Either that, or the whole rest of the truck draws so little current that you can use most all of the 60 amps for your accessories!

Like I said, I won't KNOW that I need an upgrade until I get everything installed, just like to know what my options are. Good to know though!

 

Good post, I understand the big picture better now.
I am still going to recommend a 3G ALT as it can put out about 20 amps at idle. This will be important with your cooling fans. You can still use your current wiring if you install a 60 amp breaker to limit the current to match your current wiring.
I bought a fresh rebuilt 3G at the junkyard for $9… and now you know how really cheap I am.
It really might be fine as-as, but if you are going to spend money I would spend it on a 3G ALT as it is a much better design, it has dual Y winding.
Jim

 

 

All of your logic makes sense to me, and it's entirely possible the ammeter as originally designed by Ford acts as a true ammeter. If that's the case, most of the current flows through the shunt, but a small fraction flows through the ammeter (the amount of which is proportional to the overall charge/discharge current) since the shunt and the ammeter are in parallel. It would make sense that the needle deflection is directly proportional to the current through the ammeter itself.

I found a thread on a Mustang forum where an individual claimed to have measured the resistance of the gauge itself, and he stated that the total resistance of the ammeter (coil plus wiring harness) was 0.25 ohm. That would indicate that the shunt ammeter in this application is a true shunt ammeter. However, this was for a '66 Mustang, so it's possible the topology changed later on.

However, I was under the impression that the shunt was mysteriously long when unwound from the harness - which would make it seem as though the shunt was intended to develop a finite resistance, which would support method (2). If the gauge used method (1), the length of the shunt wouldn't matter.

Either method would make sense:

(1) The shunt is a dead short, and the ammeter (of finite impedance) is in parallel with the shunt to silently observe the current through the parallel combination. The needle deflection would be a function of the current through the gauge terminals. Or,
(2) The shunt is a finite resistance, and the "ammeter" is actually a high-impedance voltmeter in parallel with the shunt. The needle deflection would be a function of voltage across the gauge terminals.

Both methods report the same information, just using a different aparatus - therefore the purpose of the shunt is different between each method.

It could certainly make sense that the factory ammeter acts as a true current-sense meter, and not a voltmeter which indirectly reports current in a parallel circuit using a known resistance (although this is done in several applications). If that's true, though, it would refute the claims of several threads in the past, and several websites on the rest of the internet. However, just because it's in another thread doesn't make it right. The claims of one misinformed individual can quickly propagate to several sources. At the very least, some updated information would benefit several people.

What would really be helpful is if someone was able to measure the impedance of the ammeter as used in this generation of Ford trucks, and bench test it to see how it actually reacts.

I do know that the needle in the gas gauge works by the amount of current flowing through the terminals. The rheostat in the sending unit controls how much current flows. I don't see why the ammeter gauge topology would act any differently. It also would seem counter-intuitive to put a finite resistance upstream of the vehicle's electrical system.

I found another website that claimed to have actually measured the resistance of the shunt to be 0.10 ohm, which would support method (2), and this was the basis of their argument. That resistance they measured could have just been the lead resistance, when the shunt is actually a dead short. That would make method (1) the more likely candidate.

Based on this analysis, I think the rumors that have been floating around the internet about the factory "ammeter" really being a voltmeter in parallel with a finite resistance shunt are false. A true "shunt ammeter" is just that - an ammeter in parallel with a shunt. It wouldn't make sense for it to be called that if it was something else.

Thanks for the insight - reps sent

 

The internet has a really nasty habit of turning false information into gospel by repeating it often enough. Truth is I don't know the exact physics behind how it works and I really don't have the inclination to rip my truck apart trying to find out. I know that I looked at the situation, and the simplified drawings that I found with my dad, who is an electrical engineer, and we reasoned how it SHOULD work.

Ultimately I determined that in order to properly register aftermarket accessories, they need to be connected to the alternator positive and that is all I really cared to know. Honestly, that's probably all I'll ever mess with it! It's been a great discussion nonetheless.

 

Quote: "I do know that the needle in the gas gauge works by the amount of current flowing through the terminals. The rheostat in the sending unit controls how much current flows."

It is my undderstanding that the 5 volts pulsing to the sending units from the ICVR is constant therefore, making the GROUND OUT of the sending unit (where the resistance of the ground out) is what registers on the gauge making it work.

 

You are 100% correct. One side of the fuel gauge is powered. The other side of the fuel gauge goes out to the sending unit, and is grounded through the rheostat within the sending unit. The sending unit varies the amount of resistance between the negative side of the fuel gauge and ground.

For a full tank, there is 10 ohms between the negative side (what you're referring to as "GROUND OUT") and frame ground (ignoring wiring harness resistances). This increases the amount of current going through the gauge, and the needle deflects to FULL. For an empty tank, there is 70 ohms between the negative side and frame ground. This decreases the amount of current going through the gauge, and the needle deflects to EMPTY.

 

They don't hardly move ever.

There is even an official Ford TSB explaining it...

Ford Ammeter Gauges: Do They Work? | MILEPOSTS Garage (Tech Tips)

Josh

 

The Ford ammeter is just that, an ammeter.

I have one on my workbench at this moment. It has a very low resistance (calculated from other measurements at about 0.2 ohm), and requires about half an amp to deflect the needle to the first division (halfway to the C or D from the center).

As Nathan states, most the current flows through the extremely low resistance shunt in the wire harness. With these low-resistance circuits, any loose/corroded connections in the wires leading to the meter will vastly alter the sensitivity of the ammeter.

Pat in Sebastopol

 

The TSB is nice and all, but I believe the article is actually somewhat flawed. They say that the system was fundamentally flawed and why Ford ever kept using it was a mystery. I disagree. Consider this:

The ammeter in the truck, car, whatever, is showing a representation of the actual current. Really that doesn't matter. If properly calibrated it doesn't matter it the gauge reads 60 amps or 6 amps, the deflection based on a portion of that current is going to be the same. Let's just look at a picture and think about this for a minute:

Consider that the alternator on my truck is a 60 amp model. Therefore, on the off chance that somehow that alternator put out ALL 60 amps and there was no other drain, the gauge would need to be able to handle 60 amps right? So it would make sense for the middle to be 0 and the C to be 60. It would make sense for the gauge to be at least somewhat linear, so we could assume that the big D is -60. This would imply that the halfway point would be 30 amps. Halfway in between that would be 15 amps. Low and behold, my truck has 100W high beams. Assume a non running battery voltage of 12.8 volts. 100W/12.8V = 7.8 amps. There are two bulbs so 7.8*2=15.625 amps. Since there is nothing else in the truck drawing any power and the alternator isn't supplying any, we would expect the gauge to be floating somewhere around 1/4 toward D wouldn't we?

Look at the picture:


The gauge doesn't lie, the gauge isn't somehow fundamentally flawed! What is flawed is people's perceptions about how much it should move! Drain your battery to the point where it is flat dead. Then jumpstart the truck and then remove the jumper cables. You'll notice that the battery charges HARD for a little while and then as the battery gets closer to fully charged, the charge rate tapers off until it is at a rate that is barely perceptible on the gauge. If you turn on the lights, and the alternator is able to keep up, you're not gonna see the gauge move. It's doing EXACTLY what it's supposed to be doing. If you're sitting at a traffic light at night in the rain with the radio on and the gauge happens to drift a little bit towards D, that's fine. The amount of deflection gives you some idea how quickly you're draining the battery!

This is where it becomes important to know something about your battery. Consider if you had a battery that was rated at 60 amp hours, this means you could theoretically discharge it at the rate of 60 amps for an hour. So how long do you think you can idle it at that traffic light with it discharing by 6 amps? If you said 10 hours you would be right. You'll also notice that if you do sit there and let it discharge for a while, then when you hit the gas the gauge will jump to the charge side. It will charge hard for a little while and then taper off. Really watch the gauge and you'll see it really does work, it really does tell you quite a bit.

Part of the reason that I believe in the usefulness of ammeters is that my dad's 1946 Chevrolet Pickup has one and it gets driven so infrequently that the battery is ALWAYS questionable. I can tell by the behavior of that ammeter if I'm gonna be left stranded or not. Example, consider the battery has been sitting for a long time and hasn't been charge and the truck has been sitting long enough to let the fuel evaporate out of the carb. This means the mechanical fuel pump is gonna have to pump fuel up to the carb before it ever has a chance at starting. If by some miracle it manages to do that before the battery completely dies and it does start, the ammeter is gonna show one helluva charge. As I drive the truck I can keep an eye on the meter. Now the GENERATOR is not meant to put charge a completely dead battery, but I should notice some change on the gauge. If I drive for a while and notice that it continues to charge hard and never steps down, that's a good indicator that the generator is throwing current to the battery but it's not actually charging. This means that if I park the truck anywhere other than home, I'm probably gonna be stranded! Likewise if I fire the truck up and it doesn't show charge, this means that either the generator is not putting out any current or the battery is not accepting a charge. If I'm smart I won't leave the driveway! On the other hand, if I start the truck and it starts charging hard, but starts to taper off and eventually lands somewhere near 0, I know I'm just fine.

Meanwhile a voltmeter wouldn't really tell me a whole lot. It would tell me that the generator is putting out a voltage, but it wouldn't tell me if my battery was really charging or not. Just my $0.02.

 

I have a simple 3-point voltmeter test. If you use it, you will see that it can tell you if: the alternator is putting out enough of a charge or not, the battery is accepting enough of a charge or not, or if the regulator is putting out more or less than it should. As I said, this is a very simple and easy test, and it has worked for me every time that I have used it.

 

I have read through this thread, found it from a link Jim left on the elec forum. I am guilty of calling the Ford ammeter a "voltmeter" in disguise.

I will make some statements below that are my take on the subject;

I believe we are all just playing with words here as far as "voltmeter" and "ammeter". As was stated, to do work(move the needle of any type gauge) requires voltage AND current.

I believe I might be correct in saying generally a meter is called a "ammeter" when it has a very low impedance(or in this DC case resistance) compared to a "voltmeter" which will have a much higher resistance when placed in a circuit.

So if we agree on those two statements, I am puzzled by the results that are being given out when people are measuring the resistance of the factory gauge. If the factory Ford ammeter is indeed a true ammeter with a very low resistance, why would they feed it with such small wires? The wires would have much more resistance than the meter itself, which would indeed make the whole ammeter circuit design suspect. And as was pointed out earlier in this thread, a resistor would be needed to make it read correctly. Knowing Ford, possibly they used the small wires to create the resistance in the ammeter circuit? And do you see where we are going here? To make our "ammeter" work properly, we are adding more and more resistance relative to the shunt in the main wire. If you consider the small wiring or a added resistor as part of the ammeter "system", you are getting closer and closer to it looking like a voltmeter.

Like I said earlier, I think we all know what's going on, and depending on how you word it, you can look at it in different ways.

 

I know what you are saying Franklin2, but a voltmeter takes the resistance principle to an extreme. A true volt meter has a very high impedance so as to not load down the circuit by drawing an excess of current across it. The ammeter circuit in the truck is calibrated to have a very specific impedance relative to the main charging wire.

The circuit functions as a current divider and it was intended to serve the purpose of an ammeter. If it were intended to be a voltmeter, they could have connected it anywhere and not had to tap the main charging circuit, as they did in later years. I agree that both meters function on a similar principle but the connection points tell the true story.

 

Seems to me that instead of the factory ammeter reading up to 60 amps and hardly ever moving that that it would be more useful if it only went up to say 20 amps then it would show a lot more movement.
I had my 1945 Farmall H out today and it has a high and low charge setting for the generator. The gauge reads up to 20 amps and after it started I switched it to high charge. And since the tractor hadn't been started in awhile it was showing about 15 amps which is 3/4 of total gauge movement.

 

Ok, i have to admit wiring is not my cup of tea, in fact i loathe it. . In my 79 i have done the 3G swap, i have also done a diesel swap so any and all 400 wiring has been removed, if i remember right anyway. My Alt gauge wire is sittin cut off over by the starting solenoid on the pass. fenderwell.

Now my over simplified idiot question:

What can i do to make the gauge do SOMETHING....? Would simply a ring terminal on the charging post tell me when the system is charging/ discharging, like a voltmeter? I dont need a technical thesis writeup to get it done, i just want to know where i can hook it up to get some sort of info out of the gauge without frying it or something else.

Honestly, im always knowledge hungry, but when it comes to making this needle move i dont care, as long as it moves in some relation to charge or discharge, or even draw on the electrical system i would be happy. Just someone tell me where to put it lol.

I do not want aftermarket gauges, theres no good place to put em and they look tacky.

This isnt a quest for a perfect factory ammeter gauge, i just want it hooked up and to move once in a while for peace of mind and to know its hooked up and doin something.l