Mag problem Rotax

[Greg Vos]

Guys I’m reaching out to you for some advise.
we have a rotax that has a dead mag (A circuit) the challenge is it works fine when the engine is warm say 40/50 degrees C
the problem is hard starting and if we do a mag test when it’s cold and below 1950 Rpm and drop a mag B while A is off ( earth shortage condition) the motor does instantly., when it should run on the B set of plugs
Oddly when the motor is hot we can’t switch between mags and we see the normal operation, we can also switch between both in a cold condition if engine is above 3000Rpm
Once warm it delivers full power and runs fine, I’m lost as to why on,y seeing the mag fail at low Rpm and only while it’s cold

all comments most welcome

 

[Aerofoam]

I can not speak to the particulars as Rotax is fairly new to me,
BUT, I have seen circuit failures like this due to a cold solder joint (cracked) on a circuit board.
No contact cold, but hot, it expands and acts like a switch turning on.
If there is an accessible circuit board for the ignition, my first resort would be re-flow all the solder joints, but my airplanes are unmanned and I walk away from every landing........

 

[Smack]

Greg, try pre-heating the ignition module with a hair dryer / heat gun. I'd like to know if that changes anything.
It is probably an electrical connection that conducts better when hot due to the connectors pressing together harder.
Outside chance that there is a loose connection INSIDE the ignition box that conducts better after warming for the same reason.
Have you switched A/B leads? The act of disconnecting / reconnecting might 'magically' fix it.