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Repairing / Rewinding coil

Discussion in 'Technical and Troubleshooting Torque' started by mattb, Jan 2, 2008.

  1. I've been reading up a bit on mc ignition systems (I have the electrical know-how of your average intelligent 19th century gentleman). Anyway, I see that people rewind their own coils, and that you can have the coil rewound for you (usually around $150 - $200). Google has failed me; can somebody explain the process, both in theoretcial and practical terms. Eg the coil on my bike seems an enclosed unit, does one cut away to get at the coil, or what? I get the impression that it's a great length that one must coil? Is there a proper way to coil it? Is there more than one complex of coil within there?

    (My bike failed to produce spark two days before I went on holiday (well, only *very* intermittent weak spark), have purchased an ohmeter for when I get back to it - maybe this is the problem, maybe not, but I'd like to understand this anyway).

  2. Hi Matt

    Yes, there is a primary and a secondary winding, the ratio of the number of turns on the primary to the number of turns on the secondary is what provides the step up in voltage from 12 volts to many thousands of volts.

    The primary is thicker wire and fewer turns, the secondary has thiner wire and many more turns. Motorcycle coils are usually potted in epoxy resin to provide electrical insulation and resistance to water. I do not think you will have much luck cutting one open and rewinding it.

    Often the fault can be the transistor ignition unit rather than the coil itself. As you are getting some spark I would suspect the coil is intact and the transistor ignition unit is a possible culprit. If it is the SR500 that has failed I suggest you contact the SR500 club they have a lot of contacts for this kind of repair. www.sr500club.org
  3. Wow. Great reply Chris. Well done. This is what Netrider is all about.
  4. Are you sure the stuff about rewinding wasn't about magnetos or alternator stators?

    Ignition coils are cheap(ish :grin: ) and modern ones are pretty much permanently sealed. I'd think it pretty unlikely that anyone would bother to repair one. Vintage units could be another matter (originality considerations andso forth).

    My grandfather used to wind his own electric motors, magnetos and dynamos. He made up a gizmo to do it on his Myford lathe that worked similarly to a bobbin winder on an old sewing machine. Had an oscillating arm that swept the length of the core, laying the wire down in even layers. It was a terrifically intricate thing which could be adjusted for all sorts of different coil shapes/wire sizes. I suspect that a pro would have something similar.

    One problem with winding ignition coils would be the fragility of the wire used for the secondary winding. It's generally not much thicker than a human hair. A bit too much tension and it's knackered. Similarly, it's necessary to be very careful of the insulating coating of the wire as it's vital that each turn of the coil is insulated from the next.

    It's all quite an interesting subject in itself (if you're a sad git like me without a life :grin: ). If you're really interested in the theory and practice, I may be able to recommend further reading. If you just want to start your SR, I'd second cjvfr's suggestion.
  5. You're right there, Rourkster - thanks as always guys!

    My knowledge is these waters is absolutely fishy, but am I right in thinking that what you are talking about is part of a points ignition system, or at least, not part of an electronic ignition system (the latter which the SR500 has and which distinguishes it from its parent the XT500). If I'm wrong, are you talking about something within the flywheel area?

    Thanks, Matt.
  6. Yep, I am not sure on the SR500 but if it has an electronic ignition system then the points are used to fire the Electronic Ignition unit unit into the coil, the points may not be a mechanical switch but a position sensor instead. In a traditional Kettering ignition system i.e. pre electronic ignition, the points are a mechanical set of contacts and switch the power directly to the coil.
  7. Electronic ignition systems still use a coil to produce the spark. These coils are not rewound since they are, essentially, a "universal" item. While they may appear to be peculiar to one system or another, in practise many can be readily swapped around between different models.

    Frequently the same coil can be used between points and electronic ignition systems, without any problems.

    It is the flywheel/alternator/crankshaft mounted units which trigger the spark and/or provide the power for such systems to run without any battery, which are rewound.

    All the best

    Trevor G

    PS Digital multimeters are not the greatest for fault finding - because of their high (4-10 megohm) input impedance they can show a correct voltage across two terminals when, because of a poor connection, very little current will flow.

    A continuity lamp is frequently a much better device, since like the circuit being tested, it draws some current.
  8. But sometimes not. If I had $5 for everyone who I had to punch in the head for telling me CB250N coils (black box system) would work on my CB400F, I'd have....erm....$15. The N coils had a ridiculously low primary resistance. About 10% of that of F coils. Result was, if used on the F, fuses blew or wires started smoking when the points closed
    :shock: .

    Beware the advice of strangers.

    I've had that problem with analogue meters too. Volts everywhere ther should be and zero resistance tested by ohmmeter, but nothing when a load is applied
    :evil: .

  9. Hey - I thought I was becoming more familiar now... ;-)

    With an electronics background I wondered why so few mechanics or auto leckies used MMs, until I got caught the first time! Then the test light made much more sense. In the overwhelming majority of intermittents or "no goes" the test light rulz...


  10. Nothing personal Trevor, just a general observation that can be applied to many areas of life :wink: .
  11. Hoho - I thort my ;-) would do tha trick...