From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 3 Aug 2021 14:15:37 -0700
Subject: Re: [EE] Honda MT250 ignition waveforms
Thread-Topic: [EE] Honda MT250 ignition waveforms
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> I've had a bit more of a think about this overnight.
> It appears that the configuration is that of a "low tension magneto", as
> used in (some) aeroplanes etc. I've not been able to get a detailed metho=
d
> of operation but it looks to me as if the coil is used, more or less, as =
a
> simple transformer.
>
> The flywheel rotates the magnet & inducing current in the mag. coil due t=
o
> the closed contact points.
> At close to the maximum current, the points open and this induces a high
> voltage across the mag coil & the connected HT coil & capacitor.
> The capacitor absorbs some of this & then rings with the HV coil used as =
a
> step-up transformer to produce spark.
>
> So, considering your waveform with the HV coil, capacitor & ignition
> switch
> disconnected, you should see a high peak voltage as the points open.
> Probably >> 250V . Also there should be a noticeable spark at the points
> themselves. Since you're not getting this; either:-
> The mag coil is faulty (unlikely, as it is generating low voltage).
> The points or their connection is faulty. (Dirty / Worn / Corroded)
> The points are opening at the incorrect time - the woodruff key is sheare=
d
> and the timing is way out. This should be fairly easy to verify.
>
> With the HV coil & capacitor connected, you should see ringing across the
> HV coil (& get spark).
>
> Hope this assists,
> RP

Thanks for the comments! The original transformer had an open secondary,
so my brother got a replacement. I think it is not a suitable replacement.
He has now ordered the exact Honda part number. In initial testing, we
found the points corroded and not making contact at all. After cleaning,
we are able to see them close at the correct time (resistance drops to
zero when the mark on the flywheel lines up with the timing mark). For the
magneto coils, it seems like the only failures could be an open or short
(shorted windings). The coils is not open. Detecting shorted windings
would be pretty difficult. But, I think that would be an unlikely failure.
Assuming the new transformer solves the issue, I'll capture some waveforms
with the transformer connected and also with the magneto/points
combination driving a resistor (maybe 100 ohms).

Harold



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