'm pretty sure the computer controlled 4G & 6G alternators are doing something more than switching on and off.
FTE Legend
Joined: Jul 2010
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From: Northeast, OK
Yes, and 14.4 is dependent on the temp, as a battery can't handle as high of a temp in hot weather as it can cold.
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Moderator
Joined: Jan 2001
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From: Virginia
Yes, but it will operate just as originally conceived, and it will be an original gauge. The objective is to have the gauge read a discharge condition when the voltage drops below 13.7-14.2V, or a charge condition above that range. This is way beyond me at present, but it should be possible.
Turning the old factory gauge into something useful would be a nice project.
FTE Legend
Joined: Jul 2010
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From: Northeast, OK
If you use 12.8 volts as the center of the ammeter, meaning the ammeter itself isn't seeing a voltage, then it would work exactly like a voltmeter. When the voltage goes above 12.8 the ammeter would swing to the right, and indeed the battery would be charging. When the voltage goes below 12.8 the ammeter would go left and indicate the battery was being discharged, which indeed would be happening. And, it would be nice if 14.4 volts could be full scale.
The downside would be that the meter would be quite different than most people would expect, and having your "ammeter" showing heavy current all the time might be concerning to someone not familiar with the truck.
The downside would be that the meter would be quite different than most people would expect, and having your "ammeter" showing heavy current all the time might be concerning to someone not familiar with the truck.
Senior User
Joined: Oct 2012
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Yes, you guys are right. An ammeter behaving like a volt meter is kind of questionable. Plan B: when I convert to a 3G alternator, doing away with the shunt, I'll incorporate the ammeter without any shunt at all, but with a 1A fuse and see what happens. As I'll be running a 4 gauge cable between the alternator and the battery I may never shunt enough current to the ammeter to indicate anything anyway. At that point I'd be inclined to change out to a 6 gauge cable and then...the experimenting might never end.
Postmaster
Joined: Feb 2011
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jim, thats a rating of 55a CONTINUOUS. the automotive world is in the habit of using relatively small wires for things like this which only rarely carry their full capacity. they get away with this because the wire starts to heat up from overload, but is cooled down by wind, space around it, and plenty of time at low load. 6ga is what ford used. personally, i have 1 or 2ga cables to it on mine, but i tend to do things overkill, and my 2nd alternator gets to work pretty hard sometimes to charge the aux batteries when they're drained.
Senior User
Joined: Oct 2012
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I have 4 gauge cable to go in right now, which in itself should be overkill. That's pretty standard for use on starters, and I don't anyone can say these alternators can be called to put out that much. It'll be fused at ~130A anyway. If I draw anything close to that I have other issues to worry about!
Senior User
Joined: Oct 2012
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The ammeter will work with the 3G alternator upgrade.
I am basically finished with the 3G alternator upgrade I did, and as part of it I connected the ammeter and...it works fine, just like before. When running the needle is just to the right of center. With the engine off and the headlights on, it goes just to the left of center. These are minor draws, so I didn't expect it to do anything drastic, but I was happy it behaved just as it did originally.
As for the connections, I connected the yellow lead at the output stud on the alternator, and the red lead was connected to the + battery terminal.
With that...I can see no reason to say that you lose ammeter functionality with this upgrade.
As for the connections, I connected the yellow lead at the output stud on the alternator, and the red lead was connected to the + battery terminal.
With that...I can see no reason to say that you lose ammeter functionality with this upgrade.