HenryO

Member
After watching a video on Steiner titled "rebuilding a magneto" I am still not sure of what goes wrong with them. My tractor would run good for about an hour and then just quit. I replaced the Mag with a 6 volt coil, did the wiring and it runs ok. Now what went wrong in the Mag. Is it the coil or something else. What to look for. I would like to repair this mag if possible and learn something. Thanks, Henry
 
When you say "I replaced the Mag with a 6 volt coil" do you mean you added an external regular can coil and fed it via an ignition switch and used the mags points???? If so the condenser may be the problem. The most common cause of mags not firing is the points followed by condensors and only then the coils

Make sure the points aren't burned or pitted and gapped correct and if only gray oxide coated run a dollar bill through them and see what happens.

You say 6 volt coil, is it a 6 volt tractor???

John T
 
Yes irs a 6 tractor and 6 volt coil and I followed yours and olds directions to place an outside coil on it with a 6 volt coil and applicable wiring. Now. I am trying to learn more about magnetos. Since I already replaced the condenser now I am trying to find out about replacing the coil. At a cost of Approximately 70 bucks I am trying to find out if the coil is the likely suspect before I buy one.
 
You can bench test a coil and/or run a simple ohms continuity test on the LV Primary and HV Secondary and they may all pass, HOWEVER its often only under actual high voltage and/or once warmed up a coil fails. She may bench test fine but still fail under actual HV operation.

If you use an ohm meter the 6 volt coils LV primary BETWEEN THE SMALL + and - terminal should be around 1 to 2 ohms, if open its bad. The HV secondary may be in the 5000 to 10000 ohms range and if open its bad.

If you attach a hot wire to the coils LV primary input terminal and use a jumper to attach then remove the other terminal to ground, each time it was connected but then removed the HV tower should throw a good blue spark to ground 1/4 inch away via a coil wire (0r install a plug with threads grounded) from the HV tower to ground. Or have it wired up and use a pencil or insulated object to close and open the points and it should spark EACH TIME THE POINTs ARE Opened.

I don't recall telling you how to change and wire a mag over to an external coil ignition, that must have been Old so I have no idea if its right or how you did it. Basically a wire from an ign switch must send hot voltage to the coil when ON while its other small terminal connects to the points and condensor. On mags a kill switch grounds the mags terminal it does NOT send it hot voltage like on a coil ignition. Sure its wired right??????????????????

Usually its bad points or condenser instead of a bad coil but again I have no idea how it was changed over and wired so I cant guarantee anything at this point.

John T
 
"You can bench test a coil and/or run a simple ohms continuity test on the LV Primary and HV Secondary and they may all pass, HOWEVER its often only under actual high voltage and/or once warmed up a coil fails. She may bench test fine but still fail under actual HV operation."

Yes, this is completely true, especially with dry coils. The insulation breaks down with age, usually shows as cracking, especially the epoxy case coils, the spark goes to ground internally instead of where it's supposed to go.

This story reminds me of a Montgomery Ward moped scooter I had as a kid. It was always difficult to start, fouled the plug, undependable to ride. It went back to Wards many times, they were clueless how to fix it but always managed to charge for their guess work.

As I learned more about it, got to checking the fire, it looked weak even to a 10 year old! I scrounged an old 6 volt coil off of an engine in the scrap pile, wired it on with a coat hanger (pre duct tape) connected the wires...

It fired up quicker and easier than it ever did! Ran like that for years! I remember it had a spark like something out of a Frankenstein movie! Who knows what kind of voltage it was running on, I assumed 6 volt because it had 6 volt lights, no battery, just made it's own power somehow.

I assume this was some kind of magneto ignition, never went into it any deeper, just knew it worked, kept me happy for many hours!
 
If it is a fair banks mag send me an e-mail and I can send you a couple of manuals I have for them in this computer. In most cases running an hour and then dieing is caused by the coil getting hot and then shorts out internally. Either one of the manuals does a lot in explaining how a mag works by the way
 
Being that it is a Fairbanks Morse magneto. I would put my money on the coil being bad over the condenser. You hooked up the magneto to an external 6 volt can coil running off the battery? Is so, Then you have bypassed the original coil and are still using the points and condenser.

Go over to Mcdonaldcarb.com and buy at least a coil and gaskets.I would also get points and condenser. I bet it will work again. Fairbanks mags when they have a good coil will throw a spark that will clear a 3/8 gap.
 
What Can Go Wrong?

I have found the bearing on the armature go out to they would not spark. I have found bad coils. I also found the magnets had lost there magnetic pull and not make electrical energy to cause spark. I have had points that looked fine that would give fits. I have never found a condenser in a magneto bad. It can happen but I just never found one.

A condenser is a simple wire coil with one wire connecting to the points. They can short out and be a ground and you get no spark at all. Or they can open up and not control the high voltage and cause the points to burn to where you get no spark. You still get spark with a open condenser with good points.

The impulse spring can break or the pawls can wear off or stick to where it will not spin the armature to cause spark for start up.

It is again one of those things that takes a clairvoyant to diagnose without taking it apart.
 
If you really hooked up an external can coil the ONLY thing youre using out of the mag is the points and condenser. I am not the one who told you how to wire up an external coil to a mag so have no idea if you have it right or not. Assuming the new coil is okay and you wired it right????????? if theres no spark the points may be bad or burned or pitted or the condensor may be bad or shorted out. You are NOT using the internal mag coil if you wire an external coil and wired it right ?????? so that's NOT the problem. If you wired it right ??? the new can coil gets voltage when an ign switch is on and its output wires to the points inside the mag. On the mag the points connect instead to a kill switch which grounds them out to kill it.

How is it wired???????????????????????????????????????????????

JOhn T
 
Changing the coil is pretty straight forward. The two slotted set screws in the top may need heat to loosen and come out. Make sure you get all the dirt and junk out of the slots. Then warm the aluminum on the sides where the screws are. I have used a propane touch or a hot air gun. Either works. The heat gun took a little longer.
 
Thanks for the advice. Coil arrives Tuesday so from then on I be working on the mag. Would like to see if I am any good at replacing it. Henry
 
So I see you decided NOT to add another 6 volt coil as you indicated but replace the magnetos own internal coil, keeping it original is a good way to go

John T
 

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