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Ignition coils off other bikes

Discussion in 'The Garage' started by xs650newb, Dec 28, 2012.

  1. xs650newb

    xs650newb XS650 Addict

    Hey everyone,

    I have a 1980 with TCI ignition. My coil is bad and I need a temporary replacement ASAP.

    I found a local guy with a bunch of used ignition coils off of old bikes. He will sell me one for real cheap.

    What do I need to look up to know if the coil will work on an XS650? It needs to be dual output, I know that. Is my only other concern the primary resistance value? What about secondary resistance? What type of range is allowable with a TCI ignition?

    Also, my spark plug caps are each approximately 8,700 ohms. Is this too high? I thought they should be ~5,000 ohms.

  2. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    The primary resistance is normally 2.5 ohms for a TCI. However, any dual output coil with 2.5 to 5 ohms should work for you. As you go higher from 2.5 towards 4 or 5 ohms, you will have less current flow in the primary and the result will be weaker spark.

    Again, different secondary windings will produce different high voltage levels. The only way to know would be to try the coil on the bike.

    The caps are normally around 5K, and as they age they increase in value. I'd say there is not an exact number where the caps prevent spark, but as the resistance increases, the spark would becaome weaker.
  3. if non-TCI xs650 coils will work I happen to have a pair of spares sitting around....
  4. xs650newb

    xs650newb XS650 Addict

    Thank you for the offer. I believe points ignition coils have too low of a resistance for a TCI bike.

    Thanks again.

  5. hmusket

    hmusket CBMMA Member

    I put a coil from a DOHC (after 1979), Honda CB750 on the '82 I was riding this summer. I didn't make a special bracket, but had to mount it with the spark plug wires facing forward, and then curving them back. One advantage was being able to put new spark plug wires on, the disadvantage was the fact there is a positive and negative terminal on the coil. The first time I had it wired, I beleive it fried my rectifier, I reversed the wiring and didn't have a further problems.

    People also use Harley Davidson coils. Do a search for that on the forum for more info.
  6. pamcopete

    pamcopete Ride.Enjoy.Life is Simple

    The TCI manages the coil current and dwell time and it measures the current in the coil to do this, so the primary resistance should not be greater than 2.5 Ohms so that the TCI box of tricks can do its job.
  7. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    Actually coils used with points need to be greater than 4 ohms to restrict the current that otherwise would burn the points.

    A TCI black box can fire a TCI coil of 2.5 ohms and can also fire points type coils of > 4 ohms.
  8. xs650newb

    xs650newb XS650 Addict

    Well now it seems that rg and pamcopete are giving conflicting pieces of information.

    If I understand correctly, rg says that a coil with a primary resistance between 2.5 and 5 ohms is allowable.

    But pamcopete says the resistance should NOT exceed 2.5 ohms.

    Can someone clarify this?
  9. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    Pamcopete is our resident ignition expert, so please take his advice. As I mentioned before, TCI is designed to use a 2.5 ohm primary coil, but if all you have available is a 4 ohm coil, it should provide spark , but it will be a weaker spark than using the proper 2.5 ohm coil.
  10. xjwmx

    xjwmx It's just the unknown. Top Contributor

    There is a dual coil that Honda uses on lots of its bikes. You can get it used very cheaply, and it's made in Japan, not China. It's stamped MP08 and it's about 7/8" shorter than a stock coil. The resistance of the primary is 3 Ohms and it works fine. It's less than half the price of Mike's cheapest dual coil. And it's OEM.
  11. XS1961

    XS1961 XS addict!

    Ive got a Honda CB900 coil on my TCI 650. £10 used..Works perfectly
  12. essexboydave

    essexboydave XS650 Enthusiast

    Seeing as I've got one of the biggest suppliers of old Honda stuff in all of Europe just a short drive from me that's a bit of very useful information!

    Cheers :thumbsup:
  13. This is a cb900 coil , this is the type of coil that will work for tic only , or will it work with points setup to ?

    Attached Files:

  14. xs650newb

    xs650newb XS650 Addict

    Ok thanks everyone.

    I am hopefully picking up a coil from a different bike today. When I know if it works I'll post the year and model of the donor bike so others will know.

    Final question, some coils have a + and - terminal. How does that connect to TCI? If I recall, the stock coil has a red wire going through kill switch to battery and an orange wire going to TCI. Which is considered + and -?

  15. bpeckm

    bpeckm XS650 Enthusiast

    The red/white wire is the +, the orange is the signal wire from the TCI box. I have heard that it doesn't matter which is which, but since the two wires have plugs that are good-one-way-only, I always respect that!

  16. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    There is no polarity with a TCI ignition coil (or with a points coil). It does not matter which direction the current flows through the coil primary.
  17. hmusket

    hmusket CBMMA Member

    From my post above
    I don't remember how I originally wired it.
    One thing to do is wire as bpeckm suggests, put lower amperage fuses in, and monitor your reg/rec unit by touch, it will get a bit warm, but not to the point where you hand is uncomfortable. Of course, it could have just been a coincidence and my rec/reg was ready to go out.
  18. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    The direction that current flowed through the ignition coil, did not fry your rec/reg. Coincidence is the key word.
  19. I have used stock Harley coils and i may have a coil if what you get doesn't work. I think i have the same coil in the picture above.
  20. xjwmx

    xjwmx It's just the unknown. Top Contributor

    I and several others have been using a Honda "MP08" stamped coil with a 3 ohm primary for a long time with no issues. It is plentiful and inexpensive and does not make anybody any money.

    There are no precision resistors in the TCI, which says the 1/2 ohm difference completely insignificant. Not only are there no precision resistors, but all the components on the board have 30 years of aging and drift. But I defy anybody to be able to tell that from the operation of a TCI. ;)

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