My Tractor Forum banner

1 - 5 of 5 Posts

·
Premium Member
Joined
·
4,870 Posts
Discussion Starter #1
Per other thread, I rebuild a mag today to get one of my Ls running. I took one of my spares and put points and condensor in it, bench-tested, got nothing. Ok, fine. Took a coil out of another one, swapped that in, now got a nice hot spark.

No obvious damage to the dead one, and I got out my little cheesy ohmmeter and did a couple of tests, couldn't find any obvious difference. Do I need a better ohmmeter? Perhaps the only real way to test these things, at least without better equipment, is to put them in a mag and try it?
 

·
Premium Member
Joined
·
8,842 Posts
It is difficult to detect a shorted turn in a coil without an equipped laboratory. Substitution is the cheapest best test.
 

·
Premium Member
Joined
·
3,028 Posts
Between the magneto aluminum case and the button on a mounted coil or the socket in the cover you should measure 5000 to 7000 ohms. That is the resistance of the secondary windings. And that reading should be the same between the tip of a connected spark plug and the magneto case. You may have to scrape the spark plug electrode to get a reading. An open reading on the coils secondary windings is a bad coil.

Measuring the isolated primary winding, off the points, to the case will read less than 1 ohm.

Roger,
 

·
Registered
Joined
·
795 Posts
Rich is right, had a mag drive me nuts years ago only to find a bad coil. Unit would run for 15 minutes and shut down. Got new coil and worked great. If you can find a old time electrical repair shop with a mag tester it really helps also. C.V.
 

·
Premium Member
Joined
·
8,842 Posts
A single internally shorted turn will render a coil useless yet all of the resistance checks will show good. A shorted turn can be detected electrically by measuring the inductance and resonant frequency and other characteristics of the coil, but it is quicker and cheaper to just try it.
 
1 - 5 of 5 Posts
Top