Farmall 460 coil

Can someone help me out here. Does the voltage drop from 12v to 6v for the ignition coil through a resistor on a Farmall 460.

Probably, I'm still confused on that in spite of all the explanations in the archives. Anyway, you can toss the ballast resistor and get a 12V coil with an internal resistor from NAPA. That is what I put on my 460 and it works fine.

See if this helps
Resistor Bypass Start Circuit

So it says the start circuit bypasses the resistor so the coil puts out 12v when the starter is engaged giving the engine a hotter spark during startup. Then the ignition/run circuit flows through the resistor before the coil reducing it to 6v during all day running so you don't burn out the points and a hotter spark isn't needed then.

That part I can understand. That creates questions from us dummies though that I hope can be cleared up.

However, with a 12 volt internally resisted coil, doesn't it still only have one wire going to the coil and one leaving the coil going to the distributor? And if so, then how does the internal resistor know when to kick in or not and switch from 12v to 6v? Seems like a 12v internally resisted coil would need two posts for wires coming in, one from the start circuit and one from the ignition/run circuit along with the post for the wire going out to the distributor. The 12v internally resisted coil I bought at NAPA as recommended here many times only has 2 posts, one in and one out. Thus it would seem that it can only work at one voltage which is probably 6v with the resistor.

Thanks.

If you use the 12 volt coil with the internal resistor, you won't get the benifit of the 12 volt by-pass boost when starting.

Redhot, for something relatively simple, this still gets a lot of questions, so I will try to explain it all:

1) On 12 volt tractors that use a 6 volt coil, one needs an external series voltage dropping (12 to 6) Ballast resistor wired in AFTER the Ignition switch but BEFORE AND AHEAD OF the Coil so it drops 6 volts leaving 6 for the 6 volt coil so it dont overheat.

2) An advanatge of such is that while cranking when battery voltage is reduced, its possible to by pass jump around an external ballast to get a hotter starting spark, but then once shes no longer cranking, the ballast is back in the circuit so a 6 volt coil only sees 6 volts and all is well.

3) Such by pass (while cranking) can be accomplished EITHER by a starter solenoid that has the extra lil "I" terminal or else a saddle mount mechanical push start switch with the extra lil terminal. Either of those goes direct to the coil after the ballast (or on low side of ballast) so hot unballasted batery voltage is applied to the coil ONLY while cranking.

4) An alternative on a 12 volt tractor is simply use a full true 12 volt coil (what some refer to as internally ballasted) in which case NO BALLAST IS REQUIRED..... However, in thar case one cant utilize a solenoid or switch by pass system to get the advantage of a hotter spark while cranking, but on a good 12 volt system she ought to still start okay ifffffffff all else is in top condition with no 6 volt coil and ballast and start by pass system in place.


Get it???????????

John T

Adding to all this fine sparkie discussion, is the possible inclusion in the inner guts of a (((no external resistor required))) coil, of a thermistor resistor that starts out cold with low resistance, and within a half minute or so, increases its resistance to lower the coil applied voltage. Thus automatically increasing start voltage.
On a regular external resistor based system, the ballast resistor might measure little voltage drop when measured with the points poen, or not connected to the distributor. If points are closed, the drop will happen. JimN

Good question and observation.

The resistor bypass start circuit is not used on all 12 Volt systems.

Your NAPA 12 Volt coil would operate on full 12 volts (no resistor). As you observed one terminal goes to distributor other direct to ignition switch. This coil is not used with a resistor or in a bypass circuit.

Some refer to a 12 volt coil as "internally resisted". My understanding is that most 12 volt coils actually have no internal resistor, rather the primary is wound for 12 volts. (But I know there can be exceptions to every assumption.)
Bob

Yo Jim, Im not aware of any Thermistor (a resistor that increases in ohms over time as it heats up) use in our old tractors, but believe Ford or someone used it in some automotive applications?????????? It would save a lot of the start by pass and normal ballast problems fer sure.

While on the subject, many here refer to 12 volt coils as being "internally ballasted" sort of like in part of the can theres this stand alone discrete actual RESISTOR. Although a few such did exist, some of which had a lil ring around the can separating the resistor portion, and jdemaris or SoundGuy or someone once even posted pics of old coils with actual internal resistors (some of which were used in circa thirties cars I believe), MANY OF THE TYPICAL GARDEN VARIETY 12 VOLT COILS ACTUALLY HAVE NO INTERNAL BALLAST RESISTORS, but their increased resistance is simply due to more coil wire length or higher resistance wire ANYONE WANNA TAKE ISSUE WITH THAT feel free, its the best info I have available.

Maybe you or Bob M or jd or someone has details or specs or data on the subject?? But far as I know most typical 40's n 50's tractor 12 volt tractor coils DO NOT have an internal pure stand alone discrete resistor hid somewhere inside there

Yall take care now

John T

(quoted from post at 14:49:44 04/08/09) Yo Jim, Im not aware of any Thermistor (a resistor that increases in ohms over time as it heats up) use in our old tractors, but believe Ford or someone used it in some automotive applications?????????? It would save a lot of the start by pass and normal ballast problems fer sure.

While on the subject, many here refer to 12 volt coils as being "internally ballasted" sort of like in part of the can theres this stand alone discrete actual RESISTOR. Although a few such did exist, some of which had a lil ring around the can separating the resistor portion, and jdemaris or SoundGuy or someone once even posted pics of old coils with actual internal resistors (some of which were used in circa thirties cars I believe), MANY OF THE TYPICAL GARDEN VARIETY 12 VOLT COILS ACTUALLY HAVE NO INTERNAL BALLAST RESISTORS, but their increased resistance is simply due to more coil wire length or higher resistance wire ANYONE WANNA TAKE ISSUE WITH THAT feel free, its the best info I have available.

Maybe you or Bob M or jd or someone has details or specs or data on the subject?? But far as I know most typical 40's n 50's tractor 12 volt tractor coils DO NOT have an internal pure stand alone discrete resistor hid somewhere inside there

Yall take care now

John T

Click to expand...

Copper wire winding.
You don't need any external current control (thermistor, temperature dependent resistance or whatever) to obtain a higher start current than the all-day-run-current.
Example:
1.7 ohm primary, 200 turns of 26GA wire or about 42 feet

current @ 68F is 7.4v/1.7 ohms=4.35Amps

then pull hard all day & say coil is 200F (heat from current + engine it is bolted to). It won't take you long to feel of it!

copper wire resistance increases as temp rises from 68 to 200=33% or is now at 2.26 ohms and current is down to 7.4/2.26=3.27Amps

a decrease of 1.08Amps or 25% or stated another way, running hot at 75% of cold start current.

Related is the fact that Energy (the heat of the spark ) is proportional to current squared, and thus the hot all-day- run Energy is only 50% of your cold start Energy.

All that with only the copper winding of the coil.

NOTE: Nothing above considers the loss in battery voltage while starter sucks large current.

That makes sense JMOR but what were the original coils with external ballast made of and also the new ones that I supppose don't say they are internally resisted?

With respect, the resistance does go up as it heats, but not fast enough to keep serious point erosion from happening. There are many operating situations that will not heat the coil enough to reduce the point damage. Winter and short operating times are two. There are real resistors in the can. JimN

I've never opened one up that wasn't copper wire. I have no doubt that they all use copper wire of varying gauge, turns, length to achieve the desired properties. A surprise to many is that the use of an external resistor actually has a stabilizing effect on coil current, helping to keep it more constant as it's copper wire resistance increases with temperature, than no resistor and copper windings only. This very likely has a lot more to do with the industries use of resistor plus coil than the idea that they just added the resistor in order to be able to use same old 6v coils on 12v systems. It also made it nice to be able to bypass the resistor during cranking when starter drags battery voltage from 12 down to perhaps 8 or 9 volt range or from 6 to maybe 4.