Where is the ammeter shunt located?
 

Currently, (no pun intended) I have my fuel pump, ignition system, radiator fan, headlight relays, etc all pulling power from the battery side of the starter solenoid. When any of these items draw power, the ammeter shows a positive current flow. I'd like to rewire all of these through a new fuse block that would be tapped in on the opposite side of the shunt, so that the ammeter accurately shows current flow to and from the battery only, and so that all these accessories don't have to draw power through the fusible link that's intended only for charging the battery. Make sense?

I have a haynes manual, but the wiring diagram only shows wiring for an alternator idiot light.

Is there a central junction point somewhere, or will my best bet just to be connecting to the alternator output wire? Thanks!

In the wiring harness.

I figured that much, I just wasn't sure where in the wiring harness. I'm going to just wire the thing to the alternator output and see what happens.

So far so good. I wish I'd measured voltage before and after, but it seems like everything is getting more power now, and the ammeter reads as it should.

My understanding was that the ammeter "reports" the current by looking at the voltage drop across a long length of wire bundled in the harness. If you don't get any more bites here, I'd ask in the Electrical forum; I'm sure Franklin2 knows. The only wiring diagram I can find is for the ALT light, without the gauge. My truck doesn't have the gauge either. I run an aftermarket voltmeter because it's much more useful.

Hmmm... my understanding of the function of an ammeter was to measure the current flow between the battery and the alternator, so it only indicates if the battery is being charged or discharged. If all the electrical accessories are going full blast, the ammeter would still read neutral provided that the alternator is able to keep up.

I would imagine then that the length of wire you're referring to would be the battery charging wire, no?

It seems to be acting the way I want it to... With the ignition, fuel pump, and rad fan on, but the engine off, the gauge moves about 1/4 of the way to the negative, indicating that the battery is being discharged. As soon as I crank the engine, it moves slightly into the positive side, (to me) indicating that the battery is being recharged.

It is in the wiring harness behind the engine running along the firewall under all that tape.

Your understanding of the ammeter behavior is correct, but I'm not sure if the shunt and the charging wire are the same. I still can't seem to find a wiring diagram.

What I meant to say was that the charging wire would be the one in which the voltage drop is being measured, using the shunt (not that the two are one and the same).

Just to be safe I'll go ahead and measure voltages at the battery, fuse block, etc... and make sure nothing is getting cooked. I think the important part is that everything is running off of the correct side of the charging circuit now, and not bottlenecking through the charging wire.

The fusible link is intended to carry the full charging output which will be close to what you're electrical system is consuming at any given time anyway. Why wouldn't you want you're ammeter to indicate the total output from the alternator? How is the charge wire a bottleneck? I think you'll find that if voltage is below 13 or so idling, that it's not being dropped across the shunt, but rather the alternator just doesn't have enough output to support all the stuff you're running, specifically the fans. I think that if I ran electric fans and an electric fuel pump that I would run the 3g conversion- of course that bypasses the ammeter anyway. Furthermore, the fusible link protects the wiring between the alt and fuse block- unless you've moved your fuse block very close to the alt and battery, I wouldn't bypass that circuit protection. oh well, its your truck.

Connect your accessories to the alternator positive.

I found a website that gives a fairly over simplified drawing of the wiring. The ammeter is a shunt that parallels the charging circuit. It ties in somewhere into the main power lead from the alternator and connects at the side of the starter relay. Thus if you connect anything to the starter relay or the battery, only the charging current goes across the gauge, not the discharge.

If you connect to the alternator, all of the current goes across the gauge. Now the alternator has a very short post so it's hard to connect a lot to it. Thus I suggest using a terminal block such as this.

http://home.insightbb.com/~nathan.pl...inal_block.jpg

I wired this up the other day to make ready for my future electrical upgrades. They are hard to see, but the right three terminals are jumped together and give 12v+ all the time. The left three terminals are jumped together and connect to a very solid chassis ground. The terminals in the middle are un-used but I will eventually wire a relay to provide some terminals for key on hot. I even thought about using the alternator's factory choke output to use another relay to provide engine-running hot.

For what it's worth I did use a clip lead and an automotive bulb to test my wiring and anything connected to the terminal block positive does show a discharge across the meter.

I used #8 wire which is supposed to be good for 50-55 amps. Since it's only a 60 amp alternator I figured that should be plenty for anything I would ever want to connect. It is not fused currently nor do I really intend for it to be. Any accessories I connect will be individually fused. All I am really using the terminal block for is an extension of the alternator's positive terminal.

Here is a very very very basic explanation of how a shunt works.

Consider I have an ammeter that only reads between 0 and 1 amp because it is very small, yet I need it to read say a 20 amp circuit. All I need to do is make a parallel circuit and make sure that the parallel circuit has more resistance than the main circuit. Essentially we make a current divider. Look at the following illustration and math:

http://home.insightbb.com/~nathan.pl...f100/shunt.jpg

The top portion of the circuit has a resistance of 100ohms while the bottom portion has a resistance of 1ohm. The majority of the current is going to flow across the bottom but some will flow across the top. Voltage drop across the two will be the same so we can do some neat things. First on the left I figured out the total resistance, notice that resistors in parallel net a total resistance LESS than that of either of the two. With the total resistance and the voltage drop known I can calculate total current.

Now on the right I know the voltage drop across both sides is 12 volts and I know the individual resistances. Thus I can calculate the individual currents. If I have done everything correctly, the sum of the currents should add up to the total current I arrived at on the left. I did and thus it works. The meter on the top is not a glorified volt meter, it is not measuring voltage drop, etc. It is a real ammeter but it is only measuring part of the current.

Now, on the right instead of having just a ground, wire in all your truck's accessories and the alternator. If the voltage of the alternator exceeds that of the battery, current will flow "backwards" across the main charging wire and the shunt to indicate a charge. If the voltage of the battery exceeds that of the alternator, current will flow "forward" across the main charging wire and the shunt indicating a discharge. This is also where the voltage regulator comes into play. It adjusts the voltage of the alternator output and thus affects current flow. Your accessories will load the system and draw the voltage down. If you get to a point where the voltage regulator basically has the alternator wide open and you still don't exceed battery voltage, the current will flow from the battery and thus you will discharge it.

Some things, such as the starter, draw so much current that they would peg the meter and destroy it. Since we KNOW it will be showing a discharge while cranking, we don't really need the starter to be metered do we? In this case we can effectively connect it directly to the battery and bypass the gauge.

Hope it helps.:-X22

People argue that ammeters don't tell you a lot and are dangerous because the entire electrical load of the vehicle goes across the meter. This CAN be true, but it certainly doesn't HAVE to be true. I will contend that an ammeter tells you more. Example, you can have a dead battery and if your alternator is putting out properly and your electrical load is low your voltmeter can still show 13.5 volts. That tells me NOTHING more than that the alternator is putting out 13.5 volts. I can have a hot battery, dead battery, or no battery. An ammeter on the other hand will tells me if the alternator is working and if the battery is accepting a charge. It also tells me what kind of load I am putting on the system, and if I understand how it is wired, where that load is coming from (to an extent). Not that a voltmeter is "useless" but it really doesn't tell you that much. Honestly, I would prefer to have them both, but I prefer an ammeter if I can only have one.

Thanks for all the information! That diagram brings me back to high school physics. I thought it was fun stuff then, but it was long enough ago for me to forget, lol!

I ended up just running a 6 circuit fuse block, with the input wire connected directly to the alternator with an 8 GA wire, similarly to your terminal block.

sc00ter, the reason I wouldn't want the ammeter to show the full output of the alternator is that I wouldn't know if all my accessories were drawing more current than the alternator could produce. (And, I'd have to move all the factory wired stuff like heater, radio, dash lights, etc to pull power directly from the battery for it to function that way anyway).

I have read a lot about the 3G alternator swap, but with everything wired correctly now, my ammeter will (in theory) tell me if I am exceeding the capacity of the factory alternator. Like you said, the fan is obviously the biggest current draw, but since the ammeter still reads slightly positive with it on and the engine idling, that indicates that the alternator is indeed able to produce enough current for it.

The factory alternator wire may contain a fusible link, I don't know. But the only other wire on the battery side of the starter solenoid is a 12 or 14 GA fusible link going back into the wiring harness. That tells me that anything I connect there has to draw power through that small fusible link, or else discharge the battery.

Staysbroken,

In another thread that I was discussing this on somebody pointed out that there are several aftermarket alternators out there that have a higher capacity and are still externally regulated and would thus very likely plug right in. If they plug right in then odds are your meter would still work properly.

I intend to install HID headlights and an electric fan, and I will re-wire my ignition box to come from my terminal block. I will also someday have an aftermarket radio, etc. If I ever get the money to do the supercharged 347 stroker I want, I'll probably run an electric fuel pump too. I intend to do exactly what you are doing, and that is to use the ammeter as an indicator as to if my alternator can keep up. If I start seeing a discharge it will be time to upgrade.

In my humble opinion though, an "upgrade" that causes the ammeter not to work, is no upgrade at all. Having a gauge that doesn't work just looks ghetto to me.

I prefer a voltmeter over the factory ammeter. An aftermarket voltmeter works the same way as the factory ammeter, just with a wider dynamic range.

Another reason is that an ammeter cannot give you relative information. If your alternator is not powerful enough and you're sitting at a stop light with too many accessories running, you will drain the battery. All the ammeter will tell you is that the battery is discharging, but you have no way of knowing how close you are to draining the battery. With a voltmeter, you can watch the voltage decrease as the battery drains. You have a way of knowing if it's safe to drain the battery slightly before you need to throw it into neutral or shut some things off.

Because of the voltmeter's wider dynamic range, it gives you a much better idea of the load on the electrical system. If you turn the heater on full blast, you can actually watch the voltmeter drop momentarily, and rise again as the blower motor's impedance decreases. Because the factory ammeter is a shunt ammeter, you won't be able to see this kind of accuracy on the factory ammeter.

The third reason I believe the voltmeter is superior is because it delivers a true representation of what the voltage regulator is seeing. If you're having issues and you glance down at the voltmeter and it's telling you that you have 18 volts on your electrical system, it immediately points you to a voltage regulator issue. A similar situation wouldn't be as easily translated with an ammeter, and impossible to see on the factory shunt ammeter.

In my opinion, the only thing that the factory ammeter can tell you can be done by a voltmeter, and then some. A true ammeter may have additional opportunities to prove useful, but is impractical for an automotive electrical system. That's my 2 cents at least; everyone is different.