I was reading through another forum where they said to bypass the amp meter. Why would anyone want to? Does it cause problems that you have to replace it or bypass it? I know newer vehicles have voltmeters but what harm does the amp meter do by leaving it as it is? Thanks-Dan

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1968 Valiant 100, 4dr, 225 - FOR SALE TO A GOOD HOME!

You do not need to bypass your ammeter. Here, read through this thread for the lowdown.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I had been going over that wondering why people were bypassing it. Plymouth wouldn't have put it in there if it didn't serve a purpose. No disrespect to you Dan, but after reading those instructions for adding a shunt now I know why people bypass it. I could just keep it hooked up like it is but after doing all this work of fixing burned up wires I don't want any hint of a potential problem.

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1968 Valiant 100, 4dr, 225 - FOR SALE TO A GOOD HOME!

There are 2 primary problems with the ammeter as implemented by Chrysler.
(1) They were designed for a 35 or so amp charging system while modern systems are 65 amps ar better. Without some kind of shunt they can overload and burn out, particularly if they are asked to support extra loads in the form of sound systems and additional eltronic and lighting accessories.
(2) the connections where the ammeter loop passes through the firewall are subject to corrosion and loosening which can lead to overheating of the wires and a short circuit with potentially catastrophic results.

The ammeter circuit can certainly be modified to overcome these issues and when that is done there is nothing wrong with retaining it. To some people, and it's purely a matter of preference, a voltmeter gives them the information they need without the extra work required to modify the system. Just be aware that the 2 systems measure different things. Each has it strengths and weaknesses so you just "pays your money and takes your choice." The ideal setup would be a remote sensing ammeter with just a low voltage signal powering the gauge itself but I don't know of any readily available units like that for automotive use.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

What you're close to describing here is the external-shunt ammeter system phased in by Chrysler starting in '71 (A-bodies didn't get it til the final year, '76). These don't work well at all, probably as much due to Chrysler's build and materials decisions (cheap) than anything else. I think the ideal setup for maximum utility and dependability is probably a stock-type ammeter with an active shunt.
Naturally, but there are potential problems and then there are potential problems. The ammeter and its related connections and components certainly bear careful inspection and occasional maintenance (remember, we're dealing with cars way beyond their expiry date!), but they're hardly ticking time bombs.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I believe this is what I have (an external-shunt ammeter) in my 1974 Dodge Monaco. The ammeter in the instrument cluster takes connects to light gauge wiring, and the wiring diagram shows that this circuit is in parallel with heavy gauge wire between the alternator and the battery.

Here are pics of the ammeter, and the relevant page of the wiring diagram:
http://s955.photobucket.com/albums/ae40/sigmfsk/

Here's a pic of a NOS ammeter net yet installed in the instrument cluster:
http://cgi.ebay.com/ebaymotors/ws/eBayI ... vi-content

I'm wishing to rewire the ammeter to the electrical system (I'm rewiring the entire car), so it seems that I'd need to use an external shunt to drive the ammeter. Something like one of these:
http://bluesea.com/products/9228

My question is: to what shunt specifications is the ammeter manufactured? Do I need a 50A / 50mV? How could I determine this? I checked the wiring harness and didn't see any obvious external shunt. Based on this thread:
http://www.ford-trucks.com/forums/10106 ... ost9564961
I checked the resistance of the wiring on the ammeter side of the parallel path, thinking that the wiring harness itself could include a resistor, but didn't find anything there.

For a final verification of a setup, I can put a load on the system, use a clamp-on ammeter to see what is really being pulled, and verify that it reads correctly on the gauge. If the gauge isn't reading correctly, then I could swap for a different shunt. But I'd like to at least have an idea of what external shunt range to try. Or even verify that my plan holds water.

all thoughts appreciated,
thanks,
arthur

The shunted ammeter design sent, if I remember correctly, less than about 8A through the ammeter rather than the whole load; while the ammeters looked more-or-less the same, they are not interchangable. The contacts for the shunted one in my '75 Coronet were just clips that obviously couldn't carry a lot of current and didn't seem to always make a decent connection. I used a small piece of steel to reduce the restoring fixed magnetic field at the needle in order to make the ammeter more sensitive and actually read something - however, the needle also was more sensitive to turning-G forces (less restoring force).

I second the fact that the Mopar shunted-ammeter setups I looked into never seemed to indicate anything - even their 4cyl's years later had the same problem.

It is very hard to balance the ratio of very low resistances well, especially when you're going through a number of connectors. A calibrated external shunt would work fine, but not with the factory ammeter - you'll need a mV meter of some kind. Your signal will be small, so I'd recommend using twisted-wire or coax to carry the signal to the meter, or you could run the signal through an op-amp as a differential amplifier.

I'd start with the most senstive shunt - even at 100A (a lot!), you'd only have 0.05V of signal and that voltage drop is small w.r.t. the 0.4-1 V drop you often see in the harness. Even cranking at 220A you'd still hardly notice the drop; the internal resistance of the battery is much higher.

There are solid state devices that will measure large currents without inserting any resistance by measuring the magnetic field with a Hall Effect sensor - prices vary widely with the specs.

Thanks, KBB. A lot of good info there.

> The contacts for the shunted one in my '75 Coronet were just clips that
> obviously couldn't carry a lot of current and didn't seem to always
> make a decent connection.
That sounds just like mine.

> ... 0.4-1 V drop you often see in the harness

So it sounds like the meter may take somewhere between 400mV and 1000mV to read full-scale, vs. the typical shunted ammeter of 50mV (or 75mV or 100mV).

> I'd start with the most sensitive shunt...
I think I understand what you're saying. If a 5A shunt produces 50mV at 5A, then it should produce 1000mV at 100A (assuming that the output is linear, and assuming the shunt doesn't melt down).

This page
http://www.galco.com/scripts/cgiip.exe/ ... amily=BASE
shows the Crompton FN series base mount shunts from 1A up to 500A.

This datasheet
http://www.crompton-instruments.com/shunt828.pdf
shows that all the shunts from 1A to 200A are the same physical size - so maybe it wouldn't be inappropriate to run 200A through a 1A shunt if one wanted greater mV output.

> There are solid state devices that will measure large currents without
> inserting any resistance by measuring the magnetic field with a Hall
> Effect sensor
Interesting. Here's a guy using an Arduino to read a sensor:
http://www.arduino.cc/cgi-bin/yabb2/YaB ... 90470622/7

But before I go off on a huge science experiment, I ordered a surplus 2A 50mV shunt that I'll play around with on a bench setup. I'll post back when I get some numbers on what it takes to drive the ammeter and if Crompton has a problem with overdriving a shunt.

thanks,
arthur

Interesting.

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DRIVE IT,DON'T HIDE IT

A long while ago I ordered a solid-state limiter for the instrument cluster from these folks:
http://rt-eng.com/mediawiki/index.php/RTE_limiter

At the same time I posted here, I e-mailed them over the weekend asking if they had any thoughts on my ammeter situation, and I got a pleasant and educational response today having to do with putting a load on the car and measuring the current on the main lead and the lead to the ammeter. I don't actually have the harness installed in the car, but while thinking about rigging up a harness across the floor, I had a different thought.

The ammeter is measuring current by being in series with a lead in the car (as compared to a voltmeter which would be in parallel with the battery). Now it turns out that the lead is in parallel with a lead that handles the majority of the current, but the ammeter is still measuring current by being electrically connected in series.

So I set up a bench power supply for 12V, connected a headlight to it, then cranked the power supply current limit down to 0. I then added the car ammeter in series, and put a clamp-on ammeter on the circuit. I slowly increased the current - and the car ammeter reads full-scale at 0.2 amps. Due to the limited precision of my clamp-on ammeter, this could be anywhere from 0.1A to 0.3A, but at least it gets me in the ballpark.

I added a pic here showing the car ammeter at full scale while the clamp-on ammeter measures 0.2A.
http://s955.photobucket.com/upload/albums/sigmfsk/

My first thought was "my goodness, that factor harness is a scary setup". If the main lead in the wiring harness was to break (R68 in the wiring diagram), all the current would go through the ammeter and burn up the car.

The thought seems related to this sentence:
> You also need to be very careful with the wiring, so, if a fusible link
> opens, all current doesn’t pass through your new ammeter.
http://www.moparaction.com/Tech/quest1/ ... PERES.html

I don't know that it would "burn up the car", and it had to be unlikely that R68 would break while the ammeter wiring would remain intact, but it still seems scary.

I then thought about using a hall-effect sensor to drive the ammeter. I want something that reads a primary current of 100A and produces a secondary current of 0.2A, or a ratio of 500:1.
Now I see a lot of AC Current Transformers with a ratio like this, but it looks like I need a DC Current Transducer.

Here's one that takes 200A and produces 0.1A (2000:1 ratio)
http://www.lem.com/docs/products/la%20205-s.pdf
So close!

And here's a catalog page that shows one that takes 100A and produces
100mV:
http://www.flex-core.com/pdf-files/CTL-CTA.pdf
so that's specified similarly to a shunt.

So I still don't know what to do yet, but I'm getting some good data.

your friend in writing down ideas before I forget them,
arthur

Scary stuff.
There may be some input for you here. I personally think Chuck's advise in the last post in that thread is the best.

Olaf.

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Aspenized

Hi Olaf:
Thanks for the reference. I can understand there's a whole lot more going on when one runs all the current into the dash just for the ammeter. So my situation is slightly less complex since it uses an external shunt, and based on what I found today it may be significantly less complex!

The two dc current transducers I mentioned each require an excitement power source, so that's a strike against them (in my application) as compared to a shunt. And the flex-core transducer is over $200, so I pretty much shutdown further investigation into them.

Crompton didn't respond to my e-mail inquiry about overdriving their shunts. I found this document:
http://www.datelmeters.com/data/meters/ ... shunts.pdf
that talks about how one can overdrive a shunt for short periods of time. But it seems that overdriving is just not a good idea ... especially since it seems not to be necessary!

I got my 2A / 50mV shunt today, and tested it with my ammeter. The shunt and my ammeter played together quite nicely! The ammeter went full scale around 2.6A, or 2.6A / 2A * 50mV = 65mV to go full scale.

I added pics of the ammeter throughout the range (and beyond the range) here:
http://s955.photobucket.com/upload/albums/sigmfsk/

By "range" I'm indicating the needle going to the painted indicators on the gauge. The needle could actually continue quite a bit beyond those indicators.

So it seems that if I get a 100A / 100mV shunt, the shunt will produce 65mV when it reads 65A current. I have an 80A alternator. The alternator will rarely, if ever, be putting out 80A. And if it did, the shunt would produce 80mV which would be equivalent to what my ammeter saw today at 80mV / 50mV * 2A = 3.2A. The ammeter needle moved all the way through 4.4A, so this 80mV position (which I produced today at 3.2A with the 2A / 50mV shunt) seems completely in the safe zone for the ammeter needle movement.

I'll tentatively call this problem solved!
Thanks all for working through this with me,
arthur

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DRIVE IT,DON'T HIDE IT

sigmfsk,

How do you get to the photos? The link goes to a log on screen and a generic add page.
Where did you mount the shunt? parallel to the amp gauge on the back of the instrument cluster?

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Hi Ted:
Please try this link:
http://s955.photobucket.com/albums/ae40/sigmfsk/

The pics are all just of my bench setup showing the calibration of the gauge; I haven't actually mounted it in the car yet. I expect to mount the shunt in the engine compartment right next to the battery, so I won't need to run the main current wires through the firewall.

After the new year I expect have more pics. I found that the 1974 Monaco ammeter is actually a different part number than a 1976 Monaco ammeter. The ammeter used in all my pics and experiments described so far is of a 1976. The 1974 looks the same, but has less turns on the coil, so requires more current to move the needle to full scale.

I put some more Monaco-specific stuff here:
http://www.bluesbrotherscentral.com/for ... eters.html
and not to slight my Bluesmobile brethren, but this slant six site seemed best for discussion of ammeter wiring specifics, even though I don't have a slant six.

Being that I have to switch the gauge face from my 74 gauge onto the 76 gauge (so I have the correct face with the "best" coil wiring calibration), I'm thinking of using some 22 gauge enameled wire to add some turns on the 1976 ammeter, so I can use a 100A / 50mV shunt, increase the sensitivity of the ammeter by adding turns to the coil, and calibrate it exactly to read 80A at full scale.

Please let me know if the photobucket link doesn't work and I'll try again.
thanks,
arthur