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Understanding Ignition Coils

Discussion in 'Technical and Troubleshooting Torque' started by mattb, Jan 21, 2008.

  1. Hey All.

    I asked a specific question re my coil recently, and got some great replies. Just filling in another gap in my knowledge, and this info might be useful for others too:

    There are universal ignition coils about, but at the same time you can't just grab any coil from any bike and pop it in yours. What differentiates the coils? Can there be as many types of coils as bikes, or is there a general set of types, such that if you want to buy a universal coil, you've just got to order the right type which your bike uses (just like you would for a spark plug)? Following from that, how does one determine what type they need?

  2. I'm not qualified to comment on new bikes, but I'll offer what I observed when running bikes from the points/electronic changeover era (late '70s, early '80s bikes, owned by me in the early '90s).

    Coils seemed to come in two flavours physically (single or dual HT output) and two electrically (high primary resistance or low primary resistance), making a total of four basic varieties in all.

    Single output units were used on singles and some two-stroke twins (maybe all), each firing a single plug. Dunno what most triples had as I never owned one, although I am aware that BMW K75s had three single output coils.

    Dual output coils were fitted to parallel twins, firing both plugs simultaneously, and to fours, each coil firing a pair of cylinders.

    Electrically, my own experience suggested that coils fitted to bikes with points ignition had a higher (~10x) primary resistance than those for electronic ignition bikes. Exact figures may have varied between brands but the general rule when swapping coils was to keep electronic compatible coils with eelectroic ignition and vice versa.

    Putting a low res coil on a points ignition bike created a big short circuit when the points closed, resulting in a blown fuse, a burnt out coil or a fire. I assume that electronic ignition black boxes had some form of current limiter to avoid this.

    As I said, there may have been detail variations but, amongst bikes from this era, those four seemed to be the only ones to worry about. If, like me, you were poor, coil choice was generally dictated by what was available within budget. Beyond gross differences there was no scope to be fussy. Such coil failures as I experienced were mechanical (terminals snapping off due to corrosion or fatigue usually) rather than electrical.

    When coil hunting, my technique for determining suitability was to check the primary resistance with an ohmmeter and then work out the potential primary current with closed points using Voltage = Current x Resistance.

    I can't remember the figures that I deemed acceptable. Probably about 1 Amp max. All the bikes I had to find coils for were points ignited so I didn't have to worry too much about fussy black boxes.

    I've also just remembered that at least one reputable aftermarket coil manufacturer only offered two varieties of coil to cover electronic and non electronic ignition bikes, so it appears that they hadn't found too many variations either.
  3. Coil resistance is usually included in the workshop manual specs for any bike I have ever looked at.

    Changing from standard does not provide a performance improvement by itself.

    The MG 650 twins use a coil with around 3 to 4 ohms resistance, which can mean up to 4A flowing through the coils.

    I think you will find that is a much more common figure for non-electronic ignition. It probably is not uncommon for electronic IGs either. For instance the primaries on a Honda Spada are 2.6 to 3.2 ohm. That could mean a current close to 5A.

    Cars used to use a ballast resistor in series with the coils, to drop the running voltage down to about 8 volts. The resistor is bypassed for starting to give the full 12v, because in reality, under cranking conditions, the coils will frequently only see around 8-10v at best.

    Why are you asking?


    Trevor G

    PS Current = voltage divided by resistance.
  4. Thanks Pat and Trevor, some helpful info to get my head around.

    Both out of a simple desire to understand it - I really enjoy doing my own mechanical work on both mine and others' bikes - and especially because I want to renew the ignition system on my SR500, which still runs all the original 1978 parts (when you've got to kick-start a 500cc single, good ignition counts!). For a start, that means a new CDI (unlike the XT ignition, the SR is electronic), and a new coil. Trouble is, I'm on quite a tight budget, meaning it will take months to replace the dozen parts I need to renew on the bike (swing-arm bearings, chain and sprockets, oil-line mod, etc etc). I've located a good CDI (Hyperpak, $200 on ebay), but I think I should try to replace the coil first. The OEM coil is $190. Here are some ebay examples (item numbers) I found that might fit the bill, at a fraction of the cost:

    Here are the SR coil specs http://www.siue.edu/~rsutton/SR/78-79.htm#Ignition coil
  5. That looks like what I referred to earlier as a low resistance coil. If you put that on a points ignition bike, you'd get a current of about 12A through it when the points closed. Way too high.

    However, the good news is, I can't see any reason why any single output coil with about 1 Ohm primary resistance shouldn't work. That includes car coils.

    If you're interested, somewhere around I've got a (brand new) replacement coil for an EFi XF Falcon. It's quite a compact single output unit designed for an electronic system. I'll stick a multimeter on it when I get home. If it's about 1 Ohm, you can have it for the cost of postage.

    It's no good to me. The only reason I'd buy another Falc would be to put it in a crusher :evil: .
  6. That'd be great! :grin: Thanks!
  7. Well, I went and dug around in the shed and found the Falcon coil. I thought it was unused but it looks like I may have fitted it to the car in course of my increasingly desperate troubleshooting efforts (long story, coil not the problem). Wouldn't have been in use for more than a few hundred kms though.

    Anyway, it's a Bosch, with a primary resistance of 0.9 Ohm (according to my cheapy Dick Smith multimeter) which should be compatible with your ignition unit as this is what the black box "sees" and the spec you linked to says 0.98 +-20%.

    Secondary resistance is a little low at 8k Ohms but this is less critical as it doesn't interact with any expensive electronics.

    PM me your details and I'll get it into a Jiffy bag for you.