Ammeter and protection

[Joe Guilbeau]

I won't deny that. I'm just trying to think of a good use for that wire.

Got any wells?

[jsinajeep]

Best of both world is both meter. Very true. And that is a very good write up Joe. That well help a lot of people that do not under stand the different of the two meter.

[sungoesdown]

Was that the place across the street from Sound Advice's Audio Garage that had the bombs out front? I think it was on W. Fairbanks.

Yep. I miss that place. http://www.skycraftsurplus.com/index.asp

[caionneach]

Now I can put an upgraded ammeter in the place where the stock oil gauge once worked and have both voltmeter and ammeter.

[mdill]

Personally,

I would change the cable choice to SAE J1127 SGX insolated cable (From Waytek) cheaper, and rated to 125 Degree's rather than 105, and the insolation is made to live with oil/grease ..
(On my short list of projects, which mean's it will be awhile )
Chevy has used shunt ampmeter's for a long time, usually they used the alt output wire as the shunt, and don't forget to fuse the shunt to meter leads lest you don't end up back to where you started from burning your wiring up.

Mike D.

[reddawn222]

Joe,

Your write up on protecting the ammeter in our FSJs is very informative and appreciated. I do have a couple of clarifying questions:

1. Your write up discusses replacing the ammeter with a newer gauge. Can the stock ammeter be retained and is it protected, or is the upgraded gauge mandatory?

2. Can you provide more detail on the function of the shunt? I got the impression the shunt converts volts to millivolts and this is central to protection of the gauge.

Thanks for any clarification.
Gary

[djongeward]

I would like to know when Jeep switched to voltmeters? I seem to have one in my 89.

[mdill]

Volt meters came in with the new dash in 1986,
The stock amp guage is a full flow though set up, all the current to-from the battery goes though the guage.

Joe is recomending replaing the guage with a shunt style amp meter (really a millivolt meter but calibrated in amps with the proper shunt)
with this set up only a small (ma level) current goes though the guage most of the current (like 99+% ) goes though the shunt.
If you do this there is no easy way that I know of to modify the stock guage, (from a quick look it does not have an internal shunt that could be removed) to work when doing this modification.
Remamber to fuse the new ampmeter leads (low amp rating fuse(s) ) or you just trade one possable smoke generator for another.

Mike D.

[jaber]

Thank you for the writeup.

[Joe Guilbeau]

Volt meters came in with the new dash in 1986,
The stock amp guage is a full flow though set up, all the current to-from the battery goes though the guage.

Joe is recomending replaing the guage with a shunt style amp meter (really a millivolt meter but calibrated in amps with the proper shunt)
with this set up only a small (ma level) current goes though the guage most of the current (like 99+% ) goes though the shunt.
If you do this there is no easy way that I know of to modify the stock guage, (from a quick look it does not have an internal shunt that could be removed) to work when doing this modification.
Remamber to fuse the new ampmeter leads (low amp rating fuse(s) ) or you just trade one possable smoke generator for another.

Mike D.

Thanks Mike, that just about covers it. When a current is flowing through a wire, any resistor that is connected in series with that wire will now have current flowing through it.

When current flows through resistance a voltage is present across the resistance. This is why a voltmeter can measure voltage, you place the leads of a voltmeter across, say battery terminals, and now you will have a current traveling through the meter leads. The meter reads that voltage that is developed inside the meter (it has, what we can consider for this brief topic, a resistor inside it).

So, now we look at the ammeter, it would be expensive to manufacture a meter that actually reads the current passing through it, so what the engineers do is to to place a metal bar across the leads of a milli-voltmeter and that metal bar (the shunt) develops a voltage across it when current flows through the bar (from one post of the "Ammeter" through the shunt to the other post.

The mill-volt meter (Ammeter) now reads that small voltage that is developed across the shunt resistor and the meter movement transfers it to the ammeter gauge needle.

What you are in reality looking at is a milli-volt measuring device that is calibrated to display a specific millivolt for a specific amount of current that it senses going through the shunt resistor.

The folks who manufacture the meter gauges themselves put a nice circle of paper inside the face of the meter that shows -60 -0- +60, and just the same as you would reverse the meter leads that you use to measure the voltage on the battery, if the current flows from the battery without sufficient charge from the alternator to replace that current, then the "Ammeter" will indicate a negative voltage shown as a discharge of current.

This is why it is useful along with a voltmeter, another way of looking at it is that your battery is the Water Tower that supplies drinking water to the community. The pressure differential between the top of the water column in that tower to the base is the "Voltage" the size of the pipes delivering that water under pressure may be thought of as the resistance... 16-inch main pipe has "less resistance" than your pipes at your sink. That is, the smaller pipes have a greater resistance to water flow than the larger pipes due to the fact that they are less resistive to the water flow.

The higher the pressure in that tower (full of water) the higher the pressure in the water lines, it that tower was only 3/4 full (about 9.56 volts on a 12.75 normally charged battery), then there would be less available pressure and some homes (circuits) would not be able to get enough water pressure to run the shower, dishwater etc...(circuits in the vehicle).

Maybe I am just rambling on here, if this answers you question, then great, otherwise post up and we will get it right the next time.

Yes, I would absolutely replace that Ammeter Guage in the dash, due to the fact that numerous folks have had a problem with the ammeter heating up due to no maintenance on the wiring scheme that allows current to flow through the meter posts, and/or an internal problem with the guage that burns up with the truck when a problem occurs.

As far as I know, there have been no reports of problems when folks just take one lead off of one ammeter post and place it on the second post, thus "Bypassing the Ammeter".

Will these two wires connected to a post eventually cause a problem, **** right. The good news is that this will be long into the future in a galaxy far, far away.

[joe]

Ammeters failing to not cause fires. Loose or dirty connections at the ammeter creates heat and WILL cause fires. Keep the connections clean and tight and you'll be fine.

[jaber]

so what the engineers do is to to place a metal bar across the leads of a milli-voltmeter and that metal bar (the shunt) develops a voltage across it when current flows through the bar (from one post of the "Ammeter" through the shunt to the other post.

So, with me not knowing the details, couldnt you put a bar between the posts of the stock unit and keep it from melting?
And if done, would it still read amperage?

Sorry, I know I'm an idiot when it comes to amps....

[Joe Guilbeau]

So, with me not knowing the details, couldnt you put a bar between the posts of the stock unit and keep it from melting?
And if done, would it still read amperage?

Sorry, I know I'm an idiot when it comes to amps....

Yep, I have to agree... you are too ignorant to attempt this.

Why in the world would you attempt to do something of this nature not knowing what the consequences of such actions might be.

Yes you could put a bar (what kind of metal and how in the hell do you intend to measure the resistance of the piece) of metal in the shunt position, but why?

If you have got a 8 digit Fluke calibrated Digital meter that has been warmed up for a minimum of 45 minutes.....

If you have a calibrated current source to place a very small amount of current through the bar of metal...

With a calibrated multimeter and a lab quality QA'd current generator you could place a current through the bar, then you would have an idea of the measured voltage dropped across that bar.

Now all you have to do is figure out what bar of metal will give the same resistance as the unit that you are replacing.

Do you know how much current that that bar of metal that you will be using will withstand? Without heating up the 20 year old Ammeter and associated wiring in the general vicinity?

What if your calculations are off when you make this shunt bar of metal, make some more, until you get it right?

Are you trolling here?

[jaber]

Yep, I have to agree... you are too ignorant to attempt this.

Why in the world would you attempt to do something of this nature not knowing what the consequences of such actions might be.

Well in the first place I asked a question to see if you would try to help a fellow jeeper out, but I guess your ego got in the way. It was asked to find out what the consequences would be. NOT to be BASHED upon!!!!

THANKS FOR YOUR HELP.

[Joe Guilbeau]

Come on Jeff...

"...So, with me not knowing the details, couldnt you put a bar between the posts of the stock unit and keep it from melting?
And if done, would it still read amperage?

Sorry, I know I'm an idiot when it comes to amps...."

Geeze, I guess I caught you in a bad mood.

In all honesty, I was bashing my head a bit and well, I guess I may have stepped over the line....

No blood, no foul.

You gotta admit, I did try to answer questions on the Shunt, but to attempt to make one out of metal?

Here is what I will do as my pennance, I will work with you to make a metal shunt to add to whatever ammeter that you have installed in your Jeep.

How's that? After all, you claim to have asked this in order for someone to reply to help a fellow jeeper out. So, I must conclude that you will, indeed attempt to do this.

Now, to your point... the bar between the posts of the ammeter will have a current passing thru the bar, that current will develop a voltage drop across the resistance of the bar between the two posts on the ammeter. That voltage is read by the milli-voltmeter (substitute ammeter) and the graphics on the front of the gauge are calibrated so as to give a relative millivolt reading in direct proportion to the amps passing thru the bar between the posts.

Could we come up with a bar (substrate material) with the exact doping required to allow 100 amps to run thru it while at the same time giving an accurate millivolt reading relative to the amps passing thru the bar?

Perhaps, but with manufacturing facilities it would be very difficult.

Perhaps I took you too seriously and misunderstood your question to me.

If so, please accept my apologies for running roughshod over what must be a sensitive and gentle soul...

I admit I can be rough around the edges, but in all honesty... you can't have no idea how little I care.

Ooppsss. there I go again with my pithy comments.

Jeff, I don't have a problem with you, and in all sincerety, if you feel that I bashed you, then I really do apologize...

Fair enough, all ego's aside... yours AND mine?