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Dual Ignition Coil Resistance Test

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  • Dual Ignition Coil Resistance Test

    Recently purchased a 1986 Yamaha 25hp, model 25SJ. Am having issues with very weak spark. I found some specs online and began testing the resistance of various parts of the ignition system. Everything checked out until I got to the ignition coil. This model has the dual ignition coil like in the attached picture. I tested the primary resistance across the orange and black wires that connect to the CDI box and got 0.4 ohms which seems acceptable. I tested the secondary resistance by touching the leads to one plug wire and the other to the orange wire. I got nothing on my meter when touching either plug wire. I then swapped my other lead to the black wire and tried both plug wires again and still nothing. I can't see how I would be doing it wrong - there are only so many possible combinations here lol. Am I missing something or is the ignition coil shot? For what its worth I questioned whether or not my multimeter was working correctly for higher resistance so I checked the coil on my 15hp Johnson and got 260 ohms which is within spec.

  • #2
    You can't test a coil that way. The orange and black wires each go to a primary winding inside with other ends of the windings connect to ground. The same for plug wires attached to secondary windings and the other ends tied to ground. There is no physical connection between primary and secondary winds, so you won't get a reading.

    What you measured between the O and B wires is the resistance of both primary windings. Measure between the two plug wires for resistance of both secondary windings. To check individual primary or secondary coils, measure from a wire and ground or one of the mounting rings. It's possible the primary is grounded to one mounting ring and the primary to the other ring.


    Is your 15 HP motor a single or dual coil?
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    • #3
      Thanks. Yes the 15hp is a normal, single coil. This is the first time I have ever messed with a dual coil like this.

      I did not consider that the mounting bolts could be the ground and that both black and orange wires were primary leads. I guess since one was black instead of another color helped make my assumption and off I went. I'll recheck and update. Thanks again.

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      • #4
        I pulled the coil and bench tested it. The image below is my coil and I numbered the 4 leads and the 2 post attachments.

        1 to 2: 0.4 ohms
        1 to 3: no reading
        1 to 4: no reading
        3 to 4: 5 ohms
        5 to 6: no reading
        5 to 1/2/3/4: no reading
        5 to 1/2/3/4: no reading

        When I pulled it, I noticed the orange wire is burned through exposing the wires. This matches up with the story the head tells in that it was seriously overheated at some time to the point where the paint was entirely burned off. The thermostat was also stuck open and the stem had broken. The burned wire must have been on the head. The burned wire must be what was causing the audible electrical clicking I was hearing as I was trying to pull start it.

        Is it true what I read elsewhere that with these dual coils both cylinders fire simultaneously, with one being a wasted spark while that cylinder is a BDC?

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        • #5
          Don't know if this motor is wasted spark or not.

          I think the coil is bad. In the specs you found, does it include resistance for the checks you made in the last post?

          The part no. is 689-85570-21-00, which is superceded from 689-85570-20-00

          I found it on Ebay for $38.80, new. Cheaper than used coils on ebay. https://www.ebay.com/itm/JETUNIT-OUT...p/264341270633


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          • #6
            The pics below are all I have found. Looks to be taken from some Clymer type manual.

            I saw that coil as well. Im always a little reluctant to buy non-OEM electrical parts but at over $125 for a coil, this is probably worth a shot for sure.

            I'm going to repair the orange wire and reinstall and see what it does. Then test the CDI per the attached image's checks. Will likely buy the coil you linked if the CDI checks out. If CDI shows bad too I may dig into it some more.

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            • #7
              Im going to refrain from doing any more as I am not 100% confident in my multimeter right now. I tested all 3 coils on my Yamaha 30 per the factory service manual and all showed open on the secondary coil resistance but it runs like a champ. Ill pick up a new meter as soon as I can.

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              • #8
                If the 30 HP coil is the same, try swapping if the wire repair doesn't fix it.
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                • #9
                  Originally posted by scout-j-m View Post
                  I pulled the coil and bench tested it. The image below is my coil and I numbered the 4 leads and the 2 post attachments.

                  1 to 2: 0.4 ohms
                  1 to 3: no reading
                  1 to 4: no reading
                  3 to 4: 5 ohms
                  5 to 6: no reading
                  5 to 1/2/3/4: no reading
                  5 to 1/2/3/4: no reading

                  When I pulled it, I noticed the orange wire is burned through exposing the wires. This matches up with the story the head tells in that it was seriously overheated at some time to the point where the paint was entirely burned off. The thermostat was also stuck open and the stem had broken. The burned wire must have been on the head. The burned wire must be what was causing the audible electrical clicking I was hearing as I was trying to pull start it.

                  Is it true what I read elsewhere that with these dual coils both cylinders fire simultaneously, with one being a wasted spark while that cylinder is a BDC?
                  Update to this.... I am now getting 3300 ohms between the spark plug leads (#5 and #6). The spec I found said secondary resistance should be 3000-4000ohms. All the others I originally measured as open still show open. Starting to think the coil may not be the issue now (aside from the burnt wire which I temporarily taped up while troubleshooting.

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                  • #10
                    I also checked the CDI per the image I attached above and pretty much everything read open. Only time I got any resistance was when connecting the ground to the brown wire and the black wire that goes to the coil.

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                    • #11
                      I don't know if this would make any difference, but bolt the coil back on motor if you haven't. Then you know it has a good ground to use for readings.
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                      That is what the forums are for.
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                      • #12
                        Originally posted by GA_Boater - Wash Hands - Don't Touch Face - Separate View Post
                        I don't know if this would make any difference, but bolt the coil back on motor if you haven't. Then you know it has a good ground to use for readings.
                        Will try that. I'm pretty sure the pulser and charge coils under the flywheel are good as they have resistance in spec. I also noticed when testing them that the grounds are good since instead of measuring from red/white to black and brown to black I also went straight to the ground that CDI uses instead of the black and got the same readings.

                        One other thing possibly worth mentioning is that there is spark currently, it is just super weak and takes a crazy hard yank on the rope to get it. My past experiences with a bad CDI has always been acceptable spark or no spark at all - never a case of a weak spark. I always just associate weak spark with a coil issue, be it a bad ground or melted coil.

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                        • #13
                          I bought a used CDI off ebay and installed it yesterday shortly after it arrived. It is now jumping a 3/4" gap with a nice thick, blue, snappy spark. I am going to run it in a barrel today and see how it does. If it appears to be fixed I will double check the timing settings, adjust the idle mixture screw, and then take that burnt wire on the coil and add some solder to the bare wire and cover with some heat shrink. Will replace the coil in the near future but will see how it performs for a few trips before doing so.
                          Last edited by scout-j-m; April 9th, 2020, 08:15 AM.

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                          • #14
                            I love my old school magneto ignition.

                            Got my fingers crossed for you, Scout. Go boating and catch some fish.

                            Stay safe out there!
                            BOAT SPECS | FORUM HELP | STARCRAFT FORUM | SHOP iBoats
                            Please, no PM's (Private Messages) regarding boat/engine problems.
                            That is what the forums are for.
                            Only forum/moderator issues will be answered in PM's.

                            Comment


                            • #15
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                              Originally posted by GA_Boater - Wash Hands - Don't Touch Face - Separate View Post
                              I love my old school magneto ignition.

                              Got my fingers crossed for you, Scout. Go boating and catch some fish.

                              Stay safe out there!
                              Thank you sir! You were a big help here!

                              Think I'm finally gonna reassemble the old 1967 9.5 OMC I have in pieces just so I have at least one old school ignition system I can count on!

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