H 12 Volt Coil??

FBH44

Well-known Member
Many years ago I put Delco alternator and 12 volt battery etc. on my Farmall H. In the parts box I now find a new coil, that I had spray painted a nice red, thinking I guess to preserve it. Question is, is this a NOS 12 volt coil, or n old 6 volt coil I saved from seomplace. Question is, what simple SIMPLE! electrical test could I do to see which voltage it is? Bear in mind, I have long thought been unable to understand electrical things.
 

If it is an original Farmall style coil, it is undoubtedly 6 volt. If it is a more modern appearing coil, it is probably still a 6 volt, unless it is one of those with that is marked "12 volt, no external resistor needed"

Even on factory 12 volt systems, the ignition still operates on 6 volts, either with an external ballast resistor, or an internally ballasted coil.
 
fbh, good question "what simple SIMPLE! electrical test could I do to see which voltage it is?"

MY ANSWER: take a good quality Ohm Meter that's accurate at low ohms range and measure the LV primary resistance between the coils little + and - (or BAT and DIST) terminals. If its in the 1 to 2 ohms range (1.5 typical of many not all) its probably a 6 volt coil, but if its more like 2.5 to 3.5 ohms range (3 typical of some but not all), its probably a 12 volt coil. That's due to the fact points shouldn't carry and switch much over 4 amps or they will burn up prematurely. I'm talkin old stock original type coils here NOT after market !!!!!!!!!!!!!!!!!

If its for an old H likely its a 6 volt

If its labeled theres an answer WELL DUH LOL but then you don't need to measure it now.

If its a 6 volt coil its fine for a 6 volt tractor or add an external series voltage dropping (12 down to 6) Ballast Resistor for use on a 12 volt tractor. More then likely (for the hundredth time) if its an old farm tractor coil it DOES NOT have any discrete stand alone resistor tucked away inside the can, even if some call them Internally Ballasted

Maybe other fine gents have other "simple electrical tests" as you asked to determine the coils voltage, but that's mine.

John T
 
A direct 12 volt (no external resistor) coil does not have a separate internal resistor. A 12 volt (external resistor required) or 6 volt coil usually have 1 1/2 ohms primary resistance between the + and - posts. A direct 12 volt (no external resistor required) has about 3 ohms.

There is no separate resistor inside the can of a 3 ohm coil. The difference is in the type and amount of wire used in the windings. A coil is actually a transformer with one side of the primary and one side of the secondary tied together.
 
"A coil is actually a transformer with one side of the primary and one side of the secondary tied together."

I've read where that configuration is referred to by some as an AUTOTRANSFORMER, versus a 2 in and 2 out with NO physical connection whatsoever TRANSFORMER, but NO WARRANTY I'm too long retired from practice lol

John T
 
John, Autotransformer is the term I learned so I think we would be safe to give it at least a "tail lights" warranty!!!!!!!

I have dissected several old coils, both 1.5 and 3 ohm and I believe that the negative post was the common post on all of them.
 
I have seen coil diagrams that show the - as the common (Primary to Secondary) and others as the + so darn if I know lol but I think hard core technically speaking with the physical wired connection they are indeed AUTOtransformers FWIW.

Fun sparky chatting with you

John T
 
(quoted from post at 16:01:45 07/10/16) I have seen coil diagrams that show the - as the common (Primary to Secondary) and others as the + so darn if I know . . .
Doesn't it depend on whether the coil is designed for a positive or negative ground system? Shouldn't the secondary winding connect to the end of the primary that connects to the distributor?
 
Jim, what you say makes sense. The LOW (NOT from battery and switch) end of the LV Primary goes to frame ground when points are closed and the plug fires from the electrode to the frame ground strap fed by the coils HV Secondary so that's how Id figure its done, but I know I have seen diagrams with it wired on both ends???

Take one apart and tell us all lol

John T
 
Jim, logic would dictate that, however, I don't believe that's the actual case in the "real world".

The primary resistance is so low compared to the secondary resistance it really doesn't matter which primary terminal is connected to the "low" end of the secondary winding, and a given coil will work equally well when the (-) terminal is connected to the distributor (- ground system), or the (+) terminal is connected to the distributor (+ ground) system.
 
From way back when---didn't Delco ignition systems
have the spark plugs fire from the block/ head to
the center electrode???

On IH & most other ignition systems, the high voltage
spark went from the center electrode to the ground/
block as I understood it.
Jim
 
Brownie, its easier to emit electrons (negative charged particles) from a hotter (plugs electrode) to a relatively cooler (plugs ground strap) surface (is it called Thermionic Emission ??). That's why they used a heating element under a vacuum tubes Cathode where electrons are emitted up to the cooler Plate. If a coil is wired correct to match the tractors polarity the spark electrons flows off the hot tip to the cooler ground strap, if not, she still fires but its not as efficient of a system. If you pull a plug wire off the plug slightly and have a lead pencil tip between the wire and plug and run her in the dark you can observe the direction of electron flow and you want it towards the plug not towards the distributor.

NO WARRANTY its been a while but that's my best recollection but may be wrong as rain grrrrrrrrrrrr

John t
 
Interesting read.
So can one say that all older tractors fitted with a points ignition have 6 volts at the points, and the coil is either 6 volts, 12 volts with an internal ballast resistor OR 12 volts with an external ballast resistor?
My Farmall 340 has a 12 volt system from the factory. The coil is not original, and got crushed when the bracket was over-tightened. I'd like to replace it, however it has no markings to indicate an internal ballast and there's clearly no external resistor in the wiring.
 
"12 volts with an internal ballast resistor OR 12 volts with an external ballast resistor? "

Once again, I will tell you there's NO SUCH THING in the mainstream/modern era as a "12 volts with an external ballast resistor" coil.

Simply call it a "true 12 Volt coil" and realize it's primary is wound with enough turns of wire of an appropriate gauge so that it DOES NOT HAVE/NEED a "resistor", either INTERNAL or EXTERNAL!

K?
 
(quoted from post at 07:30:49 07/11/16) On IH & most other ignition systems, the high voltage
spark went from the center electrode to the ground/
block as I understood it.
Jim
On the rotating magnet type magnetos (like the IH H-4 and J-4), doesn't the polarity reverse with each spark? Has anyone seen a pattern of 2 plugs opposite each other (in cap position) fouling more than the other 2? The only general pattern I have seen is the ones farthest from the radiator (lower temperature) fouling more.
 
"So can one say that all older tractors fitted with a points ignition have 6 volts at the points, and the coil is either 6 volts, 12 volts with an internal ballast resistor OR 12 volts with an external ballast resistor?"

NO

A 6 volt coil is wound with a primary resistance of about 1.5 ohms and will operate with a 6 volt battery and no added resistance. The points will see 0 volts when closed and battery voltage when open. There will be 6 volts across + and - posts of the coil with points closed.

The 6 volt coil (same as 12 volt, external resistor required) will operate in a 12 volt system with an added resistor with value near equal to the primary resistance of the coil. The points will still see 0 volts when closed and battery voltage when open. There will also be 6 volts across + and - of the coil with points closed.

As mentioned by others, in the distributor systems of tractor world we are dealing with, there is no such thing as an internally ballasted coil.

A 12 volt coil (no external resistor required) does not have an internal resistor. The primary is wound to give about 3 ohms resistance through one continuous coil of wire so that it can operate with direct application of 12 volt source. It is not wound with 1 1/2 ohms of coil and an added internal resistor.

With this coil the points will also see 0 volts when closed and battery voltage when open. There will be 12 volts across + and - when points are closed.
 
Yep, a magento has alternating North and South magnets, so one plug fires at incorrect polarity and I bet the coil voltage has to rise just a bit higher before that one plug fires. On a standard ignition if the polarity is correct the voltage doesnt have to rise quite as high before current arc jumps the gap as I understand it and a physicist friend (the late Duane Larson) once measured and confirmed.

John T
 
Bob, I have told that for YEARS so good luck if you can convince them, I never could lol

John T
 
(quoted from post at 14:00:27 07/11/16) "So can one say that all older tractors fitted with a points ignition have 6 volts at the points, and the coil is either 6 volts, 12 volts with an internal ballast resistor OR 12 volts with an external ballast resistor?"

NO

A 6 volt coil is wound with a primary resistance of about 1.5 ohms and will operate with a 6 volt battery and no added resistance. The points will see 0 volts when closed and battery voltage when open. There will be 6 volts across + and - posts of the coil with points closed.

The 6 volt coil (same as 12 volt, external resistor required) will operate in a 12 volt system with an added resistor with value near equal to the primary resistance of the coil. The points will still see 0 volts when closed and battery voltage when open. There will also be 6 volts across + and - of the coil with points closed.

As mentioned by others, in the distributor systems of tractor world we are dealing with, there is no such thing as an internally ballasted coil.

A 12 volt coil (no external resistor required) does not have an internal resistor. The primary is wound to give about 3 ohms resistance through one continuous coil of wire so that it can operate with direct application of 12 volt source. It is not wound with 1 1/2 ohms of coil and an added internal resistor.

With this coil the points will also see 0 volts when closed and battery voltage when open. There will be 12 volts across + and - when points are closed.
Thanks for the concise (and polite) explanation.
 

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