To RESIST- Or Not To Resist? Coil

I'm stumped. Way back I read that a ballast resistor wasn't needed if you had an ignition coil with an internal resistor.
** I should note that I'm dealing with a '53 Jubilee that had been converted to 12V when I bought it.**
Last summer I left the ignition key on, popped the coil. Got a new one, internal resistor. All seemed well. But after some time the Jube would just quit, after maybe 15-20 mins. running. I chalked it up to carb issues, which seemed to have been cleared up for a time.
Recently went to fire the ol' JuJu-B up...... key had been on for quite some time. I got another coil, same as last. Engine fired up after a little coaxing, but then stopped suddenly, while idling at 900 RPMs for 11 mins. Coil was not real hot to the touch, but clerk at store said the external resistor is still needed!! He said the coil gets hot, and shuts down?
Just now, I read on this site;
"The total resistance of the coil plus any external resistor (if used) should be 2.7 to 3.3 ohms for a 12 volt system. This level of resistance will maintain the correct voltage and amperage in the ignition circuit. Coil resistance is measured between the plus (+) and minus (-) terminals."
Well, the resistance of both coils I bought-- are 3.7 ohms.
So, what's the issue here? Still need the external resistor?

1. Coils don"t have "internal resistors". Coils designed for use in a 12 Volt system are wound with more wire than coils that need an external resistor.

2. Coils do not "shut down" when they warm up. If they do, they are defective.

3. 3.7 Ohms is more than adequate primary resistance IF your meter is accurate at these low resistances. What does the meter read (in Ohms mode) when you short the test leads together?

ditto what Bob said. coil primary resistance is determined by size and number of turns of wire. not a resistor..

That's what you get for listening to the store clerk...

a napa IC14SB is a correct coil for that tractor on 12v.. NO RESISTOR

We're getting a little too deep into the analysis side of resistance, and whether my meter is adequate. The coil specs state "No external resistor required"! I've since come to understand that this "built-in" resistance isn't for the purpose of dropping the converted 12V back down to 6V to protect and lengthen the life of the points, plugs, etc. That's why the extra resistor is recommended, and it's ohm value to nearly equal that of the coil so it halves the voltage and the wattage the coil would otherwise endure. I never imagined that the distributor and points were suited for 6 volts, even after the 12 to 6 volt conversion.
I'll keep on this issue until it's resolved, but I'm taking a closer look at another area of the electrical system, also. I need to ask; Should there be a voltage regulator on this converted system? Right now the alternator is wired directly to the battery and coil, by way of the terminal block. (Another post for this concern?)

If you didn't replace your points after you burned out the original coil, you should have. My guess is your points are burned and/or misgapped. The coil doesn't "shut down" when it gets hot, but it can burn out. And bad points can certainly act as you describe.

The resistor (or internal coil resistance) serves two purposes: it limits the short circuit current when the points are closed, and it also helps to damp the oscillations that occur after the points open. Both help to extend point life.

You do not want to add a second resistor to the circuit, as it will cut the current through the coil in half. Since the energy stored in the coil rises to the square of the current, cutting the current in half will reduce spark energy by 75 percent.

Do you have a no spark issue...

*I have spark, until...?, when engine stops cold.
*Engine quits//fuel related? It stops abruptly, no
coughing, wheezing, sputtering....
*Later, it starts right back up. As for the coil
"shut-down"; give the store clerk a break here.
Of course not a mechanical shut-down, but heated
wiring loses conductivity. Granted it would have
to get pretty hot, but even a novice working in
a specialty store is going to learn over time,
and hear horror stories from customers.
*Apologies if I should have started another thread
for this next point.(I may elsewhere);
>> Should there be a voltage regulator in this
12 to 6V conversion system??
)> Here is the most recent experience, pretty bizarre if you ask me;
1)Jube started up, easily!
2)Measured 12V+ on (+) terminal of coil.
3)Problem??>> Measured just shy of 12V on the(-)
terminal, also what is passed to points,
plugs, etc.
**Isn't that a concern here ?? After all, in the
conversion kits I've seen online, I dont recall seeing a distributor in the images. I'm told that
the distributor and all components/plugs- are still designed for 6V !?
4) On to the bizarre part; After one minute, I could no longer measure ANY voltage on the (-) distributor side of the coil !!? BUT... the ol'
JuJu-B kept right on running- and smoothly!
5) Odder still; I only measure about 4V on the (+)
side of the coil.! (Digital MM is fine,btw)
Ok, this is nuts. Before the engine quit again, I had time to measure the voltage directly at the alternator terminal. 14V. Appears to be working as it should. But it's now directly wired to the coil, with the key on. Brings me back to that voltage regulator question.**
I'll recheck all wires for breaks, but I dont think it's there. AND, btw... I eliminated the amp gauge, as it turned out to be the initial reason in getting no spark-- discovered after! buying the new coil and still a no-starter. I could still get the 12V "potential" reading on the coil, but found an ohms reading on the gauge to be above 700 !! ((is there a clue here, as to current levels, maybe on the high side? As in, coil spiking off a 12V source rather than something less- brought on by the addition of a resistor in the line?))

Measuring voltage on the distributor side of the coil while the engine is running doesn't mean much. In theory, you're measuring the point dwell, but since your meter isn't intended to measure dwell, you're looking at nonsense.

When the points are closed, there is zero volts at the distributor side of the coil, because it's grounded through the points. When the points are open, you'll see battery voltage at the distributor side of the coil. These are steady-state measurements taken with the engine stopped. When the engine is running, you need an oscilloscope to see what's going on.

A general rule of troubleshooting is "if what you see doesn't make sense, back up until you find something that DOES make sense, then go forward from there." If you see only 4 volts from the battery side of the distributor to ground, then either you don't have a good meter connection, or something is causing voltage drop between the battery and distributor. That could be a bad ignition switch, a bad connection or a resistor you didn't know about. I think it's more likely you didn't get a solid connection with your meter leads.

If your alternator is putting our 14 volts, then the only voltage regulator you need is working.

Update; I cant say for certain that the old coil was shot. A good bet, since finding the ignition key on. But I"ve redone the simple checks and measurements, and it all seems to fall in line.
1)Voltage at coil"sPlus terminal; 12V+
depending on the current charge state of the
battery. (not that I"m doing excessive crank-
ing, but I"ll keep recharging<<only to assure
I"m measuring the max. volts battery is capable
of)
2)Internal resistance of coil; about 3.2 (and
yes, I figured in the small value displayed
when shorting the DMM leads)
3)Current flow thru coil/dis-connected from
distributor, key on; 3.4 amps* (I read in this
forum that 4 amps was the max. for a 12V coil.

??**If there was a problem with the points, or plugs- would the engine run fine for a while, then quit?
??**How do the these things add up?; a 12V con-
version kit is sold with a 12V coil, but most of the discussions are pointing to around 6 volts across the points, am I right? So, add a resistor!!? The coil needs a min. of 10V to operate, true? What the coil "sees"(volts) is what the points are gonna get, yes? I don"t know anymore. I think I'm movin' on to a possible fuel flow issue. I"ll start a new thread.
((what is causing some words to be duplicated? In this forum and one previous I posted to. The word "bet", 2nd sentence of this post. It's just stupid!!))

(Is that post a mess or what? So much for trying to format your text, create paragraphs, hi-lite points, etc. And the thing about word duplication... It was there in the preview, gone now. Laptop browser junk, maybe? And to finish a point- "I"ll keep recharging to assure that I"m measuring the max. volts........")

2)Internal resistance of coil; about 3.2 (and yes, I figured in the small value displayed when shorting the DMM leads)
3)Current flow thru coil/dis-connected from
distributor, key on; 3.4 amps* (I read in this
forum that 4 amps was the max. for a 12V coil.

Click to expand...

When measuring DC volts and amps in a static situation, current=voltage/resistance, so approximately 12 volts divided by approximately 3 ohms will result in approximately 4 amps. Since your coil measures a little more than 3 ohms, the current should be a little less than 4 amps.

To recap; Acceptable Volts to the 12V coil, Amps measured thru the coil-under 4, or about 3.4. Coil isn't running hot, connections are all good.
Today I did a real cleaning to the plugs, and re-gapped them.(I did a one-time siphoning/ filtering of the gas, and returned to the tank)
Starting; It took many tries, using the choke all along, or there'd be no chance of a start. Finally, the Jube kicked in and ran,but ONLY with the choke held (closed). After a minute or more like this, it finally wanted to run without the choke. But after a while... business as usual;
Runs for 11 mins.-then stops cold! As clean as turning a light switch on and off.
I'm looking at the ignition system, now the fuel tank, gas condition. The carb was tuned last fall, when the tractor would run for hours with no problems. The great puzzle to solve here;
***What can possible be "right"- well enough to let her run for 10-20 mins., only to become "wrong" at some point- providing no clues(to my amateur senses)-and causing a cold shut-down??
Nothing over-heated, anything that can be measured checks out fine after the stop, no abnormal smoking..... The carb?? That's one area I can do little with. I turned the screws last fall, according to detailed forum notes. But that's as far as I can go with that.

(quoted from post at 23:47:48 04/08/14) To recap; Acceptable Volts to the 12V coil, Amps measured thru the coil-under 4, or about 3.4. Coil isn't running hot, connections are all good.
Today I did a real cleaning to the plugs, and re-gapped them.(I did a one-time siphoning/ filtering of the gas, and returned to the tank)
Starting; It took many tries, using the choke all along, or there'd be no chance of a start. Finally, the Jube kicked in and ran,but ONLY with the choke held (closed). After a minute or more like this, it finally wanted to run without the choke. But after a while... business as usual;
Runs for 11 mins.-then stops cold! As clean as turning a light switch on and off.
I'm looking at the ignition system, now the fuel tank, gas condition. The carb was tuned last fall, when the tractor would run for hours with no problems. The great puzzle to solve here;
***What can possible be "right"- well enough to let her run for 10-20 mins., only to become "wrong" at some point- providing no clues(to my amateur senses)-and causing a cold shut-down??
Nothing over-heated, anything that can be measured checks out fine after the stop, no abnormal smoking..... The carb?? That's one area I can do little with. I turned the screws last fall, according to detailed forum notes. But that's as far as I can go with that.

Click to expand...

ry to keep it running on starting fluid or propane.

"or propane"

Hank Hill would be proud!

(quoted from post at 23:54:16 04/08/14) "or propane"

Hank Hill would be proud!

Click to expand...

e should be!

Twice I've mentioned peculiar voltage readings at the + coil terminal. I DID check the tightness of the fan/alternator belt about a week ago, and it seemed OK. Past couple of days tho', I've ignored it's screeching noise, because it would go away soon enough. Turns out, at some point, one piece of the bracket assembly had snapped, and the alternator was now very loose, thus producing an unsteady voltage. And finally, no charge at all. Anyone tried to see how far you could get with the engine running on the battery alone? I'm having to wait until tomorrow to test this out, but it could be that the engine ran until the battery voltage fell below the minimum for the coil to operate normally? Stay tuned!

Without using the lights, they'll run for months worth of intermittent
use without charging the battery. Even on a 6V system.

(( before I get that request to Close this topic, or whatever the term, ... I've had to divert my energies elsewhere for several days. The new bracket has arrived for the alternator. Can't wait to get it on and eliminate the alt. malfunctioning as a cause for the engine stops. OR, find it was the true reason! ))

(quoted from post at 10:40:39 04/13/14) (( before I get that request to Close this topic, or whatever the term, ... I've had to divert my energies elsewhere for several days. The new bracket has arrived for the alternator. Can't wait to get it on and eliminate the alt. malfunctioning as a cause for the engine stops. OR, find it was the true reason! ))

Click to expand...

Doubtful, as you mentioned you had been charging the batteries. I would double check the wiring for "clean, bright and tight". When it does stop running immediately check to see if you have spark. That will start to guide the process from there...

Ok, I'll call it closed, I guess. The electrical/ ignition part of it seems to be ok. For clarity ; (finally, right?)
**"Self-exciting" Alternator (a new one on me), which is at
least why there's only one wire coming from the alternator.
***NO-- Resistor needed, Wasn't there when I bought the
Jube, and the ignition coils I buy- clearly state that NO
resistor is required in the circuit. In fact they state
"Do NOT use....."
***With the internal resistance of the coil being what it is, the
amps reaching the points/ plugs is held under 4- which is
the max., and less being the desired.

So, I'm concentrating on the carb/fuel concerns, in another
post of mine.
(( by the way, explain please; I find the only "Reply" button inside each posted message, not a unique REPLY button, as there is a New Post button. Odd, because these inside-the-post buttons even say they're for replying to a "specific" post, resulting in posted messages being out of sequence by date. Several times I've wanted my additional notes and points to be at the end of all that's been said, to bring it all home, or in this case-- to "close" the Topic. Will this post be at the end of the chain? ))

"Will this post be at the end of the chain?"

It will if you're using the modern view of the site.
Not on Classic view.
I use the button in the upper right to switch back and forth.