Questions about ballast resistor on 12v negative ground conversion.

[Los_Control]

I suck at electrical knowledge .... If I understand things correctly, I can use my 6 volt coil by switching the wires for polarity & adding a ballast resistor.

 

To be honest, I feel more comfortable just buying a new 12 volt coil .... The old 6 volt is just old. With all new electrical going in a new coil makes sense to me.

 

When I order the coil, do I still need a resistor?

Should I order a coil with a built in resistor?

If I need to add a resistor, should it be before the coil? Or between coil & points?

 

Searching around on the internet get a lot of conflicting opinions, The Farmer claims he switched his tractor to 12v 20 years ago & never burned a set of points with no resistor.

Others say you need it ..... I dunno.

[kencombs]

I would suggest a new coil, just because.   If you're using points, a coil from a Mopar of 12v vintage along with a resistor and start bypass wiring would be best.  Yes, one can get a coil designed to run on 12v, and no they don't have an internal resister (with very few exceptions from the 50s) but are wound with wire that provides the desired resistance.   But, if you do that the starting voltage will be lower than a coil intended to be used with a resistor.  The resistor goes in the switch to coil feed wire.

 

Edit to add:   I'll change my recommendation for coil and resistor.  Use 50/60s GM as they are more readily available and work fine.  And back in the day I saw many more Mopar resistors fail than other brands.  Don't know why, just my experience.

Edited May 30, 2023 by kencombs

[Los_Control]

Just now, kencombs said:

I would suggest a new coil, just because.   If you're using points, a coil from a Mopar of 12v vintage along with a resistor and start bypass wiring would be best.  Yes, one can get a coil designed to run on 12v, and no they don't have an internal resister (with very few exceptions from the 50s) but are wound with wire that provides the desired resistance.   But, if you do that the starting voltage will be lower than a coil intended to be used with a resistor.  The resistor goes in the switch to coil feed wire.

 

 

Thanks again Ken, this is what I threw in my cart @ Rockauto   I selected a 1963 Dodge dart for a donor year.

I remember buying a 63 Chrysler for $50 because it had a bad ballast resistor & would not start. .... So I just picked 1963 for a donor.

 

Good explanation about the "so called" internal resistor coil.

I know Mopar ran them for many years, the ballast resistor looks right at home on the firewall to me.

 

 

 

[Plymouthy Adams]

it is my finding that more resistors failed due to the position they were mounted and the sudden cooling when exposed to water when hot and wet hood lifted....I always run a dropping resistor on the older ignitions....and I cannot recall changing a resistor....shield them...you will survive the ride...

[Los_Control]

Thats good advise, I will use that info when selecting where to install it.

The old pilothouse with the butterfly hood is not exactly water tight. ..... They do have a rain gutter over the firewall though to protect the wiring on the firewall.

 

 

[maok]

The purpose of the ballast resistor is to minimize/reduce the points for burning, it steps the voltage down to about 9volts from +12v. Note, it will still work without a ballast but you would need to service/replace your points more regularly. It wasn't required on 6volt systems the voltage was low enough already. 

[maok]

Also, if you purchase a coil with about 3ohm primary resistance then you won't need the ballast. With a ballast, you need a 1.5ohm coil, the above pictured one should be.

Edited May 31, 2023 by maok

[dgrinnan]

I ran my a 6v coil for about 3 years on my converted 12v system. to be honest, I did not know/pay attention to the fact the coil was still 6v.  I found out when the coil failed and I went to replace it.  I would suggest going with the 12v coil.  Yes, you still need a ballast resistor.  Like others suggested, mount it on the firewall.   You will be going through points like crazy if you don't.  It doesn't have to be a make/model specific ballast resistor, if they even make one.  Anyone from any manufacturer will work.  I had one for a Mercury on my 54 Chevrolet that had been converted to 12v.  Ran that way for 3 years before I replace it.  I used a ballast resistor from NAPA. Part # ECH ICR13. Coil Part # MPE IC12SB 

[greg g]

You just want to be aware that some or all gm coils were internally resisted,  so you don't want to use an in line ballast with them as then the coil will be trying to operate on 3 or 4 volts instead of 8 or 9.

[Los_Control]

Just now, dgrinnan said:

I ran my a 6v coil for about 3 years on my converted 12v system.

I'm pretty sure it would work to use 6V coil ... even though I have not done a 12 volt conversion before.

 

What concerns me is switching the polarity on the coil ...... possible this might be a old wives tale.

The flow of electricity goes through the coil creating a pattern ...... would a sine wave be accurate? I dunno.

But the coil gets use to flowing in a certain direction.

So it may be possible when you reverse the flow the coil may act strange. Again I have no idea if this is true or old BS.

 

There was a guy that did this in a 12V conversion & everything worked beautiful & ran great. Except a strange miss at a certain RPM.

He tried everything, rebuild the carburetor, tried a different carb, several attempts going through the points, plugs,cap,wires .... Dog chasing it's tail.

Eventually he tried a different coil & the problem went away. ....... He was convinced reversing the polarity on the coil caused it.

 

Again I have no idea if it was true or not, I'm sure if it was true .... If we tried it on 100 coils, maybe 3 or 4 from the 100 might be a problem? I doubt it is common, but is possible.

I will keep mine around for a spare, would not hesitate to use it if broke down on the side of the road .... no plans on running it.

[Los_Control]

Just now, greg g said:

You just want to be aware that some or all gm coils were internally resisted,  so you don't want to use an in line ballast with them as then the coil will be trying to operate on 3 or 4 volts instead of 8 or 9.

Thanks Greg.

Think I was under the impression that a internally resisted coil was the same as a external .... I understand they are not the same now.

 

 

[kencombs]

3 hours ago, greg g said:

You just want to be aware that some or all gm coils were internally resisted,  so you don't want to use an in line ballast with them as then the coil will be trying to operate on 3 or 4 volts instead of 8 or 9.

What was the usage (model/year) of that type GM coil?  curious

[dgrinnan]

I did not pull a specific model year.  I did a generic search for an ignition coil and picked one that was reasonably priced.  I don't think they are really vehicle specific unless you have some specialty set up in a more modern vehicle.  I used the NAPA brand.   https://www.napaonline.com/en/p/MPEIC12SB?impressionRank=1

 

Same thing for the ballast resistor.  https://www.napaonline.com/en/p/ECHICR13?impressionRank=1

 

I have been running them for 2 years and engine runs great with them.  

 

When I had to replace the original firewall mounted 6v coil for my 47 WC (still 6v) I bought a generic NAPA brand 6v coil.  No issues with it and I have been running it for a year.  If you look on their web page it says it does not fit my vehicle but I think that is because they are trying to match the old style coil that mounts in the firewall.  

 

https://www.napaonline.com/en/p/MPEIC7SB?impressionRank=1

Edited May 31, 2023 by dgrinnan