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modifying a J 4 magneto to use an external battery

E

e. chrisjensen

New User
Has anybody tried to modify a J 4 magneto on a farmall cub to use a battery? Seems that one could use a battery, external coil, possibly change the condenser to an external condenser compatible with the 6 or 12 volt coil and disconnect the coil inside the magneto? Wanted to try this as a temporary fix.
I did this on a small Tecumseh motor that had a magneto ignition under the flywheel. Used an external coil with internal resistance, external condenser, and disconnected the magneto coil which was broken.
 
You sure can but what for. The mag is a foolprof system iv got mags that have been running for more yrs than i can count.
 
I agree with you regarding modifying the J 4 mag to use a battery. I've been helping a friend with his cub and I installed new points, and condenser and a new coil is needed. I was just trying to help a friend do something with limited expense.
I changed the ignition unit on my farmall cut to solid state and added an alternator.
Thank you for the reply: You might be interested in my website: http://www.rockisland.com/~ejensen/
 
I agree with you regarding modifying the J 4 mag to use a battery. I've been helping a friend with his cub and I installed new points, and condenser and a new coil is needed. I was just trying to help a friend do something with limited expense.
I changed the ignition unit on my farmall cut to solid state and added an alternator.
Thank you for the reply: You might be interested in my website: http://www.rockisland.com/~ejensen/
 

Okay, I just priced a coil. This site shows $43.25. Aldermans wants almost $100. Ouch!

The mag's not working, obviously, and this is a way to get the tractor running until the mag can be fixed properly. Maybe it's not "right" but sometimes you gotta do what you gotta do, just like back in the day when farmers made this exact same repair to get the tractor back in the field quickly. Mags do wear out.

My understanding is that the external coil modification is fairly simple. Battery source to coil, spark plug wire from coil to center post on magneto distributor cap.

The only thing I'm not sure of is the condensor connection. I don't think the old farmers who used to fix their tractors like this concerned themselves with having the "right" condensor. How they wired it up, though, is the question.
 
I really appreciate your reply. The idea is
to get the tractor running until the mag is repaired properly. I would disconnect the condenser wire and coil wire inside the distributor and use only the points to make and break the circuit. A condenser would be connected to the side of the coil -negative- that would attach to the terminal on the distributor. A switch between the positive side of the coil and the battery to be able to turn power on and off. Make up a coil wire from coil top terminal to the distributor center terminal . I have used this set up on smaller air cooled gasoline engines to make things work.
 
Here's a pic of that setup on an H4 mag, your J4 should be similar. You need to disconnect the wire from the mag's coil to the points. The grounding wire needs to be disconnected from the mag too. You will need an ignition switch with battery voltage fed to the switch and from the switch to your auto type coil. Then from the coil to the points. Hal
2z3zbpv.jpg
 
If the long term intention is to repair the mageto, remove the original coil when you do the temporary conversion. If you leave it in there, it will need to be replaced, whether it needs it now or not.
 
Thank you for sharing the picture. I just finished installing a 12 volt coil with internal resistance, attached a condenser, and disconnected the coil and the condenser in side the mag. Installed a switch and it works great. His tractor has a 12 volt alternator on it so it is well set up for this repair.
 
NO NEED to REMOVE the mag's internal coil. Just disconnect the primary lead that goes to the points and condensor.
 
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