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Lets get AMP'd up

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I am planning on using 14gage wire, SPST toggle switch, and a 7.5 amp fuse. Not sure yet if I will power it off the ignition switch, or a separate circuit directly from the battery. Thoughts from the teachers?

My only thought is to not limit future expansion of the light circuit with a 7.5 amp fuse.

If a short develops in the light wiring, even a 15 amp fuse will blow quickly and protect the wires from burning. I would go with at least a 10 amp - even a 15 amp fuse. If you already have the 7.5 in there, no problem.

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Alrighty, then,

Here is the autopsy victim.

CopyofIMG_1384-1.jpg

The complaint was no charge indication but the tractor C120 seemed to be working just fine. Always started without complaint but no charge indication on the ammeter.

The first check - measuring the battery voltage with the engine running - showed a charge voltage of around 14.5 volts. Fine and dandy.

With the tractor off, the headlights were turned on and only the slightest blip of the indicator was seen into the discharge region of the gauge scale. So the gauge was indicating about 1 amp of draw when it should have been about 5 - 6 amps of draw.

The ammeter leads were disconnected from the ammeter body and a multimeter set to 10 amps was placed into the wiring, effectively substituting for the ammeter.

The tractor lights were turned on and the multimeter indicated over -5 amps of current.

The ammeter has been confirmed to be defective.

Disassembling the ammeter

IMG_1386.jpg

the gauge bezel was removed, the spring tension retaining ring removed from around the gauge face and also the gauge face itself was remove.

The result is a look into the inner working of the ammeter.

compassampmorph2.gif

ammgaugelabeled1.jpg

The mechanics of the gauge look very similar to the morphed drawing of the compass into a gauge as seen earlier in this post.

Let's trace out the current pat through the gauge.

ammgaugelabeled2.jpg

Remember, the direction and amount of current flowing through the gauge will determine the direction and amount of magnetic force acting on the permanent magnet attached the indicator pointer.

In this particular gauge, the mechanics of the gauge have become so corroded and oxidized (white specs near the pivot point), the indicator pointer was no longer free to move with the magnetic forces produced by the charge and discharge currents through the gauge.

Please note the electrical function of the gauge was intact. It was a mechanical failure that caused no charge or discharge indication to be seen.

The next update will include a few hints on checking the operation of the gauge with a multimeter and how to confirm proper calibration of the gauge.

An yes, on this particular gauge, no rebuild was attempted, so for all intents and purposes, this type of gauge is a throw - away if it fails to function properly. No SOI sticker for this one.

SOI_UNIVERSITY_WAS_HERE_DECAL_DESIGN2.jpg

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how about if I yank the wire off of the battery charging thing and if it arks on the frame then it charging?Thats what my uncle moe said.

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Please let me add an important point about ammeters.

Most ammeters that read more than milliamps (ma - 0.001 amp) use something called a "shunt". A shunt is a very accurate, low ohm (0.1 ohm or less) resistor. The shunt is wired across the 2 terminals and then the meter movement is wired in parallel to the shunt.

Remembering Ohm's Law (you don't think I wasted your time on that for no good reason !) V / R = I, if you have a 0.01 ohm shunt, a 0.1 volt drop across the shunt would indicate 10 amps. And yes, that means that most ammeter are actually a voltmeter and a shunt ! Wikipedia Ammeter

How is this useful ? Well, SOI has said before that when using your DVM to measure current, you should always start at the highest scale. This is because if you start on too low of a scale you may actually MELT the internal shunt ! It is also why on many meters the highest current scale has a separate input so that its shunt can be directly connected across those terminals and not through the rotary switch which might over heat with a big load. (I actually built a kit multimeter many, many, MANY years ago.)

You can buy big (very low ohm - 0.001) external shunts, but they are expensive. A battery load tester is just a large (typically not accurately calibrated) shunt, either fixed or variable.

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Most ammeters that read more than milliamps (ma - 0.001 amp) use something called a "shunt". A shunt is a very accurate, low ohm (0.1 ohm or less) resistor. The shunt is wired across the 2 terminals and then the meter movement is wired in parallel to the shunt.

and I was surprised to find this style ammeter was something very different than I expected.

Below is a pic of the indicator pointer pivot area "surgically removed" from the ammeter housing.

ammgaugepivotpoint.jpg

The pivot point and the barrel magnet on the pointer are clearly shown. There are no wires inside to form a shunt style meter here. This one appears to be pure electromagnetic repulsion / attraction with a pendulum style return to 0 movement.

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Thanks for the surgery !

Different than I expected also.

I try to be careful (in my old age) to not use the word "all". "Most" is sufficient to cover the cases I have actually laid my eyes on or can find direct references too.

Staring some more at the pictures (great macros, what are you using?) , wasn't the indicator pivot mount (silver colored U shaped piece) cut from the over 2 silver colored pieces of metal that the terminals are riveted to ? If so that silver metal would form a "complete circuit" from terminal to terminal and, in fact, possibly be the shunt itself ? (SWAG = Sophisticated Wild A$$ Guess) Where you cut the mount could potentially be filed to calibrate the meter ?

I always remember what my good friend told me years ago, "A wise man doesn't have to know everything. He just needs to know what book to look in (or how to use Google) !"

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Now for the checkout part of the post.

How to tell if the ammeter is actually the cause of your problem.

In this particular case, the tractor appeared to be charging but the amp gauge would not register a charge or discharge. Previously we mentioned how the gauge pointer would not move to a discharge indication even with the tractor off and headlights on. Normally, the gauge should indicate about 5 - 6 amps of discharge with the engine off and headlights on.

For a very quick and simple functional check, hook up a known good 9Volt battery DIRECTLY ACROSS THE AMMETER TERMINALS. The small current capability of the 9volt battery will not allow any damage to the ammeter.

DO NOT HOOK THE AMMETER DIRECTLY ACROSS A 12 VOLT LEAD ACID BATTERY AS THE AMMETER WILL PROBABLY BE DAMAGED FROM EXCESSIVE CURRENT FLOW.

Remember, the ammeter is virtually the same as a piece of wire.

You will see either a charge or discharge indication on the gauge depending on the polarity of the battery terminals you hook to the gauge terminals. On the fresh 9 Volt battery I tried on a good ammeter, a 7 - 8 amp indication was seen. The battery will drain against the direct short of the ammeter gauge so limit the amount of time you have the battery connected. No damage will take place to the gauge, but the battery will drain quickly.

Reverse the battery connections to the gauge and the gauge pointer will swing to the opposite indication on charging state. An equal but opposite deflection should be seen on the gauge.

With an ohmmeter test, place the ohmmeter on the lowest ohms range and place the meter leads directly across the ammeter terminals. Orientation of the meter leads positive to negative is not important, but a near 0.00 ohms reading is. For all intents and purposes, the ammeter should measure a direct short to the average ohmmeter function. Refer back the pics indicating the current flow thru the gauge. Its all happening thru a fairly stout piece of strap metal.

The main failures of an ammeter could be summed up in one or two failure modes.

Mechanical failure of the gauge pointer mechanism

or

Poor electrical connections to or from the ammeter itself.

Not much else to go wrong inside this puppy ! :banghead:

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(great macros, what are you using?)

Just a run of the mill Canon AS590 point and shoot with a small ringlight attached.

I did choose the Canon over several others because of sharpness at the macro setting.

The AS590 actually was one of the least expensive cameras I looked at but had some great "old fart" features on it.

I also have a super quality stereoscopic forensic microscope but have not yet picked up a digitizing eyepiece for capturing digital pics. That piece can see the how many angels can dance on the head of a pin. I use the microscope to repair Ipods !!

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so i kind of got lost a little here,can these or the volt meters on our horses be repaired or do we have to spend the big bucks to replace them?

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so i kind of got lost a little here,can these or the volt meters on our horses be repaired or do we have to spend the big bucks to replace them?

In this particular gauge, the mechanics of the gauge have become so corroded and oxidized (white specs near the pivot point), the indicator pointer was no longer free to move with the magnetic forces produced by the charge and discharge currents through the gauge.

The good new is, this part of the meter is viewable through the front glass.

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so i kind of got lost a little here,can these or the volt meters on our horses be repaired or do we have to spend the big bucks to replace them?

Hey Don,

first off, Welcome Back !

My purpose on this post was not so much to show how to fix the gauge itself but to educate all those interested on what the failure modes would be for an ammeter.

As you can see from the dissection, there is just not a whole lot to go wrong ELECTRICALLY inside the gauge. However there can be several issues develop from condensation that could end up corroding the indicator pointer suspension and causing the gauge to fail MECHANICALLY.

On the outside of the gauge, corroded terminals and poor wiring practices will usually be all that is needed to be corrected to repair the ammeter and restore the charging function of the tractor.

Essentially, for electrical troubleshooting purposes, the ammeter should be considered as a piece of wire added into the charging circuit.

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As an afterthought, I did receive a email asking me if a 60 amp gauge is an acceptable substitute for a 20 amp gauge.

The answer is Yes. Electrically, the 60 amp scale is functionally identical to the 20 amp scale.

The one obvious downside is the 60 amp range of the gauge. Given the charging system of the tractor is rated for 15 amps, 20 at best, you are wasting gauge face "real estate" with a range the charging system will never produce. Any markings on the scale above 20 amps are just a waste. You will have a much harder time seeing a 5 amp charge rate on the 60 amp scale than you will on the 20 amp scale.

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and I was surprised to find this style ammeter was something very different than I expected.

The pivot point and the barrel magnet on the pointer are clearly shown. There are no wires inside to form a shunt style meter here. This one appears to be pure electromagnetic repulsion / attraction with a pendulum style return to 0 movement.

Actually, it looks to me like the "shunt" and the "winding" are one and the same. The winding has only one "loop".

Not what I expected, but actually a good design.

All ammeters I have examined in the past were a d'arsonval meter movement with a shunt added (at the back of the meter external to the meter proper).

This design does the job more simply, but you cannot convert it to a regular meter movement by removing the shunt.

.....learn something new every day... :ROTF:

:thumbs: :banghead:

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As an afterthought, I did receive a email asking me if a 60 amp gauge is an acceptable substitute for a 20 amp gauge.

The answer is Yes. Electrically, the 60 amp scale is functionally identical to the 20 amp scale.

The one obvious downside is the 60 amp range of the gauge. Given the charging system of the tractor is rated for 15 amps, 20 at best, you are wasting gauge face "real estate" with a range the charging system will never produce. Any markings on the scale above 20 amps are just a waste. You will have a much harder time seeing a 5 amp charge rate on the 60 amp scale than you will on the 20 amp scale.

"Much harder to see" is an understatement. I had a 60 amp meter on the first rev of my dashboard and it was impossible to detect charging (10 amp system in my 1969 K301).

:thumbs: :banghead:

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SOI, for whatever reason a visitor can't access Wheel Horse Electrical here on Red Square. Can I have permission to copy and paste your thread to a truck site that has a question regarding this topic?

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Can I have permission to copy and paste your thread to a truck site that has a question regarding this topic?

Absolutely.

Feel free to link or copy any of MY content.

Zieg, let me know if there are any more specifics you would like me to touch on.

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Chuck once again great tutorial with a wealth of information. I even learn alot from them. I wish I was could explain things, but I get to frustrated cause I can't explain what I'm thinking or doing well enough or as you can!

Thank you

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I wish I was could explain things, but I get to frustrated cause I can't explain what I'm thinking

I don't know Mav,

I have seen some of your work and I truly believe you could teach us all a thing or two. I have seen some of the ideas you fabbed up and I personally have been inspired by some of your projects.

And to do all that while raising a family - having already been thru that, I tip my hat to you my friend.

And now for a little motivational experience about being told what you can and can't do ....

defiance.jpg

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Great Post but i have been wondering on my grandpas 702 we are trying to figure out if it is charging because the amp meter is sort of working. it rocks back and forth a tiny amount. how would i go about testing the amps being put out by starter gen. Thanks,Jordan :thumbs:

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the amp meter is sort of working. it rocks back and forth a tiny amount. how would i go about testing the amps being put out by starter gen. Thanks,Jordan :thumbs:

Jordan,

best to start another post specifically for your issue. Then we can get into details. :thumbs:

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the amp meter is sort of working. it rocks back and forth a tiny amount. how would i go about testing the amps being put out by starter gen. Thanks,Jordan :thumbs:

Jordan,

best to start another post specifically for your issue. Then we can get into details. :thumbs:

Ok i will do! thanks!

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