Ignition Resistors

One of my tractors was very difficult to start in cooler to cold weather. It would crank fine but no fire. If I jumper from a running vehicle, it would fire right up.
I finally got around to study the situation. When I turned the ignition ON (not running), I measured a little less that 5 volts at the coil. I measured the ignition resistor and measured 2 Ohms. I went to the local farm store for a replacement it was also 2 Ohms. I measured the DC resistance of the coil and came up with 1.8 Ohms. I installed a 1.8 Ohm resistor and now all is well.
The moral of the story, make sure the ignition resistor matches your coil.

I seriously doubt the resistor was the source of your problem. The smaller resistor would allow a little more current into the coil, which would increase its stored energy by a small fraction. That wouldn't make enough difference to make it start; the point resistance is likely to be more than .2 ohms. Check your points.

I doubt the rated resistance was the problem. Faulty . . maybe?

Is your cranking resistor bypass working?

I don't know of any fix-rate ballast resistors sold with a rating of 2 ohms though. What exactly were you sold?

Here are a few:
NAPA* Echlin ICR11 (1.35 Ohms)
NAPA Echlin ICR23 (1.2 Ohms)
NAPA Echlin ICR34 (1.4 Ohms)
Lucas* 3BR (1.3 - 1.4 Ohms)
Mopar* DCC-4529795 (1.4 Ohms)
Accel* ACC-150250 (1.35 Ohms)
Standard* RU-4 (1.35 Ohms)
Standard RU-23 (1.2 ohms)
Standard RU-37 (1.4 Ohms


Ballast resistors don't read when cold on an ohm-meter what their actual rating is.

Here are two new resistors from NAPA that I tested out of the box.

ICR11 1.35 ohms rated. I checked and it reads 2.8 ohms.

ICR13 1.82 ohms rated. I checked and it reads 3.2 ohms.


All that counts is voltage at your coil while cranking and it needs to be 9 or more volts.

Changing from a 2 ohm to a 1.8 ohm "rated" resistor might have some effect on overall points life. It should have no effect on starting.

Most people are too young to remember how cars were wired when points, coils and ignition resistors were used. My old 1965 Barracuda used ignition resistors. I always carried a spare. Back in the day, if the engine would start and die as soon as you pit the ignition key in the run posistion, it was a bad ignition resistor.

Reason, the ignition switch bypassed the resistor and applied the full 12 volts to the coil. After car started, the ignition switch returned the 12v to the resistor.

So yes, a hotter spark is needed. The 5v you had going to the coil becomes less during cranking, because the battery cranking voltage is less.

Had the same issue with my Jubilee, so I connected the cathode end of a diode to the + of the coil. Connected the anode end of the diode to a wire that I ran to the starter where the +12 goes. My jubilee had no ignition switch like a car, it's only an on off switch, uses a button on the tranny to apply voltage to starter.

When 12v is applied to the starter, slightly less because battery volts drops during cranking, the same voltage minus .6v goes to the coil. Problem solved.

I'm still using the same resistor that applied 5v to the coil. The small resistor you installed may have solved your problem, but it will burn your points faster. So in my opinion, the biggest resistor you can use and it will still work, the better for your points.

You are correct!!!!!!! The ballast resistor should be the proper match with the coil, if you have a 12 volt tractor but are using a 6 volt coil, it should be sized such that it drops "approximately" 6 volts so 6 remains on the 6 volt coil. Thats like a 50 50 Voltge Divider and if pure resistance is all that matters, its DC resistance would be approximately the same as the DC resistance of the coils LV primary i.e. 1/2 the voltage dropped across ballast and remaining 1/2 across the coil, 6 + 6 = 12.

HOWEVER as little as 0.2 ohms difference (2 versus 1.8) shoudnt make a huge significant difference???? but I cant argue with your results, you were there not me........Perhaps there was a resisitve connection causing part of the problem which was cured when you installed a new ballast???? Also, some ohm meters arent all that accurate at such low ohms readings...

On many tractors that are 12 volts but use a 6 volt coil plus an external ballast resistor, they are equipped with a ballast by pass system such that ONLY while the engine is cranking over (starter motor engaged) the ballast is temporarily by passed so the coil sees the full unballasted battery voltage which really helps improve cold weatehr starting. I WOULD SEE IF YOUR TRACTOR HAS SUCH AND IF ITS WORKING. If so you need to measure the coils voltage when the starter motor is engaged and if the ballast by pass is functioning Id expect it to be near the battery voltage (maybe 10 to 12 depends on battery and current draw) versus the only 5 or 6 normal when shes not cranking over!!!!!!! If it has a by pass system theres an extra wire to the coil or low output side of the ballast which feeds unballasted battery voltage to the coil ONLY WHILE CRANKING

Good observation and post on your part, perhaps this discussion will help others understand the system better....

John T

The real numbers we need are what the voltage is when you where cranking. You may have another issue that your resister was masking. I have usually found that if a cranking system is pulling too many amps you need to jump it to start then your starter or cables could very well be the problem. I have even seen the solenoid cause this when the internal contact gets burn.

Also you need a crank bypass system on the tractor to help it have higher voltage when you are cranking. You did not tell us what tractor you where working on. Any of them with a Delco starter are easy to put a bypass on. There is an ignition terminal on the starter solenoid that just needs a wire ran over to the coil. No need fore diodes or anything else.

I think resistors have been talked to death here - but I'll add a few comments anyway.

12 volt systems are designed to make spark with 9 volts, not 12 volts. That because engines crank at 9 volts.

Dedicated external ignition resistors can be "fixed" or "reactive."

Some are wire-wound and some are carbon (built into a wire). GM and Deere used a lot of carbon wire resistors.

Fixed resistor usually uses a system to bypass them while the engine is cranking. Not the case though with a high-resistance coil when all the resistance is "inside the can."

Reactive resistor needs no bypass. It has low resistance until a load is on it and it heats up.
Can cause a starting problem if the ignition is left on too long before cranking. Many older Ford tractors use them.

No way is the change from a 2 ohm (fixed) to a 1.8 ohm (fixed) going to fix a no-start situation if all else is as it should be.

What do you do if your solenoid doesn't have a third terminal for ingition and you don't have an ingition switch with a starter mode?

George, good question, on the two cylinder Deeres that used the saddle mount mechanical push starter switch, the switch had an extra small terminal (besides the huge battery cable terminal) which was hot ONLY when the switch was engaged and a wire led from there up to the coil........That constituted the ballast by pass system, simple cheap and worked as long as farmer Billy Bob and Bubba kept that wire connected.......

As youre Im sure aware, what many call "Ford type Solenoids" have the extra small "I" terminal thats hot only when the solenoid is activated via the other small S terminal and Delco had a similar small terminal.

Fun chat

John T

If the starter has an external battery supply terminal then just add a small wire under it to go to the coil. You would need to install a diode to stop the regular ignition wiring from trying to back feed the starter. I have seen this done on some of the older Fords that have the single terminal on the starter.

I have also wired a small relay into the crank circuit that is only hot when the starter is cranking. You do not have to worry about a back feed this way. It depends on how much room you have on what you are working on.

"I think resistors have been talked to death here " SOOOOOOO TRUEEEEEEEEE

You can say that again my friend lol but people keep asking questions so here we go again.

Good informative post........heres a small addition for those unaware.....Two Cylinder Deeres used the wirewound ceramic/porcelain encased "bathtub" style ballast resisitors while some of the New Generation switched over to the carbon types you described and the kind you spoke of which increased resistance as they warmed up and NO BY PASS WAS THEREFORE REQUIRED.


Some years back, around 2003 or 2004 I believe, Id have to dig out the magazine, The "Green Magazine" hired me to write a full feature artice on Ballast Resistors so if anyone wants to learn even more in detail, (cant imagine why, its gets technical and boring) they can e mail me and I think I can find the word file (if not lost when I had a crash)

Boring but still fun sparky chat, hopefully we put this to rest for a while at least

PS Id define a coil labeled "12 volts" or "12 volts NOT for use with external ballast resistor" or "12 volts no ballast required" etc etc is designed and engineered to still produce a sufficent spark,,,,,,,,and dissipate the heat,,,,,
when input voltage is reduced to as low as around 9 volts (such as while cranking),,,, but still produce a spark, handle the current and dissipate the heat even when voltage is raised to 14+ by the charging system. i.e. designed to produce a spark over an input voltage RANGE from 9 to 14+ .........Call it a 12 volt "nominal" coil if you will

FUN CHAT, as always for us sparkies, take care n keep warm up your way, Im headed South real soon

Ol John T in Indiana

It really doesn't matter how small a difference there was in the resistor because it fixed the problem.

I have pulled several capacitors over the years that would only be 1 or 2 microfarad's under the rated specs and woulden't start a motor. Manufacturer's claim you have 20 percent to play with but sometimes it just is what it is no matter what we think.

Yep he says it fixed it and like I posted we werent there (have to take a man at his word I figure) but Id still really really wonder if it wasnt a loose resistive connection that cured the problem versus only a 0.2 ohms difference??? No way to tell now lol

take care M Man

John T

There is no third terminal on the solenoid of my Jubilee and the button on my IH C has nothing either.

I had thought of using a relay, however, the diode and wire was FREE. Not to mention, very simple and easy to install.

It seems like many don't know about by-passing the ballast resistor during the starting mode.

Yes, from personal experience, the wrong resistor makes a huge difference to starting. ON my 59 IH pickup which had the firewall mounted ceramic resistor I found out the hard way. The original resistor was still working but cracked so I thought I'd be safe and install a new one. It was "supposed to be" the right one for that truck but after installing the new resistor I had no end of starting problems, rough running and needing lots of choke to keep it going. Finally pushed so much raw gas into the exhaust that the muffler exploded while trying to start. Just as a guess I put the old cracked resistor with some glue or duct tape to hold it together, back in place and that solved the problem. Truck started and ran perfect again.

What matters - at least to some of us - is what the true problem really was and what really fixed it. .2 of an ohm isn't enough to create a non-starter in an otherwise good working system.

1 microfarad of difference that you mention is not even possible in a points-ignition system. Maybe you mean 1/10 of 1, i.e. .1 microfarad? .1 microfarad of capacitance isn't enough to make a non-starter either (unless the system calls for .1 and the capacitor is shorted and has 0 capacitance.

Just about all breaker-point ignition systems use .1 to .3 microfarads and they will run with anything within that range. If the capacitance is wrong - the points arc and burn out prematurely. If the capacitor shorts out - the spark and disappear.

George, if you dont have the third terminal or button, Id also go the diode route, cheap simple and easy for sure.......

John T

I worked in electronics long enough to know that just because a symptom disappeared when a part was changed doesn't mean there was a problem with the part. His problem WILL return because in all likelihood he didn't fix the root cause.

I didn't know I was going to open a bucket of worms over such a basic issue. I only intended to introduce the fact that a less than obvious problem may exist if the ignition resistor does not match the coil. The unlabeled resistor from the farm store was the same 2.2 ohms as my old resistor. I did have an error in my original post. The old resistor resistance was 2.2 ohms instead of 2 ohms.

Back to the discussion. Like Bill Clinton says, it's in the arithmetic. If any of you would apply some basic ohms law, you would find it supports my diagnosis and solution.
But I guess some of you who have never seen my tractor and have not performed any actual diagnostics on it know more than I do.
I guess there are believers and non believers, but I don't see any reason to get all wrapped around the axle about it.
My tractor starts right up like it should and I'm happy.

I would be interested in seeing your arithmetic. As they used to tell us in engineering school "no credit if you don't show your work".

In the spirit of fair play, I'll show my work first:

The energy stored in an ignition coil is the 1/2 the inductance in henries times the square of the steady-state current in amps.
(E = 1/2LI²)

The inductance of a typical automotive ignition coil is 9 millihenries, although for our discussion the actual inductance is irrelevant since you didn't swap the coil. (The inductance "comes out in the wash".)

Coil energy with original resistor:
E = 1/2LI²
E = 1/2L(V / R )²
E = .5 * .008 * (12 / (2.2 + 1.8) ²
E = .004 * (12 / 4)²
E = .004 * 3²
E = 36 millijoules

Coil energy with new resistor:
E = 1/2LI²
E = 1/2L(V / R )²
E = .5 * .008 * (12 / (1.8 + 1.8) ²
E = .004 * (12 / 3.6)²
E = .004 * 3.3²
E = 44 millijoules

So your resistor swap gained you a whopping 22 percent increase in spark energy.

Questions:
1. How is it that an engine in good running condition can go from not running to running perfectly just with a 22 percent increase in coil energy?
2. Are you confident you can accurately measure resistance to an accuracy of less than half an ohm? I did this for a living for many years, and I know I can't do it unless I have a multimeter capable of performing a four-wire resistance measurement. (I'm pretty sure your meter doesn't do four-wire.)
3. Did you bother measuring the point resistance, and if so, what was it? Half an ohm resistance across the points will obliterate increased current you gained from your resistor swap.

Look forward to reading your answers and seeing your math.

I don't think anyone claimed to know more about your tractor then you. I don't even know what it is specifically. Some of us do know of Georg Ohm's Law - one of the most fundamental laws used in electronics for almost 200 years. Also about the Kettering coil and points battery ignition system.

You didn't give any specs on your tractor or what type of resistor you were using. As I stated earlier, I never heard of any 12 volt breaker-point system using a 2 ohm or a 2.2 ohm rated resistor.

The measurements on a cold resistor with no current flowing through it are pretty much useless.

I realize you were not asking for advice - but you must expect a few opinions when posting this stuff on a public old-tractor forum.

A typical resistor used in a 12 volt system will only have 1/2 ohm when cold with no current and up to 2.5 ohms @ 3 amps at 14 volts when hot and a lot of current demand.

What you stated in your original most does not make sense according to commonly used auto ignition basics and Ohm's Law.

So yeah, I don't know any specifics on whatever tractor you have and what ignition system it presently has in it. I DO know the principles of a conventional 12 volt breaker-point system.