Initial experimentation with external shunt ammeter

[67Dart273]

With all the hoorah about ammeters, member Ironracer sent me an external shunt meter out of a 76 A body The face is marked +/- 40A (photos shamelessly stolen off the www)

Why? Because if you REALLY want an ammeter, this setup would solve the bulkhead connector and "full current" ammeter failures


Initial tests.............very unscientific, LOL. I simply took one of my good Fluke meters and wired it in series with the new meter, and then wired a second Fluke across the new meter to measure the voltage across it

"Quck" tests show that

100mv (millivolts) is across the meter and the system is drawing about .46A
107mv is across the meter at an estimated full scale of the new meter, and this is .5A

Now, commercial shunts are available for various mv ranges. On egag, I found a couple of 100A, 100mv shunts for about 16 and about 24.00US. In the same price range, found at least one 50A, 100mv shunt

And what would you do with these? Move the ammeter wires outside into the engine bay and mount the shunt out there, eliminating the big ammeter and the bulhead terminals. Then run parallel no10's back into the interior for the power feed. This will relieve the strain on the feed, because THE CHARGING CURRENT no longer goes into the interior

BUT YOU WOULD NOT HAVE to use such as one of these----you could rework the wiring to mimic a 76 A and use the main wiring itself for the shunt, the way GM, Ford, and Chrysler did

A quick search on ebay shows that this was a "mixed bag" through the years. I've known, by looking at electrical diagrams, that some models of the C barges started in early 70's with external shunt. But it turns out this was not a "sure thing" and that Ma sort of "skipped around"

Now you have to be "on your game". Some of these have dramatically higher asking prices than others, and as I said earlier, Ma kinda skipped around. YOU WANT TO LOOK for a photo that shows the "guts" and look for a coil of smaller wire like this:

This one below was advertised for a 75 Fury/ Coronet, and I cannot tell. It SEEMS to be an external shunt device, but not sure

This one below is shown as 71-73 Fury/Monaco/ Polara and there is no doubt that it is what we are looking for "electrically"

NOW YOU SAY but but bu.....b.b.b.b..bb.b.b. BUT "It won't fit my xxxx A body!!"

So, simple, go read the conversion thread, and pull the guts out of the "new" meter and adapt it to your old face, and rework the stud holes if necessary, and go to town on it

Ammeter to Voltmeter...who does it?

Now as some of you know, my ribs still hurt (be a year Aug 15th) and I am old so I move slower. But my plan is to use the 74 roof-fixed Scamp as a "test bed" when I get the time. This will not be an "install" but rather a working emperical experiment to see 'what works'

 

[Demonic]

Thanks for the post! Too many of our cars damaged from loose ammeter wires and other related issues.

 

[67Dart273]

Thanks. I've traditionally always been an "ammeter guy".

 

[70dart340]

If you're a nut like me, you can use a Fluke ammeter, a signal generator, and an O'Scope. The truth will prevail.

 

[67Dart273]

If you're a nut like me, you can use a Fluke ammeter, a signal generator, and an O'Scope. The truth will prevail.

First o'scope I saw used was in ?63? an amateur radio friend of mine. In those days two of the locals were building / modifying gear to work "OSCAR" (Orbiting Satellite Carrying Amateur Radio) I don't remember which version this was, somewhere between 63 and 66. They used 2M FM "trunk mount" radios already converted to amateur, and recrystalled to fall into the 432mhz area of Oscar. The 2M (144-148mhz) output of the transmitter was fed to "homebrew" varactor diode tripler, with a downconverter (homebrew) from 432 to an HF receiver.

The first o'scopes I had my hands on were in the Navy. In ET-A school in 68-69 we were introduced to the old OS-8, which is synchronized sweep, not triggered. At some point we were introduced to Tektronix and HP scopes.

When I went to GCA school, we fiddled for weeks learning the onboard syncroscope provided with the RADAR a horrible contraption, only to find, when I arrived at Miramar, that the factory had racked a brand new HP into the equipment. At that time a HP 130? something? or 140? I'll have to look it up. Still ALL tubes

My first scope out of the Navy was an old Dumont synco sweep and the transformer died shortly. I made a hell of a heavy fix by putting TWO junk transformers in the thing to rig a new power supply for it

Since then I've had some of the Heathkits, a few Tek, and nowadays I've still got a couple of 465 tek along with an Tek 220 series digital scope.

Here is a photo of the GCA radar training lab at NAS Glynco, GA with the awful, the huge, the heavy, the ridiculous, the obsolete LOLOL syncroscope up on the pedestal in the center

 

[70dart340]

First o'scope I saw used was in ?63? an amateur radio friend of mine. In those days two of the locals were building / modifying gear to work "OSCAR" (Orbiting Satellite Carrying Amateur Radio) I don't remember which version this was, somewhere between 63 and 66. They used 2M FM "trunk mount" radios already converted to amateur, and recrystalled to fall into the 432mhz area of Oscar. The 2M (144-148mhz) output of the transmitter was fed to "homebrew" varactor diode tripler, with a downconverter (homebrew) from 432 to an HF receiver.

The first o'scopes I had my hands on were in the Navy. In ET-A school in 68-69 we were introduced to the old OS-8, which is synchronized sweep, not triggered. At some point we were introduced to Tektronix and HP scopes.

When I went to GCA school, we fiddled for weeks learning the onboard syncroscope provided with the RADAR a horrible contraption, only to find, when I arrived at Miramar, that the factory had racked a brand new HP into the equipment. At that time a HP 130? something? or 140? I'll have to look it up. Still ALL tubes

My first scope out of the Navy was an old Dumont synco sweep and the transformer died shortly. I made a hell of a heavy fix by putting TWO junk transformers in the thing to rig a new power supply for it

Since then I've had some of the Heathkits, a few Tek, and nowadays I've still got a couple of 465 tek along with an Tek 220 series digital scope.

Here is a photo of the GCA radar training lab at NAS Glynco, GA with the awful, the huge, the heavy, the ridiculous, the obsolete LOLOL syncroscope up on the pedestal in the center

View attachment 1715767841

Wow! No wonder you're so good with our old Mopars! I served in the Marine Corps from 1977-1983 as a Crypto Tech, and both fixed and mobile. I learned tubes and digital. I remember using a roll round Techtronics O'Scope Our first color TV was a Heathkit which I helped build, in 1968.

 

[RedFish]

OK, through 1976, that gauge had the same -40 (/) -40 screen, internal shunt type or not. That's why some owners buy the wrong one of ebay wherever. Ever wonder why the cahnged it for the 1976 model? You can see it in this thread... [FOR SALE] - Standard dash clusters
They placed the amp gauge right above the headlight switch far left end. as their stamp dies etc.. got worse their water leaks got worse. Water leak from windshield seal or wiper pivot would short out the amp gauge and shut down the whole dang car. with remote shunt they could get one more year out of this old a-body. Same water leak might kill the gauge but there product isn't laying by the side of the roads that the Ford and GM drivers are traveling.

 

[67Dart273]

LOLOL

 

[70dart340]

I'm fitting in with my Mopar Brothers of similar vintage!

 

[RustyRatRod]

Talk to me like I'm the electrical dipshit I am and tell me what's the advantage here? I know there is one, or else you wouldn't be posting about it.

 

[RedFish]

Talk to me like I'm the electrical dipshit I am and tell me what's the advantage here? I know there is one, or else you wouldn't be posting about it.

The shunt or wiring to it could fail no matter where it is located. The remote shunt and connections to are somewhere besides in the dash/gauge. The remote shunt gauge has wires so small that the 76 a-body gauge is wired through the printed circuit board like other gauges. Its only sensing the current through the shunt. So 76 circuit board and wiring harness are 1 year only also ( unless the 77 Spanish Dart is the same, I don't know ).

 

[67Dart273]

Talk to me like I'm the electrical dipshit I am and tell me what's the advantage here? I know there is one, or else you wouldn't be posting about it.

Simple rusty. All the OEM cars, GM, Ford, or Mopar, that used "external shunt" ammeters, used the car's wiring for the shunt. The small wires feeding the dash instrument were tapped into the harness. So the full current of the vehicle loads, AND the charging current which goes through these cars normally, and back and forth through the bulkhead, is simply not there. In other words, the ONLY heavy loads going through the bulkhead are now just the operating loads, the headlights, heater, igntiion, etc, so with no charging current, amperage going "only" INTO the passenger compartment is greatly reduced

So no danger to an ammeter, and REDUCED danger to bulkhead connector

So far as a shunt burning up, that would be fairly "safe." And again, by using the car's own wiring as a shunt, there is nothing specific, really TO burn up other than wiring shorts that can happen anyhow.

What I hope to do, then with the 74 Scamp is:

1:.. Without tearing up the car, determine that WITH an ammeter bypass done, how easy or difficult it would be to find and perform proper harness points to splice the low current instrument into the wiring so that it operates properly
2...If that proves somewhat impractical, then use an actual external shunt in the engine bay, which would eliminate the ammeter / bulkhead problem
3...Then with that done the new instrument could be installed into an existing cluster similar to how guys are doing the voltmeter conversions

One thing I hope to avoid is the "Ford numbness." I've seen some Ford pickups owned by others, and my older Rangers, 86/87, you could turn the headlights on at rest and you had to "stare intently" at the ammeter to see if it MOVED. So I hope to avoid this situation, and come up with a truly useful reading

 

[RustyRatRod]

Simple rusty. All the OEM cars, GM, Ford, or Mopar, that used "external shunt" ammeters, used the car's wiring for the shunt. The small wires feeding the dash instrument were tapped into the harness. So the full current of the vehicle loads, AND the charging current which goes through these cars normally, and back and forth through the bulkhead, is simply not there. In other words, the ONLY heavy loads going through the bulkhead are now just the operating loads, the headlights, heater, igntiion, etc, so with no charging current, amperage going "only" INTO the passenger compartment is greatly reduced

So no danger to an ammeter, and REDUCED danger to bulkhead connector

So far as a shunt burning up, that would be fairly "safe." And again, by using the car's own wiring as a shunt, there is nothing specific, really TO burn up other than wiring shorts that can happen anyhow.

What I hope to do, then with the 74 Scamp is:

1:.. Without tearing up the car, determine that WITH an ammeter bypass done, how easy or difficult it would be to find and perform proper harness points to splice the low current instrument into the wiring so that it operates properly
2...If that proves somewhat impractical, then use an actual external shunt in the engine bay, which would eliminate the ammeter / bulkhead problem
3...Then with that done the new instrument could be installed into an existing cluster similar to how guys are doing the voltmeter conversions

One thing I hope to avoid is the "Ford numbness." I've seen some Ford pickups owned by others, and my older Rangers, 86/87, you could turn the headlights on at rest and you had to "stare intently" at the ammeter to see if it MOVED. So I hope to avoid this situation, and come up with a truly useful reading

The ammeter in my 75 F250 never moves AT ALL. I'm pretty sure the gauge is bad. It charges fine. I have a replacement gauge for it, I'm just too lazy to do it.

 

[67Dart273]

^^That might be but some of those are REALLY numb. One way to find out--do you have a load tester? Get the thing running on high idle and put some load on the battery like maybe 50A and go take a quick look at the ammeter. It obviously should be charging hard.

I once lengthened a piece of wire down by the alternator to "de-numb" a Ferd one. I've forgotten, I think I added 3ft or no10 to the wire?? This provides a bit more voltage drop in the harness and causes the ammeter to read further over DO NOT ASK HOW!!! This was decades ago (in the 80's) and hell I don't remember LOLOL

 

[Mike69cuda]

Del, I have a couple of non contact Hall effect DC amp meters I bought to play with. Haven’t had much time lately. PM me your address & I will send you one to fool with. We are moving so I got to find it first.

 

[70dart340]

Talk to me like I'm the electrical dipshit I am and tell me what's the advantage here? I know there is one, or else you wouldn't be posting about it.

You're not a dipshit. You're wading into old electrical guys territory. You're still loved for your common sense, and your ability to help others. Kudo's for all your help over the years.

 

[str12-340]

Subscribed!!! I am also an "ammeter guy" They are a great diagnostic tool. All my encouragement goes the OP to figure out a reliable, effective solution!!!

 

[BillGrissom]

I think you can use the existing red (BAT) wire to the "+" terminal of your new shunt meter. You must break that red wire in the engine bay and install the shunt in series. That is because the ammeter's purpose is to display current flow in or out of the battery. Note that it doesn't measure transient engine bay loads like the starter or horn, since they come straight off the battery. Similarly for any other "relay powered" loads you add, unless you tap their "red" source on the bulkhead side of your shunt. You won't connect the existing black (ALT) wire to the new meter. Instead, you must run a new wire downstream of the shunt thru the bulkhead to the "-" terminal of the meter. It can be a small 22 awg wire, but must be rugged, either in sheath or teflon insulation. Hopefully you have a spare terminal in your bulkhead to use, otherwise sneak it thru a grommet.

I used another approach. I installed parallel-reversed diodes in series with a new ALT to BAT bypass wire (6 awg) in the engine bay. Those flow either way in "forward bias", which is ~0.5 V drop at 50 A, and not much more at higher currents. That is about the drop across the ammeter at 30A. In effect, the diodes begin shunting current direct from alternator to battery as the ammeter reaches full scale, to keep that current out of the cabin or thru the ammeter. Ditto for high current draws from the battery. Anyone wanting details can search for my post years ago "Modernized Wiring ...". Don't use whimpy diodes. Mine are rated ~200A.

 

[RustyRatRod]

^^That might be but some of those are REALLY numb. One way to find out--do you have a load tester? Get the thing running on high idle and put some load on the battery like maybe 50A and go take a quick look at the ammeter. It obviously should be charging hard.

I once lengthened a piece of wire down by the alternator to "de-numb" a Ferd one. I've forgotten, I think I added 3ft or no10 to the wire?? This provides a bit more voltage drop in the harness and causes the ammeter to read further over DO NOT ASK HOW!!! This was decades ago (in the 80's) and hell I don't remember LOLOL

I have one around here somewhere.....but like that Fluke meter you turned me onto on Ebay several years ago.....it disappeared. I did find the Fluke meter though. LOL I'm sure my load tester will turn up.

 

[str12-340]

At the end of all this I hope that there is a set of instructions that will tell us how to eliminate the risk inherent in the non-external shunt system and still have a functioning ammeter.

 

[67Dart273]

At the end of all this I hope that there is a set of instructions that will tell us how to eliminate the risk inherent in the non-external shunt system and still have a functioning ammeter.

There might not BE a "complete" way. That is, however, an interesting thought, here's one take-off

If you have looked at the stock ammeter, the "entire meter" is really just a shunt (brass) across the two posts, which generates a magnetic field, and deflects a counterweighted vane needle assembly

So IF that can not be modified, the only real strategies I can think of are:

1....Bring the entire ammeter circuit up to as close to factory as you can, apply the factory equivalent of the "taxi fleet police" modifications and that might just be enough

2....Be happy with less than say, 50A alternator and be careful what you add. No big pumps fans, maybe a stereo. In other words, limit choices

3....Maybe MODIFY the ammeter into a PARTIAL external shunt. I think Bill Grissom ^^above^^ has hinted at this by using a pair of back-to-back diodes as sort of an added external shunt to limit full current through the bulkhead...........Maybe MAYBE this could be taken a step further...........

Maybe you could disassemble the ammeter, file down the shunt to make it a much more sensitive movement, maybe say, 10 or 20A full scale instead of 40, then add a shunt OUTSIDE the firewall to make up the difference.

Food for thought. As Crackedback says, "more than one way to skin that cat"

If and when I get the 74 Scamp into a more "runnable" car it might make a good guinea pig for this testing. Bear in mind I've gotten a LOT "older" in the last year or two, the broken ribs (TWICE!) added a lot to that. So crawling around under a crowded dash is not much fun. The Scamp presently has a bench seat, and those are not much fun to remove. I have to unbolt each back, remove the center, and then unbolt and remove the bench bottom.

 

[str12-340]

here's one take-off

I agree whole heartedly. I've followed this recipe and since I don't have a bunch of add on gizmos it has worked so far. Replacing the 50 year old wiring harnesses did a lot to calm my anxiety and finding a local rebuild shop that would actually give me an alternator with a factory output also helped. Just like the value of adding front disc brakes, I'm just looking for a way to reasonably protect these old friends for the long haul.

 

[67Dart273]

Believe me if I can figure a way to "simply" rather than "complicated" I will take that route. I really prefer ammeters over voltmeters. The 67 before I tore it apart, was converted to voltmeter, and it worked OK LOLOL I prefer either over a GM or Ford warning lamp!!!

 

[KitCarlson]

I suggest skipping shunt. Use a DC Hall sensor, conductor goes thru donut sensor.

Here is example of meter, but no direct experience with it. Less than $20, decades ago a sensor was over $100. https://www.amazon.com/dp/B01DDQM6Z4/?tag=fabo03-20 Has digital display.

 

[Ironracer]

The shunt or wiring to it could fail no matter where it is located. The remote shunt and connections to are somewhere besides in the dash/gauge. The remote shunt gauge has wires so small that the 76 a-body gauge is wired through the printed circuit board like other gauges. Its only sensing the current through the shunt. So 76 circuit board and wiring harness are 1 year only also ( unless the 77 Spanish Dart is the same, I don't know ).

My Duster , after changing to a Mopar volt reg, and your gauge, is charging, and showing it! Unless the drunk *** neighbor that claims he worked on airplanes in the Air Force, but can't fix ****, comes over and " helpfully" bends and disconnected a field wire, before " kindly " being thanked in loud cursing, then it shows a drain....