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OEM coil not in spec

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  • #1

    OEM coil not in spec

    Today I checked the primary and secondary coil resistance on the XJ I'm planning to sell soon. Specs are 2.5 ohms +/- 10% on the primary and
    11,000 ohms +/- 20% on the secondary. I got 2.6 and 11,800 ohms on the left side coil for #1 and #4, got 2.9 and 20,800 ohms on the right side coil for #2 and #3. What say the ignition experts, does the right side coil need replacing? The bike started right up and ran fine at idle before I started working on it but since it wasn't street legal and didn't have 2nd gear I never took it out for a good run.
    Billy

    1982 XJ1100, Ceramic Coated Headers, Raptor ACCT, Barnett Clutch Springs, Dremmel Fix, TC's Fuse Block, De-Linked S/S Brake Lines, 850 Final Drive, Yahman's YICS Eliminator, Pods, stock jets
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  • #2
    Originally posted by BillyRok View Post
    Today I checked the primary and secondary coil resistance on the XJ I'm planning to sell soon. Specs are 2.5 ohms +/- 10% on the primary and
    11,000 ohms +/- 20% on the secondary. I got 2.6 and 11,800 ohms on the left side coil for #1 and #4, got 2.9 and 20,800 ohms on the right side coil for #2 and #3. What say the ignition experts, does the right side coil need replacing? The bike started right up and ran fine at idle before I started working on it but since it wasn't street legal and didn't have 2nd gear I never took it out for a good run.
    The original coils AND another set of the same I had come by some yrs. back both were creating running issues. Eventually found both sets testing at 2.4ohms. In this case, I replaced them with Honda VF coils(3ohm). Tested both before purchase locally, and at 3.1ohms, was all good. Unsoldered from old coils and re-soldered the original plug-ins to the VF coils. Made up new set of wires.......plug-n-play, no mounting mods whatsoever.
    Just making a wild guess at the time when I changed mine was 2.7ohms as when they would start showing incomplete ignition issues, black plugs, etc.
    I also know that having a long time(few yrs.) low primary voltage issue didn't do either set of the OEM coils any favors either.

    Your call, but both are gradually 'leaking' internally, fractures in epoxy fill from to many decades of heat/cool cycles,etc,. I'm sure an elec. guru here can better explain coil failures.
    81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.

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    • #3
      Maybe clip back the wires a bit and see if it still gives the same readings. Check wires and coils for cracks etc just to be sure.
      2-79 XS1100 SF
      2-78 XS1100 E Best bike Ever
      80 XS 1100 SG Big bore kit but not fully running yet.
      Couple of more parts bikes of which 2 more will live!

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      • #4
        Originally posted by Rasputin View Post
        Maybe clip back the wires a bit and see if it still gives the same readings. Check wires and coils for cracks etc just to be sure.
        Just an ohm check across the two pri. connections being below specs. pretty much condems them IMO. That's only gonna get lesser, not better.
        81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.

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        • #5
          I did try clipping the wires back a bit but got the same reading, should have mentioned that.
          Just an ohm check across the two pri. connections being below specs. pretty much condems them IMO.
          My readings are coming in above the standard values, not below. (Not sure whether above or below is worse?) The upper spec range on the primary side would be 2.75 ohms vs mine reading 2.9 and the upper end on the secondary side would be 13,200 ohms vs mine at 20,800. Does this translate to weak spark because of current trying to overcome the higher than normal resistance?
          Billy

          1982 XJ1100, Ceramic Coated Headers, Raptor ACCT, Barnett Clutch Springs, Dremmel Fix, TC's Fuse Block, De-Linked S/S Brake Lines, 850 Final Drive, Yahman's YICS Eliminator, Pods, stock jets

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          • #6
            Testing a coil by checking the ohms seems to be inconclusive as heat, vibration, and load factors are not taken into consideration. If you were keeping the bike it would be a good idea to replace 30+ year coils, if you're selling that is a different story. Your coil(s) may work just fine even with an out of spec reading. JMHO
            2H7 (79) owned since '89
            3H3 owned since '06

            "If it ain't broke, modify it"

            ☮

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            • #7
              Originally posted by BillyRok View Post
              I did try clipping the wires back a bit but got the same reading, should have mentioned that.


              My readings are coming in above the standard values, not below. (Not sure whether above or below is worse?) The upper spec range on the primary side would be 2.75 ohms vs mine reading 2.9 and the upper end on the secondary side would be 13,200 ohms vs mine at 20,800. Does this translate to weak spark because of current trying to overcome the higher than normal resistance?
              Uhhh...........my reply was assuming the XJ had 3ohm coils like the 81's. If that be the case.....then YES they are headed 'south' to their demise. Yes, more is better...only to a certain extent. Tenths in this case DO matter.
              Agree with Bikerphil, if it runs good and selling it, wouldn't worry 'bout it.
              81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.

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              • #8
                An out of specs coil may fire the plugs OK, but could damage the ignition module. Out of specs is out of specs.
                The Other Earl
                80SG, 81SH, 80 standard parts bike, 81SH parts bike
                and new to me 78 standard dresser

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