12v Coil Keeps Burning Out!!??? Help

What causes A 12 V ignition coil to burn out? This coil has the internal resister built-in and I have had to change coils about five times in the last few years. The problem is that the coil gets hot and a tractor starts to miss/cut out and barely run.

Put in a resistor.

Sounds like you don't have the internal resistor you thought. I agree, put in a resistor. If you're buying a GM 12v coil they rely on a resistor wire from ignition to distributor.

You might check the voltage going to the coil. I had an alternator generate more than 16V one time.

To operate an ignition coil on a 12 volt tractor you need a full true 12 volt rated coil. Such will have a LV Primary resistance of around 2.5 to 4 ohms (many 3) as measured between its little + and - terminals.

If your coil measures say 1.25 to 2 ohms from + to - its like a 6 volt coil and will overheat badly on a 12 volt tractor plus the points will burn prematurely.

If your coil measures like a 6 volt coil above you can add an external series voltage dropping (12 down to 6) ballast resistor and that will stop the overheating. Typical old farm tractor ballast resistors (for using a 6 volt coil on a 12 volt tractor) are around 1.25 to 2 ohms.

Contrary to some beliefs, typical old farm tractor coils DO NOT have an internal discrete stand alone ballast resistor tucked away hidden inside the can. THERES NO INTERNAL RESISTOR !!!!!!!!!!!!

NOTE a full 12 volt rated coil may be labeled "12 Volts" or "12 Volts NOT for use with ballast resistor" HOWEVER if its labeled "12 Volts for use with ballast" its in reality like a 6 volt coil and needs the ballast just like it says or else it will overheat.

YOU NEED TO INSURE YOU HAVE A FULL TRUE 12 VOLT COIL !!!!!!!!!! Measure its LV Primary resistance from + to - to see if its 2.5 to 4 ohms !!!!!!!!!!!!!! If its a 6 volt coil (1.25 to 2 ohms) add a series ballast

Coils can also overheat if the voltage is too high like 15 to 16 volts CHECK YOUR BATTERY VOLTAGE WHEN RUNNING..

Coils can overheat if not heat sink mounted properly or in an extreme hot environment where they shouldn't be mounted..

Since this is a Tractor NOT an old car forum and you mentioned tractor, I PURPOSELY spoke of typical old farm TRACTOR coils Not automotive coils.


If I missed anything hopefully the other fine gents can add to this

John T

(quoted from post at 04:57:59 10/28/17) Put in a resistor.

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We put a resistor on it and thought it fixed it. Guy called and said he barely got it in the shed cause it was cutting out like it always had done. Kind of stumped here. Never had this issue occur in the 40-50 tractors of this model I've worked on

Well, it's no surprise that so-called "12 volt" coils frequently burn out. The heat generated by the series resistance has to be dissipated whether it's in the coil or in an external resistor. And coils aren't designed to dissipate heat. Leave the ignition turned on for a couple of minutes with the points closed, and you'll find the coil gets very hot. And the heat (from my coil, at least) seems to be concentrated near the top of the coil.

About all you can do, short of installing an external resistor, is be careful not to leave the key on with the motor stopped.

> Contrary to some beliefs, typical old farm tractor coils DO NOT have an internal discrete stand alone ballast resistor tucked away hidden inside the can. THERES NO INTERNAL RESISTOR !!!!!!!!!!!!

Yes, but...the resistance is created by using a smaller-gauge wire than on a "requires external resistor" coil. Not only is there more heat to dissipate, but the small-gauge wire is more likely to burn out.

(quoted from post at 18:10:28 10/27/17) > Contrary to some beliefs, typical old farm tractor coils DO NOT have an internal discrete stand alone ballast resistor tucked away hidden inside the can. THERES NO INTERNAL RESISTOR !!!!!!!!!!!!

Yes, but...the resistance is created by using a smaller-gauge wire than on a "requires external resistor" coil. Not only is there more heat to dissipate, but the small-gauge wire is more likely to burn out.

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Well, the heat is caused by amperage. Amperage is caused by voltage and ohms. If the resistance is high the amperage is low and the heat is low. John T gave the ohms for 6v and 12v coils which will prevent too much amperage. The size of the wire is pretty much independent of the heat generated so long as the total resistance of the coil is in line.

> Well, the heat is caused by amperage. Amperage is caused by voltage and ohms. If the resistance is high the amperage is low and the heat is low. John T gave the ohms for 6v and 12v coils which will prevent too much amperage. The size of the wire is pretty much independent of the heat generated so long as the total resistance of the coil is in line.

You miss the point. The TOTAL heat generated by the resistance in the primary circuit of a 12 volt ignition system will be the same whether it uses a "12 volt coil" or a "6 volt coil" plus an external resistor. The difference is that the with the no-external-resistor 12 volt coil, ALL the heat is generated INSIDE the coil, while if an external resistor coil is used with a 6 volt coil, only HALF the heat is inside the coil. That's why 12 volt coils run hot.

The other issue is how the additional resistance is built into the "12 volt" coil. As John said, there's no discrete resistor inside the coil. (I'm not entirely certain that's true, since the heat seems to be concentrated near the top of the coil, but we'll assume there's no discrete resistor.) So where does the additional resistance come from? You can't just add primary windings, since that will change the coil inductance. So you have to use a primary conductor with higher resistance per foot than is used in the "6 volt" coil. Either they use a different material, such as aluminum instead of copper, or they use a smaller gauge conductor. Either way, the primary conductor of the "12 volt" coil is more prone to burning out than that of the "6 volt" coil. Aluminum has a lower melting point than copper. Smaller gauge conductors burn out before larger gauge conductors.

Try replacing the condensor. Coils rarely burn out and if they do you shouldn't be able to "limp" the tractor anywhere. It should just die. Points are another source of intermittent ignition when the engine gets hot. You might as well replace those too while you have the distributor off.

You're right MARK like I said there's no internal discrete stand alone ballast resistor tucked away and hidden inside the can. The necessary LV primary winding resistance is a function of the wire and its resistance per unit length and the total length. NO INTERNAL RESISTOR just wire and like any wire not being a PERFECT CONDUCTOR it has some degree of resistance...

Take care

John T

OOPS MY BAD, I thought I covered it all below but I forgot to mention something else which may help....

Yet another reason why typical old farm tractor coils have the LV primary winding resistances they have (6 volt around 1.2 to 2 ohms, 12 volt 2.5 to 4 ohms) IS TO LIMIT THE AMOUNT OF CURRENT THE POINTS CARRY AND SWITCH SO THEY DONT BURN PREMATURELY !!!

And that max current is typically somewhere around 4 amps

THEREFORE in a 6 volt system the total LV primary resistance (coil + any ballast) is 1.5 ohms as 6/1.5 = 4 Amps
in a 12 volt system the total LV primary resistance (coil + any ballast) is 3 ohms as 12/3 = 4 Amps

Again a typical old farm tractor coil DOES NOT HAVE A DISCRETE STAND ALONE RESISTOR TUCHED AWAY AND HIDDEN SOMEWHERE INSIDE THE CAN the LV primary winding resistance is a function of the wire used, its resistance per unit length, and the length.

On a 12 volt tractor if the coil plus any ballast used doesn't have around 3 ohms total resistance the coil will overheat and the points burn prematurely. Of course extreme high charging voltage can also cause overheating as can leaving the ignition on when not running

YOU HAVE TO INSURE YOurCOIL (since you have no ballast) has at least 2.5 to 4 ohms LV primary winding resistance or it can overheat. If it has more like 1.2 to 2 ohms IT NEEDS AN EXTERNAL BALLAST RESISTOR

I think I covered most of it now lol post back any questions

John T

(quoted from post at 03:29:09 10/28/17) > Well, the heat is caused by amperage. Amperage is caused by voltage and ohms. If the resistance is high the amperage is low and the heat is low. John T gave the ohms for 6v and 12v coils which will prevent too much amperage. The size of the wire is pretty much independent of the heat generated so long as the total resistance of the coil is in line.

You miss the point. The TOTAL heat generated by the resistance in the primary circuit of a 12 volt ignition system will be the same whether it uses a "12 volt coil" or a "6 volt coil" plus an external resistor. The difference is that the with the no-external-resistor 12 volt coil, ALL the heat is generated INSIDE the coil, while if an external resistor coil is used with a 6 volt coil, only HALF the heat is inside the coil. That's why 12 volt coils run hot.

The other issue is how the additional resistance is built into the "12 volt" coil. As John said, there's no discrete resistor inside the coil. (I'm not entirely certain that's true, since the heat seems to be concentrated near the top of the coil, but we'll assume there's no discrete resistor.) So where does the additional resistance come from? You can't just add primary windings, since that will change the coil inductance. So you have to use a primary conductor with higher resistance per foot than is used in the "6 volt" coil. Either they use a different material, such as aluminum instead of copper, or they use a smaller gauge conductor. Either way, the primary conductor of the "12 volt" coil is more prone to burning out than that of the "6 volt" coil. Aluminum has a lower melting point than copper. Smaller gauge conductors burn out before larger gauge conductors.

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So you're saying a tail light is more prone to burning out than a headlight. I thought headlights generated more heat. And an 1157 has a smaller filament than a 6024. I think a large wire and a small wire, run at the same temperature, are probably each just as durable.

I guess we've beat it enough. I don't think I've ever seen a 12v No External Resistor Required coil anyway. I've seen them labeled 12v and I always put a resistor in and they always run fine. I've never had a 12v coil run hot because I always put a resistor in them, and have run 12v coils on 6v tractors. Through ignorance I guess.

RTR , keep all those 'burned out' coils for use on other engines wher they will very likely perform wonderfully!

Sounds to me like you either have a heat source (exhaust leak - missing heat shield) or a grounding issue. In my experience repeated failure of electrical components can almost alway's be traced back to a poor ground. Doesn't matter if it's a tractor or your vehicle grounds are the place to look when an electrical part keeps failing.

With the internally resisted coil you are using it shoulden't be getting hot. (yes I know there is no stand alone resistor - but no one said there was) Measure the ohms and take care of the grounds!

Even a coil that should have an external resistor and didn't would not fail that often if there were no other problems. I've seen quite a few wrong that ran for years before the BOOM.

> So you're saying a tail light is more prone to burning out than a headlight.

That's a silly comparison. The two bulbs have quite different designs; each is designed to dissipate the heat it generates. In the case of the two different coils, they both are nearly identical physically, but the "12 volt" coil has to generate and dissipate twice the heat of the "6 volt" coil. A better comparison would be two 120 volt, 100 watt incandescent bulbs. If you run one at 110 volts and the other at 130 volts, which one will burn out first? Most likely the 130 volt one, because it's generating much more heat than the other.

> I think a large wire and a small wire, run at the same temperature, are probably each just as durable.

The point here is the smaller conductor is carrying the same CURRENT as the larger conductor. So it is NOT running at the same temperature, it is hotter. Think fusible link: the smaller fusible link fails at a lower current than the conductor it protects.

> I don't think I've ever seen a 12v No External Resistor Required coil anyway.

Most auto parts stores carry them, or at least used to. I believe they were used as original equipment on VW Bugs, but I'm not sure about that. They are also available from everyone's favorite source of questionable tractor parts, TSC.
12V no external resistance coil

I learn a lot from you guys even when you all don't totally agree on something.
I have several old ford N and Jube tractors and all the coils say 12v but use with ballast resistor. I haven't checked the ohms but was wondering do the resistors typically wear out over time or are they pretty bullet proof. All my tractors are 12v neg ground with alternatior (3 wire) I believe. Right now laid up with cancer treatments and surgery so have forgotten a lot. Is there any advantage to keeping it like it is or should I change to a true 12v coil and remove the resistor?
Thanks much.

> s there any advantage to keeping it like it is or should I change to a true 12v coil and remove the resistor?

If it ain't broke, don't fix it. Pretty much every car made in the sixties and seventies used a ballast resistor, although for most it was resistance wire rather than a discrete resistor.