The Stovebolt Site Forums The Parking Lot Big Bolts ballast resistor voltage

I am in the process of converting my truck to 12 volt, in fact i am completely done with it EXCEPT, i tested my ballast resistor before i tried to start it. My battery was at 14 volts and after the resistor it read 11 volts. i am using a resistor that i have seen on many applications very similar to this, will i be ok? Also this is the resistor

http://www.autozone.com/autozone/ac...ng=search&itemIdentifier=253187_0_0_

yes that's the correct item - the purpose of an ignition resistor is to lower the current to the coil by dropping voltage

Bill

Thanks, but, is 11 volts too much though? The original generator only put out 8 volts and I really don't want to ruin my distributor. I'm just trying to stay safe.

The lower voltage from the ballast resistor will help your points to last longer, 14 volts would have you burning up the points relatively quickly but not doing any real damge to the distributor its self.. I like to use a ballast resistor that will give me 8-9 volts at the dizzy.

the distributor doesn't see volts it sees the coil output amps [at hundreds of volts!] - the distributor is the same for 6 or 12V, but you do need a 12V coil .... simply put, the reason for reducing the voltage to the coil under operating conditions is to keep the coil cooler and to lessen the arcing on the points so they last longer, the same thing can be accomplished with a newer style coil that has the resistance internally, in which case you don't need an external ignition resistor .... also when the engine is running that 11V will be much lower

Bill

oohhh okay! That makes a great deal of sense. Thank you all, i shall now proceed to look for a 12 volt ignition coil!

The current flow and the voltage drop across the resiistor is a function of the total resistance in the circuit. The coil designed for 12 volts will have more resistance than the 6V coil, so the total will be more. With the points closed, some of the voltage will be consumed forcing the current through the resistor, and the rest will be used by the coil. With the points open, you'll see 12 volts all the way to the insulated point, because no current is flowing. The best way to get an accurate measurement of voltage drop across the resistor is to measure voltages with the engine running. You should get close to charging system voltage upstream of the resistor, maybe 14.5V or so, and 9.5-11 volts at the coil + terminal.
Jerry

You can either buy the 12 volt coil (with the internal resistor) or use the old 6 volt coil and add the external resistor.
As far as checking the voltage was your points fully closed? You have to check the voltage with the points fully closed or you will get higher voltage as there is no current flow and resistors only drop voltage if current is flowing. But with the points closed you should get about 8 to 10 volts at the + side of the coil. (make sure the ignition is on too)
I've never tried running the engine and checking with a multimeter but to me that sounds like it would not work as now you get a form of alternating (more puseating than true AC but anyway)current which will give be at 6 to 10 volts for one split second and jump to 12 to 14 the next.I have hooked a single cylinder ignition system to a oscilloscope though and could find the pulse and measure it but it the pulse of point contact was hard to see. But with a 6 cylinder it would be more frequent may be detectable with the right DMM. But points closed is what your looking for.

Try it with an analog voltmeter- - - -the kind we used back when these trucks were new. There was no such thing as a digital multimeter back when I learned automotive electricity. I can also check point dwell with an analog voltmeter, since it reads an average of the points open/points closed voltage which can be translated into a percentage of "points closed" time otherwise known as "dwell". Us oldtimers aren't really as dumb as everyone seems to believe.
Jerry

They had 12 volt systems for years with a ballast resistor. The advantage was that the resistor was bypassed when the starter was cranking allowing better coil operation even though the starter creating a large draw. I wonder if the ballat resistors that are offered are actually for use with a coil designed for a 12 volt system. I am assuming that the coils used with the resistor, from the factory, were different from the 6 volt coils.

Yes, the external-resistor 12 volt coils have a different primary resistance than 6 volt coils. The resistor cannot drop enough voltage to protect the points if it's used in series with a 6V primary winding. I've fixed several chronic point-burners by installing the right coil.
Jerry

Alright I'll give ya that would probably work with a analog meter, but I don't know many people who have them any more particularly when you can go get a cheap DMM for $10 and though I haven't tried it I can bet it would make these go crazy. My uncle taught me the method I described and it does work, and I'd venture to say he's rewired close to 100 antique tractors installing or checking the ballast resistors like this.
However I'm confused about your last comment.


Are you saying there are three types of ignition coils. a 6V, 12V, and 12V with external resistor? Cause I've only seen two. The 12 Volt coil (with internal Resistance) and the 6V. And saying that the 6 Volt coil works fine attached in series with a resistor. I've got 2 1947 WC's running with a ballast and a 6V coil and a 1947 2-ton running with a ballast and 6 volt coil and my uncle has about 15 tractors running this way. I have to change the points maybe once every 3-5 years and that's not chronic failure, according to manufactures your supposed to replace points yearly. You do have to make sure the dummy's at the auto-parts store gave you the right resistor through. They make two types one that is meant more for accessories (less resistance I believe) and the other that is for ignition coils.

You're entitled to your own opinion, but not your own facts. Delco-Remy made 6V and 12V coils through the 1950's and 60's, and the 12 volt coils were marked as such. All the 12 volt units used either ballast resistors or nichrome resistance wire between the ignition switch and the coil. Ford used a 12V coil that did not need a resistor on some of their applications.
Jerry

1947farmer,

They do make all 3 coils you discribe 6V and 12v with or without the internal resistor.

If you're using a 6V coil with the ballast resistor wired in Series to the coil primary on a 12v system, you could do the same thing by using the 12v coil with the internal resistor eliminating the need for the external ballast resistor.

Mike B

Yeap, I did some more research on the topic. There are three coils the 6v, 12v, and 12v with internal resistor. However this is also generic description cause not all 12v coils are made equal, and neither are the 6v.
I checked the coils I have at home on my equipment. I have a 6V and 12V with the exact same internal resistance 1.5ohms, one that couldn't be identified with 4.5ohms and a 12V with internal resistance that measure 6.5ohms. I also looked up new coils, 12V coils come in various ohms of resistance.
I also talked to my uncle, and he certified to me he does have several things running with a external resistor and 6 volt coils and has no issues. However he told me he has also had to put two resistors in series on several ford tractors with 6volt systems cause they kept burning up points with only one resistor.
It's seems Jerry and I are both right and wrong about different issues here.
Learn something new everyday, usually when you just think you have it all figured out.

Thanks for the update. Once the coil resistance gets right, then it's a matter of matching the inductive reactance of the primary winding with the right-capacitance condenser to prevent metal transfer from one point to the other. That was usually done by the engineers who designed the ignition system, rather than someone at the shop level who decided to install whatever coil was handy and hope it would work.

The shop where I grew up had regular customers from 5 or 6 southeastern states, people who would go to great lengths to use my father's skills to get the best performance and gas mileage from their cars. A lot of these people were salesmen who made a living traveling, and a few thousand extra miles between tuneups or an extra 1/2 mile per gallon made a huge difference in their annual incomes. Dad is 87 years old now, and has forgotten more than I'll ever know about point-type ignition systems, but I did manage to pick up a little knowledge along the way.
Jerry