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J20 Ditch Witch ignition coil

M

mudmuckgrease

Member
I keep blowing out this J20's ignition coils. This time around, the cap on the coil was almost lifted off, so I am back ordering a replacement. I am wondering if adding a ballast resistor would alleviate this problem. The trencher has an old Wisconsin air-cooled Engine, and finding original electrical parts for it is not possible. Since it has what appears to be an old Ford transmission, I think I'll try to order a Forn 9N coil. I read somewhere that a ballast resistor will prolong the life of breaker points and I am wondering if it will also help the coil as well.
 
It seems that the nearly world standard is for about 4 amperes steady state ON current. I would measure that and adjust with added resistance as needed to achieve 4A.
 
Thanks for the reply, JMOR. Is the STEADY STATE ON CURRENT with engine cranking, or just with the ignition key on? I assume the measurement will be between the negative coil terminal and the central output terminal...?
I can measure the amps through the central post, but I still need to measure voltage to get the required resistance. I don't have the ability to measure the voltage running through the central post as those numbers, I imagine, are pretty high. Or am I looking at this all wrong: Should I measure the volts and amps at the positive side of the coil's circuit (wire from the positive battery to a positive terminal of the coil?
 
Key on. points closed and ammeter in series with either small/primary terminal and wire feeding such terminal.
 
I have a hard time believing the coil blew it's top while the engine was running?

Was that what happened?

More commonly, coils overheat and melt down or "blow their top" when the engine stalls or is otherwise not running and the ignition system is still powered up.
 
Thanks for your help. I just found a YOUTUBE explanation of the Ballast resistor: https://www.youtube.com/watch?v=PCs5gAZBbxU
It seems it will help protect this machine's coil.
 
To check the amp draw, remove either the + or the - wire from the coil, connect the amp meter leads in series from the coil terminal and the removed wire.

Turn the ignition on, if no reading, hand turn the engine until the points close.

Take the reading quickly to prevent the coil overheating.

Another test is to connect an ohm meter across the + and - terminal, out of circuit.

If the reading is around 1.5 ohms, the coil will need a 1.5 ohm resistor.

If the reading is around 3 ohms, it will work without a resistor.
 
Yes, that seemed strange to me - I wasn't running it when it blew its coil. I finished burying a water line, drove it to where I park it, turned the key off, and walked away. About a month later, I returned, and tried to start it only to find that I had a blown coil ? again! When I tried to start it, the battery was fully charged, so I know I left it with the key fully off:
mvphoto15641.png

It must have blown when I was trying to start it. This is coil number three, and when I installed it, new, I measured the ohms and saved that info in a file. Between the primary's positive and negative terminals, the resistance was 1.8 ohms. So, according to Steve@Advance, it should have had a resistor in its circuit. When I receive the new coil, I'll measure it and act according to its readings.
?Live and Learn!?
Thanks, folks, for all your help.
 
I'm in JMOR's camp on this one. Typically (NOT always) a coil distributor points ignition system limits the current the points must switch to around 4 amps.

THEREFORE on a 6 volt tractor a typical unballasted coils LV primary resistance is around 1.5 ohms as 6/1.5 = 4 amps
on a 12 volt tractor a typical unballasted coils LV primary resistance is around 3 ohms as 12/3 = 4 amps

Of course you can still use a 6 volt coil on a 12 volt tractor IFFFFFFFF you add an external series voltage dropping (12 down to 6) ballast resistor.

Ifffffffff you pass too much more then 4 amps the coil and/or the points can overheat and fail prematurely.

With a coil having 1.8 ohms if its a 12 volt tractor you need to add some series ballast (around 1.8 ohms for a 50/50 12/6 voltage divider) or the coil will overheat. Or else just get a full true 12 volt rated coil (maybe 2.7 to 3.5 or so ohms range) no ballast required........

John T
 
This machine has always used 12 volts, and that is the kind of coil I'm after. The online offers never show the internal resistance, so I will have to wait until I get my hands on it to see what I have before I do anything with its circuits. I'll work toward a 4 amp system. The battery sits idle at 12.7 volts, so 12.7/4 = 3.175 ...so if I get another 1.8-ohm coil, I will need another 1.375 (or thereabouts) ballast resistor. Hope I understand this correctly - time will tell.
 
You're on the right track for sure. If you wanted perhaps a 50/50 12/6 voltage divider so a 6 volt coil sees the 6 or so volts it was designed for and the other 6 or so is dropped across the ballast resistor THE BALLAST WOULD BE THE SAME OHMS AS THE COIL. Of course under charge a battery voltage rises so you can take that into account. A coil is designed to operate fine and not overheat at slightly elevated charging voltages.

A full true so called 12 volt coil has an LV primary winding resistance in the neighborhood of 2.5 to 3.5 or so ohms. Many 3
A so called 6 volt coil has around 1.25 to typically under 2 ohms. Many 1.5
Many "old farm tractors" used so called 6 volt coils on 12 volt tractors with the external series voltage dropping (12 to 6)
ballast resistor.
WARNING a coil labeled "12 volts requires (or use with) a ballast resistor" is more like a 6 volt coil !!!

John T
 
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