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Coil Problems on Early Ford N-Series Tractors with 12-Volt Conversions

Many people have converted their old tractors to 12 volts with little or no problems and are completly happy with the results. This page is provided as an information resource for those individuals who have not been so fortunate. Hopefully, the information on this page will help get the 12 volt systems working for some of the many people who are having ignition problems with their converted tractors. I will attempt to keep the technical engineering jargon to a minimum.

I keep smoking this over and rewriting it. Mostly because it has taken some time for me to remember some of my DC circuit and electronics training. That information was saved in my brain over 40 years ago. My brain can be a little less reliable than the average hard disk. Check back in a few weeks and this page will probably have been rewritten again.

Anyway, The early Ford 9N, 2N, and 8N tractors have a front distributor engine with a square-can coil that sits right on top of the distributor. The early square-can coils have never been as forgiving as the much more universal round-can type 6 or 12 volt coils and the replacement square-can coils seem to be getting worse. The late side-distributor 8N tractors were upgraded to the more universal round-can coils. More applications for a particular part usually means the part has to be made more forgiving of individual variations and it tends to keep the cost low because they sell more of them. The round-can coil is in a metal can and the electrical guts are in oil. This is a MUCH better design for getting rid of excess heat than the plastic case and potting compound used in the front mount coils.

I believe the main reason many 12-volt conversions are having ignition problems such as weak spark or burned coils is directly related to the weaker square-can coils and the less-reliable replacements we are getting. Buying a 12 volt conversion kit is no guarantee of success. There is a lot of competition in the marketplace and the pressure to keep prices down forces conversion kit builders to shop around for their parts. Also, what works for one person, and the coils they happen to be buying, may not work for another person in a different part of the country.

My theory to explain the front-mount square-can coil problem is simple. The front-mount coil is a marginal design, with very limited applications. Stick an ohmmeter across the low voltage terminals of a few replacement coils and it quickly becomes obvious that there is a lot of variation in them. The 6 volt coils usually measure from 0.5 to 1 ohm. The 12 volt coils measure anywhere from 1 to 3.5 ohms. Any 12 volt coil that measures less than 3.0 ohms should be run with the original ignition resistor and possibly a second resistor in series. A 6 volt coil should definitely be run with the original resistor plus a second resistor in series. Sadly, I cannot tell you the size for the second resistor or if one is required. The coils are inconsistent. Every time we replace a coil we should be checking to make sure the ignition resistance is correct for that particular coil. Any 6 or 12 volt coil can be made to work in a 12 volt tractor.

CAUTION! Most multi-meters are not very accurate on the ohms scale. first make sure you have a decent battery in your meter. Put it on the ohms scale and cross the leads. You should get a reading of ZERO ohms. If not try to zero your meter (if it has an adjustment for that). If you cannot zero your meter, whatever it reads with the leads shorted together will affect any measurement you try to get. In that case, you are much better off reading the circuit on the amps scale.

I know this is a repeat, but it bears repeating. You must have a fully charged battery and clean connections to read amps. The best ammeters have an inductive pickup. You don't have to get intimate with the current. The meter jaws clamp around the wire going to the coil. Turn key on with the points closed and read current. Don't leave the key on! Another way to do this is on the bench if your ammeter only has leads. Run a wire from the positive post of a fully-charged battery, to the resistor(s), then to the coil terminal. Then, use your meter leads to complete the circuit from the coil back to the battery negative post. Don't rconnect any longer than it takes to read amps. You want to try different resistors until you get 3-4 amps.

This will be a gross over-simplification, but lets try and forget for a moment that the ignition is an intermittant circuit that only carries current when the points are closed. The original 6 volt system with a good battery will run at a maximium of around 7.3 volts when charging the battery. The total ignition resistance in the 6 volt ignition system was about 2.2 ohms with the engine warmed up (1.7 ignition resistor HOT plus 0.5 coil). Based on those values, our ignition circuit will be consuming about 24 watts of power when the points are closed. Most of that is being dissipated by the resistor.

The typical 12-volt conversion with either a 6 or 12 volt coil should have a total resistance of around 4 ohms. Any more resistance and the spark gets too weak for reliable starting. With a good 12 volt battery the 12 volt system will run at a maximum of around 14.6 volts when charging the battery. Based on those values, our ignition circuit is now consuming 53 watts of power! The resistors are required to dissipate much of the heat and save the coil.

I have provided a handy calculator at the bottom of this page so you don't need to be an electrical guru to check my math and try different values. Just plug in any two numbers and hit the CALC button. Then hit the CLEAR button to try something else.

Plug the numbers into the calculator below, and compare the results:
7.3 volts, 2.2 ohms, 3.3 amps, 24.2 watts
14.6 volts, 4.0 ohms, 3.6 amps, 53.3 watts

Fact: the 12 volt circuit has over twice as much energy that must be dissipated by the coil and resistors. Depending on the actual coil and resistors you are using, it is very easy to end up with more heat than the coil is capable of dissipating.

How do we fix the problem?

1 - Try running a little more resistance and sacrificing some spark energy to save the coil. There is very little wiggle room here, but if you have been burning up coils with a 1/2 ohm resistor, try running a 1 ohm resistor.
2 - Rather than replacing the original variable resistor, keep it and add a second resistor downstream. This will retain the initial hot spark for easy starting and more total resistance when the engine is running.
3 - My gut tells me to stay away from the 12 volt round can coils. Let the much less expensive resistors deal with the load and additional heat of a 12 volt circuit.
4 - Remove the guts from the original square-can coil and bring the leads out to connect to a more reliable round-can coil. You still need to match the correct coil and ignition resistor to get about 4 ohms of resistance total. But the round can coil will take a lot more abuse, so you have a wider margin of error to play with.
5 - Stay with the original 6 volt system. Sorry, but that is what I decided was the right thing to do with my 52-8N. It has been over two years since I got this tractor and I have no regrets. That was the correct decision for me.


It's your tractor, if you have decided a 12 volt system is right for you, go ahead and convert it. There are plenty of happy owners of successful 12 volt conversions out there, somewhere.


Here is a handy electrical calculator. If you know any two values, it will give you the other two. This is a freebie, so no warranty is expressed or implied. One of my geek friends wrote the Java Script for me and NO this kind of Java is not coffee.


This calculator is based on Ohm's Law. Some engineers will insist that Ohm's law should be expressed using modern symbols like I=E/R. However, this calculator is intended for people working on antique Direct Current automotive/tractor circuits. It is much simpler and easier if we substitute common terms like Volts, Ohms, Amps, and Watts.

Provide any TWO values and hit the Calculate button. The calculator will provide the other two values. Hit the Clear Form button to start over.

VOLTS=Voltage, OHMS=Resistance, AMPS=Current, WATTS=Power



Content and Web Design by K. LaRue — This Site Was Last Updated 02 FEB 2023.

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