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Honda ignition voltage waveform, 7.5 and similar

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  • Honda ignition voltage waveform, 7.5 and similar

    Last summer my Honda 7.5 (model BF75A serial 1200493) died suddenly with an ignition failure. The symptom was a spark so weak I could only see it in a darkened room and that would not fire at all in the cylinder because of the greater breakdown voltage of the gas/air when compressed.

    The Honda service manual provided some trouble-shooting guidance but it was incomplete and some of the resistance readings were incorrect. What I needed, and what was not provided at all, was the waveforms and/or peak voltages at various points in the ignition circuit. While peak voltage cannot be measured with a simple voltmeter, it is easy with an oscilloscope, which is what I used.

    After replacing the components most likely to fail to no avail, in frustration I bought another motor, measured the signals on that good one, isolated the problem on the original one and replaced one additional part, the exciter coil, to solve the problem. I now have a spare motor for backup and for my dinghy, and a spare CDI module and spark coil.

    In the hopes of saving someone else the expense of troubleshooting by replacement the ignition voltages / waveforms I measured on the working engine are as follows. All relative to the engine frame. While I measured these on a BF75L they should be the same for most or all of the Honda 7.5, 8, and 10 hp motors.

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    Exciter Coil (under flywheel) - The output of this coil provides power for the operation of the CDI unit. The output is a pulse of about 250 v, about 5 milliseconds wide. When this coil is connected to the CDI module the pulse is only positive going. When this coil is disconnected from the CDI module the pulses alternate between positive and negative.

    Ignition Coil (low voltage side) - The output of the CDI module goes to the low voltage side of the spark coil. It consist of an initial negative pulse of about 250 v, about 100 micro-seconds wide, with a sharp leading edge, followed by ringing for about 400 micro-seconds. The first positive excursion of the ringing is about 150 v; the others are smaller. Only the initial 250v pulse is significant.

    Pulser Coil (under cam-shaft pulley) - The output of this coil goes to the CDI module to indicate the time at which a spark should fire. It consists of a positive pulse of about 6 v, 10 milliseconds wide.

    Ignition Coil (high voltage side) - I did not, and will not measure the voltage of this as it is too high to safely measure. A healthy spark indicates that the voltage is adequate.

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    Hope someone finds this helpful,
    Bill Wetzel
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