PO15 Coil Testing

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  • sedover


    Replies: 18
    Topics: 6
    #2854

    Dealing with testing a PO15 ignition system which has no spark. I have the components isolated. The breaker is good.

    I am not sure how to approach the coil. I am seeing primary side continuity but the secondary to ground is open. This was measured with a Fluke DMM. I have a Stevens Analog at work and will try that tomorrow. Can a secondary resistance measurement be read from the secondary terminal/wire to ground.

    Steve


    garry-in-michigan

    Lifetime Member
    Replies: 3583
    Topics: 31
    #26071

    I don’t believe there is a secondary to ground connection at the coil. Measure between the two high voltage ignition wires. In this case, the spark plugs make the ground connection. If one wire is left dangling when cranking, the electricity has no where to go. They must be connected to a spark plug or grounded to the block (or magneto base plate)


    sedover


    Replies: 18
    Topics: 6
    #26081

    Thanks for the reply Garry.

    An ignition coil is a transformer with the collapsing current flow in the primary inducing current flow in the secondary. This has to have a complete circuit in which to travel. The ratio of winding turns establishes the output voltage and current which is available to jump the plug gap. Normally, a resistance value is measurable from the secondary post to the coil’s ground point.

    I guess it should be asked, that ,if my coil has failed, where can I source one. If they aren’t available, are there conversion kits.

    Steve


    Tom

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 636
    Topics: 57
    #26082

    "Normally, a resistance value is measurable from the secondary post to the coil’s ground point."

    This is true, most of the time. But for some opposed twin outboards, it’s not the case. The Johnson coils of that era tend to be very reliable under normal use. If you haven’t done the test specified by Garry, do so before you write off that coil.

    Here’s a little more detail: Most coils have a primary and secondary winding which are grounded together. The other end of the primary winding goes to the points. The other end of the secondary goes to the spark plug. The complete secondary circuit, then, is from the secondary lead, across the spark plug gap, to the engine block, to the mag plate, and back to the secondary coil through the ground.

    The PO coil windings are independent. One end of the secondary winding goes to one spark plug, the other goes to the other spark plug. The complete secondary circuit, then, is out of one plug lead, across one plug gap to the engine block, across the other plug gap, and back to the secondary through the other plug lead. So if you are testing one spark plug wire at a time without grounding the other wire to the engine, you will get no spark. And the coil will be damaged if repeatedly tested without grounding one of the seconday leads.

    Tom


    RICHARD A. WHITE

    Lifetime Member
    Replies: 1840
    Topics: 140
    #26107

    The Merco tronic handles these by using a secondary probe. It in effect is grounding the winding you are NOT testing.

    http://www.richardsoutboardtools.com
    classicomctools@gmail.com


    Mumbles


    Replies: 5764
    Topics: 298
    #26109

    The original part number for your PO coil is 72-852 and the P/N for the square condenser is 72-864. The uF rating for the cap is .27 – .33 and if it is bad, a Vishay or Orange Drop capacitor can be used instead. To find a coil. try placing an add on the Free Classified section here or try Doug Penn at douglas.penn@gmail.com…… Here’s a link to his site;

    http://www.penn.itgo.com/

    Here’s what the Vishay and Orange Drop caps look like.

    PO 15 ignition system trouble shooting.

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