1966 ammeter with new engine, alternator, and wiring harness

[mf_luder]

I'm trying to get the ammeter working on a 1966 C10. This truck does not have its original engine, alternator, or wiring harness. I am not interested in replacing the ammeter with a volt meter.

I need to know what voltage produces full deflection in the ammeter. It's probably in the range of 50 - 500 mV. Alternatively, I need to know the resistance of the 10 AWG wire that goes from the horn relay to the positive battery terminal with high precision (it's probably a few milliohms). I'm hoping that somebody can either point me to some documentation that I haven't been able to find myself or actually take one of these measurements.

There's a lot of conflicting information on the Internet about how these ammeters work. As best as I can tell, it's basically like this:

In general, an ammeter works by measuring the voltage across a resistor or "shunt" with a resistance that's low (a few milliohms) and accurately known. In a car, this resistor is placed between the positive battery terminal and the rest of the car (except the starter) so that the voltage across it (less than a volt) will be an indication of how much current is flowing to or from the battery and in which direction (read about ohm's law if that doesn't make sense to you). Early ammeters had built-in resistors and the current being measured ran through the gauge, which can be dangerous with large currents. Newer ones run a fused wire to each end of an external resistor, so that the current flowing through the gauge is small. In 1966 Chevrolet was using the latter type, and the "resistor" was actually the length of stranded 10 AWG wire running from the bus on the horn relay to the positive battery terminal. (Copper wire does have some resistance; Not much, but that's what you want for an ammeter shunt.)

My truck has a 350 with an internally regulated 63 amp alternator on the passenger's side, for which reason the wire running between the battery and the horn relay is getting deleted. Even if it was not going to get deleted, I believe that the shunt resistance needs to change to account for the higher amperage alternator.

If I know the voltage that produces full deflection in the gauge, I can calculate the resistance I need to produce that voltage at any amperage I desire. Alternatively, if I know the resistance of the original shunt, I can calculate the voltage that it would have produced at the original alternator's maximum output and then calculate the resistance I need to produce the same voltage at the new alternator's maximum output.

Low tolerance, high amperage resistors can be a bit pricey, otherwise I would simply start with a low resistance one and and replace it with higher resistance ones until I got the result I wanted. Unfortunately it would be cheaper to buy a DC power supply that I could use to test the gauge. That's what I'll end up doing if nobody here can help.

Thanks for reading.

[VetteVet]

I wouldn't get caught up in the accuracy of these ammeters. They are basically just indicators of a charge or discharge of current from the alternator or the battery, whichever is the higher output, by measuring the voltage proportion between the two. In other words which one has the higher voltage. IF it's the alternator you see charge and if it's the battery you see discharge.

Here is a thread that gives more information.

http://67-72chevytrucks.com/vboard/s...d.php?t=393139.

This thread is on the 67- 72 trucks but it is virtually the same ans the 63-66 models.

In addition you stated the resistance on the SHUNT as a 10 gauge AWG wire.
In the factory configuration the SHUNT is a 12 AWG wire. Given the mission of the ammeter, I don't think that makes much difference. Also your understanding of the alternator ampere output is flawed. The alternator will only supply the amperage needed by the system regardless of it's total capacity.

I have my alternator (100 amp 12SI ) installed on the passenger side and I still ran the SHUNT, between the battery and the main junction with the alternator output, and the rest of the truck circuits. Your horn relay is the main junction, so you should run your alternator output there, as well as the battery charging wire, AKA the SHUNT.

The highest output you should see from the alternator will be 18 volts which will be unregulated if it malfunctions. Voltage range then will be 0 to 18 volts.

Simply measuring the resistance across the ammeter gauge should give it's internal resistance.

There should be plenty of resistance charts on awg copper wires on the internet to give you the values you need for a given length of 12 or 10 gauge wire. I would figure about 4 feet for length.
HTH

[mf_luder]

I appreciate the response. I know how the ammeter works and I know how the alternator works. There's nothing in that other thread that's new to me. You're right that I got the gauge of the wire wrong. I was reading it from a diagram that I thought was accurate, but I've looked at mine now and it's 12 AWG. I was briefly confused about how you might have got the idea that I thought alternators produce their maximum output all the time (I don't know why I'd use the phrase "maximum output" if that were the case), but I assume it's because I mentioned that I think the shunt resistance should be adjusted to account for the new alternator. I said "needs to", but I know that it probably isn't strictly necessary. I do think it's a better idea to do it than not to do it, but I can't do it either way without knowing the original shunt resistance.

To be clear, when I say that the truck doesn't have its original engine, alternator, or wiring harness, I don't just mean that they've been replaced; I mean that that they are not even remotely the same as they were originally. The wiring harness is one that I've designed and built from scratch, and its power distribution scheme is completely different. I have what's left of the original harness dissected for reference, which is how I know that the original shunt wire is in fact 12 AWG, like you said, and that it's a little bit less than four feet long. I know that you can buy complete harnesses, but I prefer to build my own, not least of all because a lot of the original routing no longer makes sense. As you know, the external voltage regulator was originally next to the horn relay on the driver's side of the radiator. Obviously, it was placed there because the alternator was on that side. With the alternator moved to the other side, next to the battery, and the external voltage regular gone, there is no reason to run a 10' wire around the driver's side of the radiator from the alternator to the battery. The horn relay bus will no longer be used. That leaves me with the need to find a replacement for the shunt wire that's not four feet long, because the alternator's battery wire is going to terminate about a foot away from the positive battery terminal and I think bundling up three feet of extra wire and zip tying it to something is an ugly solution. I'd rather have a real resistor and it's time to buy it because I'm trying to get that part of the harness finalized.

Using one of the resistance charts you mentioned seemed like a good idea to me as well, and according to those charts the shunt wire will have a resistance of 4 to 6 milliohms (let's call it 5), which would produce 0 to 275 mV with a 55 amp alternator, as I believe the truck would originally have had, or 0 to 315 mV with a 63 amp alternator. The problem with that is that the resistor pictured in the thread that you linked to, which is from a Chevy II from the same era that used a real resistor, is supposedly about 40 milliohms, which at 55 amps would produce 2.2 volts (I could easily be wrong about that. That seems extreme). Modern automotive ammeters seem to take about 75 mV most often, and sometimes 50 mV. These calculations seem to indicate that the ammeters in these trucks were very different than the ammeters in the Novas that were made at the same time, and for that reason I'm not confident that they're right. As I mentioned before, it's not cost effective to buy a resistor that will produce 50 mV and keep replacing it until I get the result I want, and I don't know how else to be sure that I don't fry my gauge. I don't trust those charts enough to put 300 mV across something that is maybe only designed for 50.

[VetteVet]

Well you've certainly thrown a curve into the normal way of wiring in a new alternator.
Your wiring plan may work well or not. I assume it will but I'm still doubting that the location of the main junction is the best. I hope you have the alternator output as the main contributor to the truck power circuits and not the battery.

I understand your concern to protect the battery gauge which is why you should use two inline fuses in the pickup wires to it. That's one thing the 66 and older models didn't do. I have a diagram for both years if you haven't seen them. As you know they are going to originate on the battery positive post and the alternator output wire at the main junction. It seems that the point of contention is the length of the SHUNT wire between the two points.

You can shorten the length but you will have to compensate for the loss of resistance, or maybe not since I have read that some posters have modified that with no change in the function of the battery gauge. These guys were installing CS alternators capable of 100 plus ohms.

If you're going to install a shunt in the battery charge wire then your two input wires will come off that and the main battery current will go through the shunt. I know you are aware of this ,but I mentioned it for other members who might read this thread.
The shunt must match the gauge if it is to be accurate, but remember, this battery gauge is not even calibrated for amperage readout. It is merely an indicator as I said earlier.
I looked up the 12 and 10 gauge wire resistances in a four foot section of wire and they list at 6 milli-ohms and 4 milli-ohms respectively. I suppose a one foot section of either wire would equal 1/4th of those figures or 1.5 milli-ohms on the 12 gauge ,and 1 milli-ohm. on the 10 gauge. I see that as a need for an additional 4.5 milli-ohms requirement in the shunt resistance. Do you agree?

I looked on Amazon and they list several in the $4 to $20 range.

To answer one of your other questions, I measured two of my cluster battery gauges and they both settled out at 30 milliohms. So now you know the original shunt resistance and the battery gauge resistances. I believe you said the gauges were in the 40 to 50 milliohm range, which is higher than mine read, so you can draw your own conclusions about that.

Here's a wiring diagram of the 65 year which will be nothing like yours but maybe it will help.

This is the engine side.



And this is the cab side.

[PGSigns]

Here is one thing to think about. I redo a lot of these clusters and the amp gauges in more than half of them are over heated or melted. If you have added high amp draw accessories to the truck to go along with the higher capacity charging system the factory gauge will go up in smoke under that much load. This is the reason the companies like American Auto wire that make harnesses for these trucks drop it from the harness. For safety and reliability I would not wire it in the system.
Jimmy

[franken]

The battery gauge is nothing more than an indicator of charge/discharge based on the battery having a more or less positive voltage than the alternator. Almost no current passes through it and there's no danger from it regardless of load on the electrical system. I deleted the next thing I typed...

[VetteVet]

Originally Posted by PGSigns

Here is one thing to think about. I redo a lot of these clusters and the amp gauges in more than half of them are over heated or melted. If you have added high amp draw accessories to the truck to go along with the higher capacity charging system the factory gauge will go up in smoke under that much load. This is the reason the companies like American Auto wire that make harnesses for these trucks drop it from the harness. For safety and reliability I would not wire it in the system.
Jimmy

Jimmy I think you're right on except that I believe you are referring to the pre 63 years where all the current runs through the ammeter. Like Franken says, the 63 to 72 years use the charge indicator. It could be that the prior 67 years do overheat if they get too much current because they are not fused. Is that what happened?

Originally Posted by franken

The battery gauge is nothing more than an indicator of charge/discharge based on the battery having a more or less positive voltage than the alternator. Almost no current passes through it and there's no danger from it regardless of load on the electrical system. I deleted the next thing I typed...

I am curious about this harness and was wondering if It's something I haven't
seen. I'm thinking, I was thinking what you were thinking, but I used invisible ink.

[PGSigns]

On a clean rewire I would still would leave it out, they are seeing with the factory harness a lot of heat. It may be the age of the windings, breakdown of the insulation or corrosion driving the resistance up but they are burning up. I'll dig a few out and post some pictures and y'all can let folks what you think.
Jimmy

[VetteVet]

I just realized that we had forgotten to welcome MF Luder to the forums. So let me be the first to do that and I hope we have gotten off to a good start. VV