resistor wire 3020 gas

A friend bought a 3020 gas, I've never seen it, maybe a 3010 that didn't run smooth right away after he bought it. It's a positive ground system so it's an earlier model. It would run well cold but after a few minutes it would start dropping cylinders till it finally died. After a lot of tinkering with it he found out it was ignition, not fuel. After replacing everything he could replace in the ignition it still ran bad after it ran a few minutes then faltered and died. He said it uses a 6V coil, that's what the book calls for so I told him the problem might be between the key switch and ignition. He made some voltage checks and sure enough, it was getting 13.3 volts at the coil and the coil was getting so hot he couldn't touch it. He then said the previous owner had butchered the wiring and it has cut off wires hanging everywhere with new wires replacing the cut off ones so obviously the resister is bypassed. In the shop manual he found out there is supposed to be a resistor wire in the system instead of a stand alone resistor and the distributor should get 7.2 volts.

So after a long winded story, do any of you guys know how much resistance this wire is supposed to have? how much resistance does it need to drop the voltage to 7.2? I told him he could maybe throw a ceramic resistor in there to get him going but I don't know what to recommend. I also told him I think I have an old Chrysler ceramic resistor in a drawer and we could try it and see what kind of voltage we get. I'm going to run over to his place some day soon to help him out and I'd like to make one trip to solve his problem. Thanks.

Mr Fixer, Heres the deal. Without going into the "exact" math and note any "exact" answer depends on the coils resistance and at what voltage the battery is typically when under charge which depends on RPM and charging system and battery etc. etc............

If it has a so called "6 Volt Coil" and its a "12 Volt System" the ballast resistor is approximately the same resistance as the coil to form a 50/50 voltage divider with 6+ or so volts dropped across the coil and 6+ or so dropped across the ballast.

IE measure the coils LV primary resistance, likely around 1.2 to 1.8 ohms if its a 6 volt coil, and use a similar sized ballast of 1.2 to 1.8 or so ohms.

NOTE if the charging system raises the battery to say 14 volts and if the ballast was the same resistance as the coil, that would leave 7 volts on the coil, close enough to the 7.2 you spoke of.

Remember there's no ballast voltage drop until it draws current which is when the points are closed. When measuring voltage on the coils input if the points were open it would read battery voltage and only drop to the 6+ to 7 when they are closed.

Now if you know the coils resistance and the battery voltage when under charge at X RPM you can use Ohms Law to compute the exact ballast resistance so you end up with 7.2 volts on the coil, but for a short n sweet n easy simple answer just use a ballast of the approximate ohms of the coil PROVIDED ITS AN ACTUAL 6 VOLT COIL ON A 12 VOLT TRACTOR

As it sounds you're well aware, the ballast goes between the ign switch and coil DUH to take the place of a bad ballast wire etc.....

John T

Thanks John. At least I now have something to go by. Will measure the coil resistance and this resistor I THINK I have and go from there.

Back behind the shed I have a 62 Rambler Classic with the 196 cu inline six. My sister drove it in college back in the late 60’s so it’s been in the family awhile. While she had it the coil went bad and she had it replaced by a local garage. It kept burning points after that, I think you know where I’m going with this.

She graduated and the car came back here to the farm to live. Soon after, I was talking to an old mechanic about the burning points and this is when I was first introduced to the coil voltage and resistor scenario. He mentioned the coil was wrong. It was then that I found out Rambler could have used either a six volt coil with resistor or a 12 volt without resistor in the same model year and same engine! This car takes a 12 volt but was fitted with a 6 volt, hence the burning points. It never was a good cold weather starter and I blame it on the 12 volt coil. If it had a 6 volt with resistor and a 12 volt starting bypass I’ll betcha it would have more starting spark for cold weather.

You're welcome. Some 12 volt US cars n trucks n tractors used 6 volt coils plus an external ballast, while others used 12 volt coils. On many 12 volt cars trucks n tractors that used 6 volt coils and ballast they had a ballast by pass circuit while cranking and others used temperature compensating ballast systems for a similar effect.

John T

Be a lot simpler and safer to just go to
Napa and get a new coil that doesn't need
an external ballast. IC 14 SB is what I
used. Seems like it's about $15.

(quoted from post at 04:42:50 07/03/18) Be a lot simpler and safer to just go to
Napa and get a new coil that doesn't need
an external ballast. IC 14 SB is what I
used. Seems like it's about $15.

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I did switch from a 6 volt with resistor and bypass to a 12 volt on my Oliver 88 when I switched it to 12 volts. It kept burning points with the 6 volt coil and resistor so I must have used the wrong resistor. Since I threw on the 12 volt I haven’t touched the points. I don’t know the characteristics of a 3020. I have a hunch the reason Deere uses a 6 volt coil is that engine needs a little hotter spark for starting.