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I have a 1953 chevy truck that is converted to 12 volts..I'm not sure why I have to use a ballast resistor on a 12 volt coil ? Shouldn't it run just fine with a direct 12 volt wire straight to the coil since the truck is all 12 volts ?

Sal,

Why do you need a ballast resistor?
Well, just because (if you are using original style points).

I am sure someone will post all sorts of logical/technical/mythical reasons why GM (and maybe others) had/has this requirement, but, just be sure you have a resistor or you will burn out points.

You can either put a ballast resistor between the ignition lead and the coil, or, use a coil that has the resistance built in. Some people say use the resistor (rather than a resistor coil), so you can wire the starting circuit (to the coil) to run at a full 12v, and then once you have started, you run through the resistor to the coil.

Short answer? Current control. Longer response? Points and condenser are matched to control arcing. This is a function of Voltage and materials. If the condenser is too small it will pull material from one contact and deposit it on the other contact. If it is too big it will pull that material the other way. Either condition is bad and shortens the life of the points. So the engineers ran tests and calculations to make that combination work as long and as well as they could. At 6 Volts. Why do all that work over again when a "ballast" resister will do it for you? And.. they are cheap. Those points open and close 3 times every time that engine goes 1 revolution. And that engine goes around thousands of times every minute it is working. That is a lot of open and close for each mile. And they last for thousands of miles. So congratulations to the original engineers for getting it right all those years ago. So? Use that "ballast" resister and celebrate the work that went on before us. Now was that helpful?

You had to go with reason, Steve

Why did they not specify/design a condenser and a coil that worked together at 12v without burning points?

A cold engine does not have 12 volts available during cranking, more like 9V with the starter load on the battery. GM designed a "12 Volt" coil to function properly and deliver a hot spark during cranking, and installed the resistor to protect the points when the charging system voltage went to 14+ volts to keep the battery charged.

Mechanical points, even with a condenser in the circuit, can't handle more than about 5 amps of current flow without burning. If a "12 volt" coil is designed to protect the points during cruise with over 14 volts available, the ignition primary current flow during cold (or very hot) starting will not be sufficient to deliver a good spark. The result is a long cranking session, and no start due to insufficient voltage at the spark plugs.

Now, before some of you armchair experts accuse me of lying, I've fixed hundreds of "normal crank/hard start" situations by installing the proper combination of coil and ballast resistor with a bypass wire to provide full battery voltage to the coil during starting. The situation was caused by somebody installing the wrong coil and eliminating that "unnecessary" resistor. This is not some theoretical pipe dream I'm having!
Jerry

I have been putting Bosch blue coils on vehicles for many years. I have them on tractors, Model T's, My 46 2-ton, forklifts in my shop that I have converted to 12V... The list goes on. If you want to buy something local have your local auto parts store look up a coil for an early 70's VW beetle. Any of the VW's from 1967 on would have been 12V and used an internal resisted coil.... You can also run a condenser from the same vehicle.

I am converting my '52 to 12 volts. When I rebuilt the starter, I installed the starter switch with the ignition resistor bypass terminal in the event I decided to use a ballast resistor. But, my plans are to use an internal resistor coil and forgo the separate ballast resistor. Ballast resistor are known to fail and are the weak link in the system IMHO. If I have starting issues (which I don't expect at 12v) then I can switch to the ballast resistor and carry a spare in the glovebox.

My 50' 3600 with a 235 converted to 12-volt runs the internal resistor coil, 4000 miles now with no burned points or starting issues? I just did not like the look of the external resistor on my firewall so went to the internal.

Forrest Gump's Mama had a saying about folks who use internal-resistor coils!
Jerry

"Ballast resistor are known to fail and are the weak link in the system IMHO."

I'm not being a smart aleck, as I truly don't know, exactly does one diagnose a bad ballast resistor? Do you have insufficient voltage to crank??

In all my years of driving I have had more condensers fail than ballast resisters. One of the former to zero of the latter.

lefty1: Put a Volt meter on that ballast resister with the key on. Should read about 6 volts if good. 12 volts if bad. That is with the points closed.
tcleaderman: Profit! Motor company gets to sell you a new part.
To the question of internal resistance coil verses external ballast resister? I can think of only one reason that the external resister is better: Hotrod Lincoln has stated it very well. My reason? Beware, opinion time, ballast resister is easy to find, replace, and test. I don't think they fail any more often than the coil will. Disclosure: I have no scientific evidence to back up the longevity claim. Just experience. And mine might not be any thing like yours.

Wow Jerry that was really additive, just send us all a PM next time in regards to your myopic opinions.

lefty1,
so if you have a failed ballast resistor and you have the starter bypass switch, the symptoms are really classic. The engine will fire or start while cranking and then immediately die. I will attest to having the unfortunate experience of having a failed ballast resistor. They are not prone to failure but everything component is the possible site of failure.

I think the 12 volt design with the bypass switch was very clever. It always gave the best possible spark delivery at cranking time.

In reading the post here I believe there is a small error in the post from Steve H. Resistors fail to an open condition, not shorted. So with key on (not in the start position), "0" volts will be the reading on the coil side of the ballast if it has failed.
My opinion: I consider Hotrod Lincoln a living textbook on how old cars work.

Amen

Does that change the fact that everyone running an internal resistor coil is stupid? Is it the intent of these forums to foster and support that type of behavior? If so the living legend should be very proud of himself and you for validating his behavior, well done. Quite simply the post was not additive to the discussion and he nor any of you are not qualified to render that judgement about anyone else here.

HotRod is correct.

There's a detail he left out, however. Without a ballast resistor, over time, metal will be transferred from the outside contact of the points to the inside contact (due to the heat caused by excess amperage as well as the opposing charges of the two contact surfaces. You actually end up with a cone shaped buildup on the inner point contact and a corresponding hole on the outer point contact.

Naturally, this changes the point gap, because the cone is closer to the edges of the hole than the flat surface surrounding it are to each other(which is what the points were gapped to). That can dramatically affect performance and even prevent starting.

The reason the ballast resistor was bypassed during cranking was to provide a higher voltage spark to the plugs, making it easier to "get the fire going". The combination of no heat and cold fuel required a hotter spark to ignite.

Once the motor is running, the plugs don't require as much voltage to ignite the fuel-air mixture because the environment is a high enough temperature to almost ignite the fuel without a spark.

The reason diesel engines don't use spark plugs is because the compression is so high that it generates sufficient heat to ignite the fuel without an ignition system. Of course, back in the old days they used to use "glow plugs" to help warm up the fuel air mixture until the motor was running. Compression was lower back then, and the engine needed a little help to get going.

In all 'MY' years of driving I've had several ballast resistors fail.
After the second time I just started carrying a spare with the extra set of points... I thought it was 'stupid' to spend more money than was needed on a part I couldn't fix or 'inexpensively' replace. (There was a clear price difference between a coil and a ballast resistor), I hope no one gets BH over that..

I have an external ballast resister on mine and it went bad in a very bad place. I had to troubleshoot it on the fly. It was an easy fix to just bypass it and down the road I went. I also don't like looking at the ballast resister on the firewall, but I am too cheap to replace a perfectly good coil (I had an extra ballast laying around). I'm trying to save my money so I can afford to buy more parts from Brad. If the coil goes bad, maybe he will have a good used one with the internal resistance.
Carl

Coils at NAPA that have internal resistor:

MPE IC277SB $22.50
ECH IC14 $31.95
MPE IC14SB $20.70

NAPA sells ballast resistors at prices starting at around $4.00

Ok, I have decided to use the ballast resister. Where would GM have mounted the resistor when the trucks went to 12v from factory?

Here's where the ballast resistor is mounted on my '55 3100. Upper right hand corner of pic.[URL=http://s1348.photobucket.com/user/capnduane/media/55Chev634_zps52e17604.jpg.html]photobucket[/url]

The resistor was usually mounted to the firewall about valve cover height or slightly above where it could get good airflow for cooling. Any resistor develops some heat- - - -that's why the housing is ceramic, to protect nearby parts from heat damage.

If there was not a good reason for running a low-resistance primary winding in the coil for producing a hot spark during cranking, would GM have gone to the effort and expense of designing their ignition systems that way? I can't think of any manufacturer of any product that will choose to make their product more complex and expensive than necessary, and then produce millions of that product. Just because a few self-appointed experts have declared themselves to be more intelligent than the General Motors design engineering department doesn't seem to be a good reason for me to ignore 50-something years of experience working with these vehicles. "Your mileage may vary".

Jerry

Just ordered a new resistor from Rockauto... $3.24. I'll put it in the toolbox I keep under the seat. The one in my truck looks original, 59 yrs old! I took it off and stripped about 15 coats of paint off of it, and a little wire brush work. Still works fine?

I like to travel with a set of small jumpers and a little VOM meter. I haven't used it on my truck yet but have helped other cruisers.

Yup, those free multimeters from HF fit right in the glove box! "One never knows"

beltfed: We might be talking the same thing but different? With the points closed and the key on with a voltmeter across the resistor you should see 6 volts if all is well. If the resistor is open, and that is how they fail, you will see 12 volts. If you put the voltmeter at the coil to ground you will see 6 volts if good and 0 volts if bad with the points open. If the points are closed, well you will see 0 volts cause you have a ground. Still, 2-3 dollars and a possible on the road fix is better than 20 dollars or so for a coil and no on the road fix. Just my way of thinking. Both will work. And both are as reliable as the other, I think.

I love bench racing and keeping it friendly here goes....with points closed and one lead of the voltmeter at ground potential and the other on the feed side of the resistor you will read 12 volts not 6. If you move the voltmeter lead to the other side of the resistor you will read about 8 volts assuming you have a 1.5 ohm or so ballast resistor. If the resistor is bad(open) you will read "0". That is the important point..."0" volts on the downstream side of the ballast resistor means failure.
The ballast resistor is in series with the primary side of the coil to reduce the voltage (current) to it in the run position.
I think I have it right but never know. I have let the smoke out of many wires before!
Maybe Hotrod Lincoln will jump in here and correct me.

beltfed, sounds rite to me, except I think the "about 8 volts" is actually 9 volts

Bill

Yep! More than one way to skin the same cat..:) Across the resistor or + side to ground...Get the part or the whole circuit. + side of coil to ground or _ side through the points. 12 volts and "about" 8 volts if you measure from the + side or the _ side of the resistor. It does matter where the volt meter goes. However you do it will work as long as you know where that voltage is coming from and headed to. Volts and continuity. We are gonna make some one nuts with all this stuff. But hey this is fun!

They still use glow plugs in diesels.

I read 5.58 volts how is the engine going to run on 6 volts when the coil says 12 v ?

You're using the wrong coil.
Jerry