The Stovebolt Site Forums The Shop Area The Electrical Bay Ammeter and shunt

I'm connecting and ammeter on my '69 Chevy C10. Currently a 3 hole bezel and cluster circuit, but am adapting a new 7 hole bezel and wiring gauges separate, keeping the 3 hole cluster circuit. This original cluster circuit has nothing for an ammeter or voltmeter (I'm familiar with the electrical differences).

My question comes as a result of contacting "Autometer" which is the brand of gauge I will use. They say I do not need a shunt for their ammeter, yet all the YouTube videos instruct you to use a shunt. I can test it before installing it, but do I risk burning up an $80 ammeter? Also have not been able to find a shunt on over a dozen classic parts sites, nor Auto zone, nor O'Reilly's.

I'd contact Autometer and ask if they built a shunt into the ammeter. It is very possible they did and since they've told you already you don't need a shunt for their ammeter, I'd say you're probably good to go. I'm not using one and I have a 100 amp alternator and the original AD 6v ammeter. Despite warnings to the contrary I haven't created a fire under the dashboard yet.

Good, thanks Jon.

The shunt idea has been cussed and discussed for close to all of the 20 years that I'be been on this site, and nobody is willing to comprpmise their incredibly biased viewpoint. If you'd like to send me a PM, I can describe the home brewed one I've been using for a long time- - - -which costs less than 5 bucks, and works.
Jerry

Autometer insists I don't need one, but if things go awry I will pm you.

By then it might be too late- - - -once you let the smoke out, electrical devices don't work any more. What's the full scale amperage reading on the face of the ammeter?
Jerry

If Autometer says you "don't need one", they must mean that their meter has one? No external shunt needed. Did you ask if it has one built in? That would be the question to ask. It has a shunt, don't worry.

Alternator is a 42 amp output, and ammeter is a 60 amp.

I installed a shunt as recommended by HotRod Lincoln. Here is the link to my post on how I did it. https://www.stovebolt.com/ubbthread...link-for-12v-alternator.html#Post1310217

Link doesn't work for me.

This is probably what Phil was referring to.

To elaborate on what Jerry described, the distance between the alternator terminals would be 50% of the total distance between the outer terminals on the shunt to for the ammeter to deflect 50% of the current thru the shunt. You could use other ratios as well.

Me either.

Since most dash mount ammeters (including expensive ones like Autometer) have the approximate accuracy of "Measure it with a micrometer- - -mark it with chalk- - - -cut it off with an ax", putting that simple loop of hose clamp stainless steel into the mix protects the ammeter from overload and still allows it to be a charge/no charge indicator. Using an "ammeter" just allows some people to feel superior to those who rely on an idiot light for the same information. Add a voltmeter, and some way to monitor the specific gravity of the battery acid, and you might be able to make an educated guess about the health of your charging system!
Jerry

I should have used the word "approximately" in my description of shunt "calibration". Jerry's right about the ammeter just being a charge/no charge indicator. Unless it's a precision ammeter (definitely not the stock one), it's little more than an "idiot gauge".

Here is the post I was referring too. I originally copy and pasted the whole post but apparently the link broke.

[LINK]

phak1 No link yet

klhansen Have used the stock gauge in the oh about 30 trucks in 50 years,always go to 12 volt,work over my alternators so they prime at a low RPM. You can also compound 2 belts so it will charge at idle.

Hotrod Don't you think the light bulbs are a pretty good voltmeter ?? Anyone with experience with these trucks can tell if it's "charging" An alternator with the right rotor and stator will charge at idle and shortly catch up to a routine start drain on the battery.

FYI, while searching for an old fuel gauge, I ran across an AD ammeter (from a 1950 or 51 truck). Looking at it, the whole thing is a shunt device and I had never noticed it before. Clearly set up that way...see image below.

Yep- - - - -adding the extra shunt when a high current alternator replaces the generator just prevents the meter movement from deflecting as far when 3-4 times the current the generator could deliver passes across the gauge.
Jerry

I'd be greatly surprised if adding an extra shunt would ever make that particular ammeter register correctly. Slightly closer, possibly but you're still measuring the distance to Mars with a yardstick. I've been using a 100 amp alternator in mine for 4 years or so, Jerry and the ammeter has never even felt warm. Back when I mentioned I was doing this conversion, I was warned I was going to turn my truck into Krakatoa.

Jon, Parallel resistors (which is what a shunt is) reduces the current flow thru a circuit. If the ammeter has a certain resistance and the shunt has the same resistance, then current flow thru each would be 1/2 of the total. Adding another shunt of the same resistance as the ammeter and original shunt would divide the current flow thru each element by 3, or 1/3 of the total. Theoretically, you could get it spot on, but why bother? Again, the original ammeter is a pretty rough indicator (your yardstick measurement to Mars. ) If your truck hasn't burnt down yet, I don't think it will in the future.

I understand that well, Kevin. Thanks for mentioning the parallel resistor scheme, however. I've discussed that on this forum, but I get the idea it isn't in everyone's comfort zone. What I meant to say was that a shunt might help somewhat, but given the design of this ammeter and the internal shunts (there are actually 2), the numbers don't make adding a third (or a second if you want to look at the ammeter as a primary shunted device) that easy. And the thing is any ammeter must be (or remain) linear in delivery and I'm not convinced this one ever was all that linear.

Small correction, but the shunts in this case are only reducing the current flow through the ammeter itself. They form sort of a freeway of current that is intersected by a small sidewalk that is the ammeter itself.

You can see the shunt design below. First is the brass framework cage (goes from post to post continuously) and second is the 1/16 thick piece of steel joined to the top (right beneath the gauge face). Which as I said possibly could be seen as one shunt device. The resistance of the OEM gauge is .003, so with 50 amperes we have about 1.5mA current flowing through the meter and we probably need something like 2mA with 100 amperes. At least that would match most of the internally shunted 100 amp ammeters I've seen.

I agree that the parallel resistor thing is a headache. I'm trying to swear off the math for figuring those out. With identical resistances, it's easy, but once you introduce different resistances, it becomes a real PITA, IMO.

I didn't realize there were more than a single shunt in the ammeter on our trucks.

Exactly...when you introduce different resistances it either becomes a headache to calculate or a pure crapshoot. I was hoping you'd pull out a slide rule and say "oh we just need an extra shunt with this rating ______ and a new face plate showing 100 amps (or the understanding 50 now equals 100).

I didn't realize that either until I was hunting for an old fuel gauge and saw this one. Ah well, as I've said all the predictions/warnings of Mount Vesuvius didn't happen in any way so it probably isn't worth too much worry. Maybe this winter I'll convert the ammeter to a voltmeter and just be done with it.

Thanks again for your thoughts and comments.

LOL! I actually still have a slide rule. But do I remember how to use the dang thing?

I still have mine, too!

Ed

Kevin, I thought I had kept mine, but I looked for it 5 or 6 years ago and it was gone. I think it disappeared when we got rid of an old rolltop desk. I noticed my old Roll-A-Dex was gone, too. However...the other day my wife handed me some clear plastic envelopes she found in a drawer made to store 3.5 inch floppy disks. My digital camera (a Sony Mavica from the late 90s) uses those. If you look at images I post, you'll see the prefix is usually MVC followed by 3 digits and another letter the camera will assign. The MVC stands for Mavica.

To finish up, I had contacted Autometer and they said I didn't need a shunt for this ammeter. So, I finally ran a #10 wire from the starter solenoid to the ammeter, and from the ammeter to the alternator, putting a fused link on both runs, and "voila", it works beautifully! Thanks again for everyone's input. I wouldn't have done it without you.

Tagging on to this thread because the previous discussion is relevant to my question. About to order some wiring supplies and one of the questions I need to answer is this:

Ammeter - full current or shunted?

Prior to reading this thread my (limited) understanding was that my 51 truck would have been setup as full current. If I read Jon G’s posts here, it looks like maybe the factory setup is already shunted? How would I know?

For background, the truck has presumably original gauges but was converted to 12v generator sometime in the distant past. I’m perfectly satisfied with the ammeter being a simple charge/discharge device…not striving for precision. I’m also willing to add a shunt if that’s a best practice. But for now I’m just a little confused.

The stock ammeter has an internal shunt, so the answer to your question would be "full current".