Highly technical question about ignition coils
 

Gents, I am working on a 53 Allis-Chalmers Model G with positive ground and I can't get spark. I know it's not a cub but lets pretend it's an old Farmall. My question is about 6 volt ignition coils. I'm trying to determine if it matters which side connects to battery and which side connects to points. From the searches I've done, I read that I'll get spark if it's connected either way but I'll get better spark if it's connected correctly.

1. Are coils made differently for positive ground systems?
2. Does the minus terminal always go to points, or always go to negative DC power?
3. Does it matter which way + and - are connected?

As an aside,there is some misinformation out there about how coils work and this may fire up a contentious debate, but here goes. One misunderstood fact is that coils generally don't have a ground. When the points open and the magnetic field collapses around the secondary winding a current is induced to create the spark. Since most coils don't have a grounded case the current flows from ground through the battery, through the ignition wire to a coil terminal (plus or minus) through the secondary winding, out the high voltage spark terminal, to the distributor, through the rotor to a spark plug wire, through the spark plug and across the gap finally back to ground.

Thanks for taking the time to address my question.

Since it's positive ground the + side will go to the points and the - side gets hooked to the hot wire coming from the key. Most 6v coils I've dealt with required an external resister and they do go bad, we run into that issue on the Ford's because nobody ever thinks to replace it.

Thanks for the info.

I've read that the 6 volt coils are usually 1.5 ohms and only need the resistor if used in a 12 volt system. Coils for 12 volt systems are about 3 ohms and aren't good for 6 volt systems.

Let me know if that's wrong, but it makes sense to me.

I can't give you any technical data but all the old Ford's were 6v and had an external resister.

The condenser is the trigger part , also new points with better connection lets more current to the coil magnetic field and cond.


A quick test is , stick your tongue to a coil wire and crank it over ....ha ha

Points will work without a condenser however they will burn out from arcing without a good condenser. I will replace the points if I can't clean them up.

That's the dumbest idea ever! How will I reach the key with my tongue on the coil?

I forgot to mention that the distributor wasn't locked down, therefore, I suspect that the points didn't have a good ground. I plan to add a ground wire and report back.

I'll take some pics as well. This pic is similar to the restored model I'm working on.

Clyde keeps his Model G out at Rough and Tumble. If I can get to it ( it's covered for the winter ), I'll get some pictures and try to trace some coil wiring next time I'm out.

That would be great. I do have the service manual with a schematic but the coil polarity isn't legible.

Web search this to get a better explanation and maybe some pics or a video.

You can check coil polarity by holding a sharpened pencil between the spark plug and the plug wire; spark will jump wire to pencil to plug. You want a clean spark between the wire and pencil, a flared spark between pencil and plug. If it goes the other way then the polarity is wrong.

I think Tim answered this in posts 2 and 4 but there still seems to be some question of how it works. All positive ground systems work the same regardless of voltage. There are of course different voltage coils and ones with internal and without internal resistors. Most 6v I have seen use external.

Anyway. Here is a basic positive ground diagram. Hope it helps