confused over coils

I have a new 6 bolt coil that says doesn't require an external resistor. I didn't know a 6 volt coil ever required an external resistor. I'm planning to put it on a 12 volt system so I assume I still need an external resistor. I've had much better luck on my 12 volt conversions with a 6 volt coil and external resistor than I have buying 12 volt coils that don't need a resistor. Can someone educate me on what I have and am I correct needing the additional resistor on a 12 volt system.

You determine what you have by reading the resistance on the coil primary. That is all there is to it. Many car and tractor companies used the same coils on their 6 volt systems as their 12 volt systems. Resistance is resistance. It does not matter if it is all inside the coil, or half inside, and half outside via an external resistor. Main point is - a 6 volt coil can be used on a 6 volt or a 12 volt system. A 12 volt system-only coil can NOT be used on a 6 volt system.

A 12 volt system that tends to run at 14 volts typically has a total primary resistance of 3.2-4.2 ohms. It can all be built into the coil, or part in the coil and part in an external resistor. John Deere used a 6 volt coil with 2.4 ohms resistance for 6 volt systems. When they changed to 12 volt, Deere used the same coil but added a 1.8 ohm ballast resistor into the wiring.

Very good and logical explanation! Is it not very common to bypass an external resistor while cranking to give a hotter spark? I thought that was one reason for the external resistor.

Many car companies determined that the starting process in cold weather (Detroit) was a problem. The starter could draw enough current that the system voltage might drop to 4.5 volts. The coil was only given that amount, causing weak spark. The solution was to use a coil that was designed for 3.5 volts or so giving the starting process a good chance. This starting voltage was delivered to the coil from a connection direct to the starting circuit. this is only supplied when the starter is turning the engine. (several methods are used to do this "bypass" circuit) When cranking results in a start, the bypass voltage is shut off. The ignition circuit through the switch has been on all this time, but it runs through a resistor dropping its voltage to the 4.5 or so volts for the coil. Remember that the running voltage of the system is a bit over 7 volts, charging the battery. This keeps the coil happy, and allows cold starts to be much more successful. Jim

The only true way to know is to check the resistance across the primary terminals, out of circuit.

If it's down around the 1.5 to 2 ohm range, it's a 6 volt coil.

If up around 3 to 4 ohm, it's a 12 volt coil.

Even if the can is factory stamped specifically what it is, best to check. They often get reboxed, counter people confused, incorrect listing, and aftermarket unknown values.

Yes it is. One way to do that if there was never a bypass, assuming a ballast resistor is in the circuit, is to run a 14 gauge wire from the starter side of the big starter switch, to a diode (must be there), then to the coil side of the resistor. That diode prevents electrical flow from the coil to the starter, but allows it from the starter to the coil. Jim

(quoted from post at 07:12:59 04/25/20) Very good and logical explanation! Is it not very common to bypass an external resistor while cranking to give a hotter spark? I thought that was one reason for the external resistor.

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There are two types of "external ignition primary resistors".

The first type is a simple resistor who's value doesn't vary much as it heats up. This is the type that is typically used when there's a system to bypass it during cranking.

The other type is a "PTC" (positive temperature coefficient) resistor that starts out cold with low resistance, then warms up and it's resistance quickly drops. That allows higher ignition voltage for starting, and protects the points and coil after startup. The Ford 9N, 2N, and 9N used this system, for example.

If you look in a generic ignition parts catalog (Standard Ignition, as an example) you will note that a wide variety of primary ignition resistors was used by the OEM's, with varying specifications.

INDEED many 12 volt tractors used a 6 volt coil PLUS an external series voltage dropping ballast but they by passed the ballast ONLY WHEN CRANKING which could improve cold weather starting when battery voltage may be reduced.

John T

Charlie, great question myself as well as others have covered this extensively over the past thirty years, but no harm in re educating everyone.

1) On a 12 volt tractor you can use EITHER a full true 12 nominal volt rated coil orrrrrrrrrrrrrr a 6 nominal volt rated coil PLUS an external series voltage dropping (12 down to 6 volts) ballast resistor. If you used a 6 volt coil on a 12 volt tractor (with no ballast) it can overheat badly PLUS the points burn quickly

2) A Coil designed to operate at 12 nominal volts can produce a spark if during starting voltage is reduced to 10 or so volts ORRRRRRRRRRRR if under a high charge battery voltage may be 14+. It will operate, produce a spark and not overheat if operated at those brief lowered starting or higher charging voltages.

3) A so called 12 volt coil may be labeled "12 Volts" orrrrrrrr "12 Volts NOT for use with ballast" orrrrrrrrr "12 Volts no ballast required" etc etc HOWEVER BEWARE A coil labeled "12 Volts for use with or requires external ballast" is more like a 6 volt coil and as it says requires an external series voltage dropping (12 down to 6) ballast or it can overheat!!!!!!!!!!!!!

4) "Most" old tractor 12 Volt coils DO NOT have an internal stand alone discrete "resistor" tucked away inside the can even if they may be referred to by some as "Internally Ballasted" I am aware some old cars (maybe tractors???) used coils that indeed had stand alone discrete "resistors" (maybe temperature compensated) in the bottom of the metal can some with a noticeable ring around the perimeter.

5) A typical 12 Volt coil may have a LV primary winding resistance of 2.5 to 4 (many around 3) ohms.

6) A typical 6 Volt coil may have a LV primary winding resistance of around 1.2 to 2+ (many around 1.5) ohms.

7) Some tractors might use the SAME 6 volt coil on EITHER a 6 or 12 volt tractor, its just that on a 12 volt they add the external series voltage dropping (12 down to 6) ballast resistor.

8) Some 12 volt tractors used a 6 volt coil with an external ballast PLUS A START BALLAST BY PASS SWITCH/SYSTEM to improve cold weather starting.


FINALY TO YOUR SPECIFIC QUESTION " I'm planning to put it on a 12 volt system so I assume I still need an external resistor."

YES if the coil is 6 volt rated and you want to use it on a 12 volt tractor you need the series voltage dropping ballast or it could overheat and the points will burn quickly


"Can someone educate me on what I have and am I correct needing the additional resistor on a 12 volt system"

SEE THE ABOVE

Hope this helps, best wishes

God Bless America, keep her safe strong and great

John T

Ran into the same question after buying a 6 volt coil at TS. Seems at times meanings get lost or confused in translation in country of manufacture. My 6 volt coil also said no external resistor required. Using above listed resistance readings for a 6 volt coil, found it was just a standard 6 volt coil that DID need an external ballast resistor. So guess they meant. (If your system is 6 volt), no external resistor required?? Because questions like these cause me to have concerns for the overall quality of the coil. Spent a bit more money for a 6 volt coil from Bosh made to fit an old BMW motorcycle. Such as a 1975 BMW R90/6. My 1940 Farmall H loves it.

Good Luck

John - I've read a lot of your threads about various things over the years and they have all been real helpful. Maybe that's why I thought I understood coils until this one came along and made a statement about resistance which I wasn't expecting and had never seen on a 6v coil before. Thanks for the detailed explanation. I'm going to print it out and save it for the next time I'm running into coil issues. Thanks to everyone else that replied too.

John T, A question. Back in my youth I put a 1956 chevvy 6 in a 1951. The 51 was 6 volt but the 56 was 12. I left the coil on the 56 & used it on 6 volts. That engine always seemed to start hard especially when cold. I didn't know any better at the time but I think I should have put the coil on from the 51. What do you think?

I guess its "possible" in translation and in labeling things might? get confusing if different countries are involved ? Another important and limiting factor when it comes to deciding how much total (coil + ballast) ignition circuit resistance is required is to consider the current the ignition points much pass and switch. Typically on many old tractors (sure depends on points design and construction) the current the points must switch is limited to around 3 to 4 max amps, otherwise they will burn prematurely.

THEREFORE at 12 volts if there's 3 ohms of total (coil + ballast) resistance the points current would be 12/3 = 4 amps, while if at 12 volts the total (coil + ballast) resistance were 4 ohms, the current would be 12/4 = 3 amps. As far as ballast resistance, if you wanted a 50/50 voltage divider and used 12 volts as the baseline (higher if charged) and wanted to drop 6 volts across a 6 volt coil and 6 across a ballast, the coils LV primary resistance would be the same as the ballast resistance. However duty cycle and points dwell all play into this equation, and battery voltage will vary subject to temperature, load and state of charge, and charging parameters etc. etc.

Coils (design, voltage and resistance etc) for certain cars or tractors or motorcycles or lawn mowers are certainly NOT all the same...?...

Nice sparky chatting with yall

John T

You're welcome Charlie, and thank you also. Like you I don't recall such a label on a 6 volt coil but I haven't seem allllllllllll of them lol

All that label tells me as an engineer is if 6 volts is applied to that coil it will produce a spark AND NOT OVERHEAT IE NO ballast required just like it says

John T

Hey there Teddy, if I understand the question (51 or 56 and which) a 12 volt coil on a 6 volt car could still produce a spark but it would be weak and even weaker when engine cranking drew the battery voltage down causing hard starting. However, a 6 volt coil unballasted on a 12 volt car would eventually overheat and damage the coil plus burn the points soon.

You already know all this but its still fun sparky chatting with you anytime

John T

Using a 6V coil and ballast resistor allows for bypass starting.

That's a great idea to use on a starter selenoid with no terminal to bypass a resistor!

Don't think my comments on 6 volt coil labeled no ballast required, was taken seriously. Plus comment on using bosh motorcycle 6 volt coil was dismissed as foolish. The first item should be taken seriously. If new coil is mislabeled as no ballast resistor required, a burned up new coil and being stalled miles from home could be the result. The values that are in question on coil mislabel are as follows. Normal spec for 6 volt coil resistance is 1.25 to 2.0 ohms. Since the ballast resistor is in series with the coil, if a ballast resistor is built into the 6 volt coil as the label infers, internal resistance should read between 4.65 to 5.4 ohms. Ballast resistors for tractors I've worked on measure 3.5 ohms. But the new 6 volt coil in question measured 1.3 ohms well within spec for a common 6 volt coil with no internal ballast. If coil is labeled correctly, where did the added resistance of the internal ballast resistor go? Proof of my conclusion was a weak spark with new coil, no ballast resistor. With ballast resistor, spark was much stronger blue in color. And would produce a spark across a 2 3/4 inch gap on an ignition spark gap tester. Inferring lawnmower, car motorcycle coils being used also minimized the facts of my comments. The spec sheet for a Bosh 6 volt coil built for a 1975 BMW R90/6 motorcycle fits the specs and physical size for a 6 volt tractor coil. Similar power, points ignition. Voltage is 12 volts, but two coils are in series so no ballast is required. With all that said, just saying I stand by my comments and suggestions Mr John T.

(quoted from post at 10:16:48 04/26/20) Don't think my comments on 6 volt coil labeled no ballast required, was taken seriously. Plus comment on using bosh motorcycle 6 volt coil was dismissed as foolish. The first item should be taken seriously. If new coil is mislabeled as no ballast resistor required, a burned up new coil and being stalled miles from home could be the result. The values that are in question on coil mislabel are as follows. Normal spec for 6 volt coil resistance is 1.25 to 2.0 ohms. Since the ballast resistor is in series with the coil, if a ballast resistor is built into the 6 volt coil as the label infers, internal resistance should read between 4.65 to 5.4 ohms. Ballast resistors for tractors I've worked on measure 3.5 ohms. But the new 6 volt coil in question measured 1.3 ohms well within spec for a common 6 volt coil with no internal ballast. If coil is labeled correctly, where did the added resistance of the internal ballast resistor go? Proof of my conclusion was a weak spark with new coil, no ballast resistor. With ballast resistor, spark was much stronger blue in color. And would produce a spark across a 2 3/4 inch gap on an ignition spark gap tester. Inferring lawnmower, car motorcycle coils being used also minimized the facts of my comments. The spec sheet for a Bosh 6 volt coil built for a 1975 BMW R90/6 motorcycle fits the specs and physical size for a 6 volt tractor coil. Similar power, points ignition. Voltage is 12 volts, but two coils are in series so no ballast is required. With all that said, just saying I stand by my comments and suggestions Mr John T.

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"confused over coils"......certainly a proper title for this thread!

A basic understanding of electricity, voltage, magnetism, and resistance takes a lot of confusion out of the equation. It also
helps tremendously to know how to use a multimeter, particularly the voltage and ohms functions. A multimeter takes something invisible and makes it visible. A basic understanding of electricity, voltage and resistance lets you know if the now visible voltage or resistance is a good or bad reading. Leading you hopefully down the proper path to fix your problem. There are so many ways to learn just watching an on line video. If however one choses to use the Hail Mary method of guessing or what feels right, odds are confusion will continue to rein.