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Thread: Fairbanks Morse magneto coil

  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
    80

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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.

  7. #7

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    Quote Originally Posted by skipsoldbikes View Post
    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.
    I hardly consider in 2019 a magneto used 59-69 as modern but it's all semantics. Fairbanks mags used by HD for the sportsters and the XR models have coils that will probably out live us all

  8. #8

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    Hi Tin,
    I think you may have mis-spoke causing the confusion. Magnetos on antique motorcycles ( or any magneto for that matter) typically have a Five elements :
    1) A magnet, that may be either stationary or rotating.
    2) A double-wound wire coil, where current generation and amplification occurs. A primary winding, consisting of a few turns of comparatively coarse wire, and a
    secondary winding, consisting of many turns of fine wire, are wound around a soft iron laminated core.
    3) A breaker (also called interrupter or points), which interrupts current flowing through the primary circuit, causing the magnetic field within the coil core to
    collapse, thereby inducing a high voltage into the secondary winding.
    4) A capacitor (also called a condenser), connected across the breaker contacts, helps prevent the contacts for arcing.
    5) A distributor, which routes the high-tension current to the appropriate spark plug.

    in this conversation, lets concentrate on #2 in the list, the coil. The double-wound wire coil, contains a primary winding, has a few turns of comparatively coarse wire, which seldom failures due to its bigger gauge. Then we have the secondary winding, which has many turns of fine wire, over a mile of literally hair thin wire wound around a soft iron laminated core. It is this hir thin secondary wire that is the number one cause of magneto failure in most old magnetos.

    Keep in mind we are talking about copper wire that is only insulated with some shellac, and over time this shellac breaks down causing internal shorting. Another common cause of failure is the adhesive used to glue the coil together was vacuum sealed originally, over time decomposes & allows the coils to move when energised, causing the shellac to wear off the wires, and again causing the coil to short internally. A third & also very common cause of failure on magneto coils is moisture, now this is very important to this thread. Nearly all magneto coils when wound were coated with insulating varnish and put through a vacuum/pressure impregnation process to keep the wires from moving when energised, then covered with a special egyptian cotton coil winding cloth, then recoated & vacuum sealed again. This works great, but on a 50-75 year old coil, this material wasn't as good as todays materials, and it beaks down allowing moisture to get into the hair thin wires making them a solid bunch of green goo. this is by far the most common cause of magneto failure I see in my shop, with bad condensers a close second. Every single magneto shop in the country will tell you this same thing, it is a fact.

    Where I think your comment about coils are seldom bad may have come from is with the MODERN coil replacements made by Joe Hunt, and Morris. The MODERN replacement coils are completely impregnated in a hard plastic shell, which is impervious to moisture & coil movement. This is a good thing, but the original Fairbanks Morse coils were not made that way and failure was common. I have personally taken apart several FB mags that the coil coating was melted off laying in the bottom of the mag, so I feel you and I are talking about two entirely different items, I am taking about all magnetos, you are talking about a MODERN replacement coil that will fit an old magneto.
    As a side note, Many magnetos have a fixed magnet & a rotating coil, while some like the FB mag in this discussion hace a rotating magnet, and a fixed coil. The theory of how the both work is the same. I am including a link to my friends website in the UK who also rebuilds magnetos & offers an excellent view on how he winds his magneto coils, thought you might enjoy it: https://www.themagnetoguys.co.uk/Rew...-Magneto-Coils

    Also, I am including a good link on rebuilding a Faibanks Morse magneto. It is a mag for a tractor, but 90% of the rebuild is the same. You will see a discussion about the impulse coupling which you can ignore. Enjoy !! https://www.youtube.com/watch?v=2fmnP700qXc
    Last edited by skipsoldbikes; 10-03-2019 at 11:28 AM.

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