Magneto Drop
in 2 Stroke Technical Questions
14 years ago
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by Richard Jowsey » 14 years ago
We toggle the magneto switches to find out whether one of the CDI ignition systems has failed. If so, grounding out the other (working) ignition circuit would kill the engine. The normal small drop in revs is irrelevant to this test; it's all-or-nothing, i.e. the engine keeps running, or dies. You'd certainly hear a big change in engine note if that happened: the Sound of Silence!
The incorrect reading given (sometimes) by a Rotax tacho when flipping the mag switches is *annoying*, but it isn't a "critical" fail. I was able to fix this problem by increasing the AC voltage supplied to the rev-counter when a mag coil was shorted (using a mid-point tap on the magneto windings). That suggests that the most likely reason for bogus tacho readings (503/582) is insufficient voltage from the factory-standard tacho tap (grey wire) when that coil is switched to ground.
This issue appears to have been fixed in the 912 engines.
Hope that helps,
Richard
Thank you said by: Lorenzo Bernareggi
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by Clive Wilton » 13 years ago
.....I discovered a 50% tap on the ignition coils, so I tried connecting this to the tacho (instead of the 10% tap) -- now there's no bogus drop of indicated revs when toggling the mag switches....
Good news Richard, thanks for the very useful info.
Could you possibly clarify where and how you found the 50% tap and how you connected to it ?
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by Richard Jowsey » 13 years ago
I had to remove the magneto coil to fix some random breakage. I noticed a mid-point tap on the rear of the coil assembly, to which I soldered a (grey) wire. I secured this wire with a glob o' hot-glue, threaded it out through the wire-wrap tube, and crimped a spade connector on the end. After checking that the voltage was "as-expected" while the engine was running, I connected this mid-point tap to my tacho, with very satisfactory results.
The attached pix are "before and after" shots. The tap point is at the 1-o'clock position, between #3 and #4 ignition coils.
Hope that helps!magneto-0.png (You do not have access to download this file.)magneto.png (You do not have access to download this file.)
Thank you said by: Clive Wilton
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by bill martin » 13 years ago
Great fix! Thats one I'll have to remember
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