Big-time Mag Drop

Abbreviated session with plane today.  Carefully inspected all visible wires for chafing, breaks.  I wanted to eliminate the panel switches as a possible factor so ohm testing and inspection of connections on that circuit showed all was well.  Started engine and experienced big RPM drop on the new (second replacement) module.  Dropped from 2,400 warm up RPM to less than 1,000.  Shut it down.

Seems to me we have two good clues: module test OK with reinstallation of old (1,000 Hour) module; and, until today, big drops with new modules were only AFTER flight.  OK preflight tests until today.  Rotax eccentricities aside, I'm looking for some kind of wire abnormality.  Maybe heat or vibration in flight is an issue.  BUT...none of this squares with the no-issue performance of the reinstalled unit in troubleshooting earlier.

I will plan on a long day tomorrow with the Heavy Manual troubleshooting and will report any pertinent findings.  If Roger or anyone else has thoughts on what I might look into, please pass them along.   TIA       Jim

Hi Jim,

Once upon a time I was working with a quite old 912UL with similar deep but not always repetitive drop.

The culprit was a one module charging cable (the red one). When I pulled slightly the connector...... have a look on the picture.

S.

OK, I think I may have found a smoking gun.  I decided to do a more thorough check of the wiring associated with the ignition modules.  Lacking a borescope, I used a penlight (in my teeth) and a dental mirror and needle nose pliers to closely examine the wires coming up from below, under the lower connector for the bad side module.

Whaddaya know?  Found two abraded wires right where they exit from the cloth insulation around the wire bundle.  One was cut through completely and the other only partially.  I couldn't identify the wire colors for sure, but I believe at least one of them went to the trigger coil.  I determined that the wire bundle rested on a replacement bolt I had installed several hundred hours ago to better secure the support arm for the RV-12 module structure.  The fault was mine.  I had put a rubber cushion under the bolt head because the original support arm had broken and I was trying to isolate the vibration.  Moved the bolt head too close to the wires.  Unforced error on my part...

OK, I know broken wires under vibration can make troubleshooting difficult, but... how come when I replaced the apparently defective new module with the old one, everything worked fine?  Coincidence or bad luck?  Murphy's Law?

The question I have right now is:  Can this problem be fixed without replacing a very expensive Rotax part.  The wiring bundle in question disappears under the electrical stack of modules and coils.    Before I disassemble that, I would like to know if I need to order something or if I can get access to the problem area and splice the bad wires.

A second question would be:  Does the revelation of damaged wires suggest the cause of my huge RPM drops when testing modules?   Thank you in advance for input on this vexing issue.    Jim    

Hi Jim,

In my case I was able to cut the cable before it fully enters the cable bundle and just solder a similar new cable with a connector. Some heat shrink tube and it was done. BTW, the related module was also faulty. I think that that the bad cable might be the cause, too.

It is difficult for me to say how it is in your case without any pictures :)

Anyway, maybe you can try to remove some of the bundle isolation to get to enough failed cable length and repair it.

Sometimes without a significant disassembly work you can't say what you need to order. Sometimes the amount of time to be spent to repair something is not economically justified because it is cheaper to buy a whole new part and exchange it. Sometimes it is illegal or not allowed to repair something and it has to be exchanged. 

S.

Thank you, Stefan, for your input.  The wire bundle in question--on my RV-12--is very difficult to reach.  It is down under the ignition stack and only an inch or so of it protrudes out.  Were I a heart surgeon with Lilliputian soldering equipment, I'd make fast work of it.  As it is, I was lucky to spot the abraded wires with a dental mirror.

I agree with you that sometimes replacement of the part is the best practice and I would very much like to hear from anyone who has effected a fix or executed remove/replace on the wire bundle's source. 

I value the advice given on this forum and thank you for participation.     Jim