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Thread: Fairbanks Morse magneto coil

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  1. #1

    Default Fairbanks Morse magneto coil

    Is there a way to test coils with a multi meter? The manual states to switch with a known working coil.

  2. #2
    Join Date
    Mar 2008
    Location
    New Jersey
    Posts
    81

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    A magneto coil is nothing more than a coil of insulated wire. The suggested test procedure is what is called a comparative test. So if you take a good magneto coil and test it and it reads 5.7 ohms it would be expected your coil should read about the same. I am a bit surprised the manual does not give you a value. Keep in mind this is only a continuity test which at best is not very good. Under load ,in service, that coil could open and be no good. The insulation on the coil could also be faulty causing it to be shorted to ground. If the insulation has failed and allows it to short to ground you might detect it by connecting your ohm meter from one or the other of the leads to the body of the mag, it should read infinity, no continuity. A better way of testing for a grounded coil is with a Megger but few people have one of these. If the insulation has failed, not to ground, the coil of wire will still read a resistance value but it would have shorted turns and the resistance would be lower than the known good coil. Maybe you could find the information for the coil resistance on line, if not I would bet Morris Magneto's in NJ would help you out. Always keep in mind resistance tests on generators, alternators, starters, mags etc. are at best not definitive IMHO.

  3. #3

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    Mag coils are seldom bad. I probably have three or four I saved because someone needlessly replaced them

  4. #4

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    nearly every magneto coil I test is bad, and I test every one that comes into my shop. Most are either "open" or have a resistance that is way out of spec due to moisture encroaching into the copper coils which are hair thin. NEVER re-use a 50 plus year old coil. Oh, and they need to be tested hot, not at room temperature, thats when most of the problems occur.

  5. #5

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    Quote Originally Posted by skipsoldbikes View Post
    nearly every magneto coil I test is bad, and I test every one that comes into my shop. Most are either "open" or have a resistance that is way out of spec due to moisture encroaching into the copper coils which are hair thin. NEVER re-use a 50 plus year old coil. Oh, and they need to be tested hot, not at room temperature, thats when most of the problems occur.
    Funny Dr. Dick who was a founding member of Morse magnetos says send him all the old coils you have removed. He like me has discovered they are seldom bad.

  6. #6

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    with respect to you and him, I am talking about magnetos in general & old magnetos. He was referring to modern morse magnetos. I work on Bosch, Dixie, Lucas, BTH, etc, and I see 40 plus a month in my shop, so I am positive I see more then Mr Dick does, and for sure more brands. If you think about it, and where these mags are stored, moisture is the enemy & 99 percent fail on my tester, which tests "in use" at operating temperature. Every single magneto shop I am aware of that works on vintage magnetos says the same & you cant beat experience. Now if you want to talk about only late model Morse, or Hunt mags, well that's another kettle of fish. If you wish, I can show you many green corroded magneto armature bobbins that are junk.

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