Misfires Module B

Ok,  have licked my wounds and started in on the engine misfiring on B ignition.  The current advice here is that I need to swap the plugs from the A and B coils to A and B ignition modules.  Easier said than done, at least it looks that way.  I was able to get the to electronic modules  off, including the soft start module, and I disconnected the connectors at the back.  I can see the A and B coil plugs coming up from the bottom, but I can't figure out if it is even possible to disconnect and swap them.  The plug release tab on the connectors is on the bottom.  So, do you have to unbolt the A and B coils to gain access, and of so, what do I need to remove to get there.  Looks like this is going to be a time consuming job.   Any help appreciated.

By the way, I'm a saver and I have a short article that Roger Lee wrote, who is a renowned  Rotax expert.  Roger has saved many a homebuilder with Rotax issues.  If I follow his troubleshooting logic, and I am experiencing only a 300 rpm drop on the ignition check, my problem is more likely to be the plug wire itself, rather than a coil, or ignition module.  Since I just replaced the spark plug and connector, I should be moving to the wire itself, but I am not sure how to get it off of the coil.  Any experience here would be great.  I will post Roger Lee's tip article below.........Tom

   Ignition failure clues

 For hard to find issues like this a good 1,2,3,4 step diagnostics makes things easier and usually cheaper because most issues aren't some exotic problem. Jumping around usually takes much longer to find an issue. There are usually simple solutions and common causes to learn from others. Replacing plugs is a good start for a larger than normal mag drop. Then trimming back the spark plug wire ends by 3/8" that go into the threaded prong plug cap. Poor grounds or wire wire rubbed through that goes under something on top of the engine is another good place to check. If you drop on plug then that is around 300 rpm mag drop. To be an ignition module that would be 800-1000 drop and it may just stop the engine. Ignition CDI modules rarely go bad for the 26 degree BTDC normal running phase. Where they do go bad at times is on the start circuit at either 4 degrees BTDC older modules or the soft start at 3 degrees ATDC. The soft start module seem to be holding pretty well. The failures seem to be more of the older CDI modules.

 Recap:

 300RPM mag drop on a single ignition.  Spark plug or wire

800 to 1000RPM drop                           Ignition module

You are correct that it is more likely to be a wiring issue than a coil issue, but the drill is the same.

You can swap the wires and if that doesn't work assume the coil is bad, or swap the coils and if that doesn't work, assume the wire is bad.

Either way, you will need to remove the coils in order to disconnect the wires.

I haven't personally done it but according to the manuals, the 912ULS Spark Plug wires Un-Screw from the coils just as the spark plug connector ends do.

- - -

The 912is plug cables supposedly Un-Plug from the coils but I have yet to have the nerve to pull hard enough on the cable to get it to disconnect.

If it truly Un-Plugs on the 912is engines, it is not an easy pull by any means.

Thanks Bill.   I used your suggestion on measuring the resistance between the plug connector pairs, such as top 1 to top 2, top 3 to top 4 etc.  My readings were from 15.1K ohms to 16.4K ohms, so I think the high voltage side is good.  That brings me to the ignition modules A and B or the low voltage side wiring.  Looks like you have to remove the carb and intake manifold to get at the A coil and B coil plugs. Also, it looks like new ignition modules are $1000 each.  Ouch!!

The low voltage (primary) side is more likely to have a problem than the high voltage side.

The primary windings are well less than 1 ohm.

This means that if any part of the circuit has even as little as 1 additional ohm of resistance, more than half of your spark will be lost.

2 or 3 ohms is almost unmeasurable but can cause a total failure.

Do NOT go out and spend a thousand dollars on any parts until you are absolutely certain that the part is defective.

The next relatively easy step would be to remove the primary connectors from the coils and make sure that they are holding very firmly onto the coil connectors.

A gentle squeeze with a pair of needle-nosed pliers can be used to firm up the connectors and it won't cost a cent.

Hi Bill.  Thanks again for you advice.  Yesterday I used a field effect circuit tester to identify spark while running the engine.  I had intermittents all over the place.  My Milwaukee tester is probably not the best tool for that.  I ordered an in line spark tester and it will be here Saturday.  I still am not positive that the bad circuit is #3 top.  All that I know for sure is that it is Ignition B.  I appreciate you advice about not jumping to conclusion and buying a $1000 part.  I did not realize that there were additional connectors at the coils.  Makes sense when I think about it though so I searched and found a picture of a coil and seen the short primary leads  I still want to identify the intermittent firing plug/plugs so that I have a better idea of where to look.  That will probably be later this weekend though.  One note of interest.  The higher the RPM when doing the ignition test, the worse the miss.  Please stay tuned as I really appreciate your help.......Tom