Question about ballast resistors

mngreen

mngreen

Senior Member
I put in a ballast resistor many years ago after repeatedly frying points on my 12V ignition circuit after getting advice to do that without really knowing what kind of coil I had in the car. I recently tried to test the coil impedance but my ohmmeter is not sensitive enough to tell. With the resistor installed, my car runs well, which it also did before installing it.

What happens if a generic ballast resistor is installed in a 12V ignition circuit with a coil that already has an internal ballast? What would be the effect(s) on the running of the engine? Would it even run? What about the condenser?
 
You will not get the full voltage to the coil as ballast resistor drops the voltage due to its resistance. Not getting the full voltage to the coil will not produce the desired flux (magnetic field) in the windings so when the points close will not produce the desired voltage in the secondary coils due to less magnetic fields collapsing consequently less voltage to the plugs. The vehicle will run but at a somewhat reduced power, the condenser will work the same and the points will last a little longer due to less current flowing through them. The replacement coils are Echlin IC9 for 6V and Echlin IC14 for 12V. See if there is a number on the coil.
 
hmmm , i did that also & am running the same coil as always . no problems yet ...
 
jrelliott said:
You will not get the full voltage to the coil as ballast resistor drops the voltage due to its resistance. Not getting the full voltage to the coil will not produce the desired flux (magnetic field) in the windings so when the points close will not produce the desired voltage in the secondary coils due to less magnetic fields collapsing consequently less voltage to the plugs. The vehicle will run but at a somewhat reduced power, the condenser will work the same and the points will last a little longer due to less current flowing through them. The replacement coils are Echlin IC9 for 6V and Echlin IC14 for 12V. See if there is a number on the coil.
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Kindly please define "somewhat reduced power"--would it be a really noticeable difference? Reduced top end? Lousy acceleration? I will look for number on the coil but I don't think there are any as the coil is chromed.
 
I would think the fact you kept frying points without the ballast resistor indicates the lack of one in the coil.
 
The only time it would give reduced power is if the voltage going through the coil is dropped enough to make the spark at the plugs weak enough to start missing under loads such as full throttle to pass or climb a hill. If you don't have any missfirings, then your doing fine with the ballast and as mentioned the points should burn less.
 
mngreen said:
Very good point [pun intended]!

What else might cause the points to fry?
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Probably oil or grease on the contacts will make them burn more. You want a good flat clean area for full contact. Too small of a contact spot makes for localized heating (burning). So, you also want good point alignment.
 
How can you tell which coil you have? Many that are sold as 6V or 12V aren't marked. I know I have a 6V as I put the coil on, but I have had others in our club that have had coil issues and have no idea what they had.
 
Every coil that I've purchased with 'internal resistor'.... has white printed words 'internal resistor'. You certainly wouldn't want 'too much' resistance....by having BOTH internal and external resistors.
One bad consequence of too much resistance....HEAT, which shortens coil life.
 
Under a load is when you would see reduced power, like pulling a big hill or trying to accelerate rapidly. In the Model A with its low compression would probably see an rpm drop due to less spark. If you are wondering about lower spark, why do pilots on small airplanes check both magnetos and notice an rpm drop when going from both to one magneto? Two mags are for redundancy so if one fails still will run.
 
jrelliott said:
Under a load is when you would see reduced power, like pulling a big hill or trying to accelerate rapidly. In the Model A with its low compression would probably see an rpm drop due to less spark. If you are wondering about lower spark, why do pilots on small airplanes check both magnetos and notice an rpm drop when going from both to one magneto? Two mags are for redundancy so if one fails still will run.
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Dual ignition will give a different flame pattern across the piston top vs a single ignition point, thus the different RPM.
 
I jumpered out the ballast resistor on my car because a plug had fouled and the plug recovered. However, with electronic ignition, it can only be done temporarily or the module goes up in smoke.
 
Heavy moisture or condensation in the cap before crank up CAN cause the points to glaze. This turns the contact material into an insulator. You then have to file through it to reach the unglazed point contact material to get them to work again. I know it’s not a burnt condition but I thought I’d mention it only because it happened to me. The reason I knew to check for it was because of an Airboat friend of mine had had it happen when he sank his boat. After he cleaned his engine, changed the oil, and blew out the cylinders, it still wouldn’t start because the points in his Bendix magneto’s were glazed. BTW there are only two types of Air boaters. The ones who have sank their boats and the ones who will. lol :D
 
Milton said:
I jumpered out the ballast resistor on my car because a plug had fouled and the plug recovered. However, with electronic ignition, it can only be done temporarily or the module goes up in smoke.
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Next time you foul a plug just pull the connector to that plug away from the dist cap just enough to make the spark to jump a gap. It will clear a plug in seconds. This is because when a spark jumps a gap it intensifies and burns hotter. Try it next time, works great. and easier.
 
I am surprise that no one came up with a bad condenser as a main reason that points will keep burning out.
 
James Rogers said:
Next time you foul a plug just pull the connector to that plug away from the dist cap just enough to make the spark to jump a gap. It will clear a plug in seconds. This is because when a spark jumps a gap it intensifies and burns hotter. Try it next time, works great. and easier.
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Great tip! I would also recommend that you have somebody else pull the connector! ;)
 
edzaha said:
I am surprise that no one came up with a bad condenser as a main reason that points will keep burning out.
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Yes, I am too.
For some reason I thought it was mentioned early on, but I just went back through the replies and see no mention of the condenser.
 
mngreen said:
What happens if a generic ballast resistor is installed in a 12V ignition circuit with a coil that already has an internal ballast? What would be the effect(s) on the running of the engine? Would it even run? What about the condenser?
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I'm not real sure but I think I'm finding out. When I got my A it was 12v with a Pertronix module and no ballast resistor. I switched it over to a Mallory recently, added a ballast and figured all was good. Well now I have what I thought was a fouled plug, since each hole I put it in stops firing. I am starting to suspect that the plug was just the weakest one and that the ballast that I added is limiting the current flow to such a point that the spark is failing. Logic, and Pertronix instructions, tells me that I need a ballast so the module doesn't fail. Since there wasn't a ballast when I got the car methinks the coil has an internal ballast. I tried to ID the coil to no avail. If I shake it I can hear fluid inside, it has "AJA" stamped on top and a "1" by the - post and a "15" by the + post and a "4" at the 12 oclock position. I think it may be foreign, like Bosch or such, any clues?
 
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