Ignition coil voltage

Several years ago I went to local JD dealer and bought a new ignition coil for my JD 2010 gas. It is a 6 volt coil for a 12 volt charging system. Parts guy explained a 6 volt will help the points last longer. Now, I just replaced the coil in my dad?s TO-20 with a 12 volt coil. I converted that tractor to 12 volt years ago. But, I pulled off a 6 volt coil, which I must have replaced at some point over the years. So, what do you guys recommend? Match the charging system or use 6 volt?

Makes no difference as long as you have a resistor to lower the voltage for the 6volt coil.

parts guy is full of prunes

"Parts guy explained a 6 volt will help the points last longer."

Parts guy was BADLY misinformed! A 6-Volt coil used in an ignition system WITHOUT having a resistor to compensate for it will KILL the breaker points.


The 2010 had a resistor on the back of the ignition switch, or some had/were changed over to a length of resistance wire, making it OK to use a 6-Volt coil.

<img src = "https://i.imgur.com/f6EAHIn.jpg">

(Note the starting bypass connection in the drawing.)


The advantage to using a 6-Volt coil AND a resistor in the primary circuit is that the resistor can be bypassed (by a connection to a special post on the starter solenoid) during cranking to make for a "hotter" spark by compensating for the dropped battery voltage that occurs during cranking.

On the other hand, if NOT using a resistor in the primary circuit you MUST use a "true" 12 Volt coil designed with more primary windings to allow use on 12 Volts without drawing excessive current that leads to burned points and/or coil failure from excessive heat.

A "six volt" coil is identical to a coil marked "12 volts external resistor required". A coil marked "12 volts NO external resistor required" may be used on a tractor converted from six to twelve volts, eliminating the need to add a separate resistor. It just depends on how you did the conversion.

When in HS, I had a 1965 Barracuda, 12v. It had a ballast resistor. No one seems to remember why ballast resistors were used. I carried a spare resistor in glove box because ballast resistor failure was a common thing thing back then, along with replacing points, condensers, spark plugs annually.

So who remembers why ballast resisters were used other than to limit current so you don't burn points? Most 6-12v conversion circuit diagrams are leaving out the importance of the ballast.

Hint, it has something to do with delivering a hotter spark during cranking.

The resistor bypass (not the resistor) is used to deliver hotter spark while cranking. While the starter is cranking and drawing high amps thus lowering battery voltage then the current to the points were to go thru the ballast resistor voltage would be lower and too low to supply a good spark during cranking. So this is where the bypass comes in to supply battery voltage(non-reduced voltage) to the coil.

You are right. So why don't the conversion circuit diagrams show bypassing the ballast?

On the subject of the Deere 2010, some had Presolite ignition instead of Delco and the Prestolite had horrible ignition points life.

As to coils? No such thing as a purely 6 volt or 12 volt coil. 12 volt system must make good spark when cranking at 9 volts and must make reliable spark at 15 volts. When it comes to the coil - either all the primary resistance is built into the coil, or part is in the coil and part is in an external resistor. 2010 used a coil with an external resistor.

Good question, if I have posted this once I have 1000 times over the past nearly 30 years, but here goes again. Save this for future reference.


On a 12 volt tractor you can EITHER

A) Use a full true 12 volt rated coil (NO ballast required) OR

B) Use a 6 volt rated coil PLUS an inline series voltage dropping (12 down to 6) ballast resistor so the coil only receives
the 6 volts its rated for. The ballast drops 6 volts leaving 6 for the 6 volt coil

NOTES

1 A typical full true 12 volt coil may have a LV primary resistance between its + and - terminals around 2.5 to 4 ohms,
many around 3 ohms.

2 A full true 12 volt coil may be labeled "12 Volts" or "12 Volts NOT for use with ballast resistor"

3 A typical 6 volt coil will have a LV primary resistance between 1.2 to 2 ohms, many around 1.5.

4 A coil labeled "12 Volts for use with ballast resistor" is in reality more like a 6 volt coil and requires the
ballast just like it says on a 12 volt tractor or it will overheat and the points burn rapidly

5 If you use a 6 volt coil with no ballast on a 12 volt tractor it will overheat and the points burn quick

6 If you use a 12 volt coil on a 6 volt tractor the spark will be weak

7 If the 12 volt tractor has a ballast installed and in the LV primary ignition circuit you need to use a 6 volt coil
as a 12 volt coil PLUS the ballast would produce a weak spark


A reason some tractors used a 6 volt coil on a 12 volt tractor is they could add a ballast start by pass system so the coil receives unballasted battery voltage when cranking which improves cold weather starting


NOW TO YOUR SPECIFIC QUESTION "Now, I just replaced the coil in my dad?s TO-20 with a 12 volt coil. I converted that tractor to 12 volt years ago. But, I pulled off a 6 volt coil, which I must have replaced at some point over the years. So, what do you guys recommend? Match the charging system or use 6 volt?

FWIW as a retired electrical engineer I recommend: If the tractor was converted to 12 volts and assuming there's no ballast in the ignition circuits LV primary I WOULD USE A FULL TRUE 12 VOLT RATED COIL. A 6 volt unballasted would overheat badly. If you insist on using a 6 volt coil add an inline series voltage dropping (12 down to 6 volts) ballast resistor.

Hope this helps post back any questions

John T

GM starters have the resistor bypass right in the starter solenoid. My old Dodge had a resistor on the firewall. Ceramic thing with a coiled wire inside.

Uhh how does a 6 volt system start? That theory dont hold water there would be a switch bypassing the resister if it increased voltage upon starting.never seen 2 wires go to a coil from a switch before.somebody post pictures please.our 52 A john deere has factory delco remy distributer with a resister to coil,6v coil. The switch is pretty simple turn key fire straight to coil.it has 1 hot wire 1 wire to coil,and 1 to amp meter

George, the reason the bypass circuit is not often shown is because it's not necessary with a small, low compression engine.

That concept was used mostly on the large displacement, high compression engines.

All I can come up with on the concept of using a 6v coil with a resistor, the auto mfgs could use the same coil, same wiring harness for all vehicles, then add the bypass circuit when the build called for a large engine.

Having an external ignition resistor and a way to bypass it when cranking is very common in "12 volt" systems. Not as common in 6 volt systems. I have no idea what you are calling a "theory" here.

Some systems use a heat-sensitive external resistor that needs no mechanical bypass. It automatically increases or decreases resistance by how much current goes through it and how long. Deere used a built-in mechanical bypass built right into the ignition switch on many tractors. GM used a bypass built into the starter solenoid. The reality is - a tractor with a 6 volt battery only has 4.5 volts available when cranking, and a 12 volt battery around 9 volts.

Just for reference, here are some 6 volt systems showing the extra resistance unit built into the coil and thus no extra wires for a bypass. The extra resistance when needed is determined by heat and load.

RWB, great questions, let me try to clear up any confusion for you.

1) If your model A does (if its 12 volt but uses a 6 volt coil) NOT happen to have any ballast by pass while cranking feature, then its correct there's ONLY a single wire to the coils INPUT and a single wire from the coils other OUTPUT terminal to the distributor. And yes there's NO SWITCH by passing the resistor. Its "possible" on a 12 volt tractor to use a 6 volt coil plus a ballast and not use any ballast by pass when cranking feature. Its also possible to use a 6 volt coil with ballast on a 12 volt tractor and have the ballast by pass when cranking feature.

2) HOWEVER later model 12 volt John Deere tractors such as 50, 60, 70 etc that are 12 volt but use a 6 volt coil DO HAVE THE BALLAST BY PASS WHILE CRANKING feature and do have TWO wires to the coils INPUT. How the ballast by pass while cranking feature works on that series of tractors is there's an extra small side terminal on the saddle mount mechanical push switch on the starter that's hot ONLY WHEN CRANKING and it sends unballasted battery voltage up to the coil therefore directly by passing the ballast to improve cold weather starting.

3) Many tractors and brand of tractors and other 12 volt vehicles used 6 volt coils with a ballast PLUS had the ballast by pass when starting feature. They worked in different ways including but not limited to

1) A by pass switch/terminal on the mechanical push starter switches to send unballasted voltage to the coil by passing the ballast but ONLY when cranking.
2) An extra I terminal on an electric starter solenoid that by passed the ballast
3) Temperature compensating (Thermistor??) wire from switch to coil that was low resistance when cold but resistance increased once up to temperature IE NO SWITCH OF ANY SORT WAS NECESSARY
4) Ballast resistance in the form of resistive wire link versus the traditional white bathtub ceramic looking units
5) Such ballast devices may be stand alone or mounted on the ignition switches or wire links from switch to coil

A purpose of 6 volt coils plus ballast on 12 volt tractors or cars was the ability to by pass the ballast when cranking to improve cold weather starting AND MANY MANY CARS TRUCKS N TRACTORS used one of the methods above.

As temperature decreases and its hard to crank a tractor and doing so draws extreme high amps the battery voltage drops. THEREFORE that's a good time to help increase coil voltage by by-passing the ballast when cranking. That's why you see 6 volt coils on 12 volt vehicles and the ballast by pass when cranking feature IT HELPS COLD WEATHER STARTING !!!!

Hope this helps, post back any questions, here are a couple pictures for you

John T

Steve,
Don't agree, I bypassed the ballast on Jubilee and it fires off on first or second crank.

Bypassing the ballast becomes more important as battery ages too.

There are many ways you can wire in the bypass if your switch or solenoid isn't equipped to bypass.

It's as important today as it was in 1965.

The push button switch has 1 wire in 1 wire out. And cant be seen but a resistor is under dash.it has 1 wire from ignition headlite switch to resistor then wire from resistor to coil. No double wire off push button switch.we have owned this 630 since new.dad bought it 1 year old new old stock tractor.big discount due to 10 series. Its original wiring wise except where mice ate insulation off n new wire tied in.
Must be a rare 630 then according to what you say n what i see.who knows

Yes, it has the prestolite distributor

So, a few replies: thanks for your info. The 2010 has a Prestolite distributor. The key switch has a resistor, which I never really paid attention to. Now I know what it is. The Fergusons do not have resistors so I must be buying the internal resistor type. Heck, I just buy the coil listed for my tractor and rated for my charging system.

The following is for information only. I can veryify that it is accurate.

Scopes, meters, and testers may be inaccessible from time to time, so we at "Bubba's Garage" have come up with this here test for Ignition Output:

1.Hold medium wrench in one hand, Sears wrench preferred for accuracy of test 'cause other smooth wrenches are too slick for throwin'!

2.With engine runnin', grab hold of the plug end of an ignition cable with the other hand and hold on tight!:shock:

3.This will more than likely cause you to throw wrench! KV output is based on how far the wrench wuz thrown!

10 feet = approx. 10,000 volts
20 feet = approx. 15,000 volts
30 feet = approx. 25,000 volts
Over 30 feet = Caution! Must be one of them new
dis-is-it ignition systems,
use bigger wrench and retest.

WARNING! No side effects have been reported due to testing at this time (although it has been known to kill chiggers after berry pickin')

Bubba Sez: Electricity will not usually hurt you, instead it is the back of your neck going through the hood latch that really ruins your day!!

Noah W