GDM

I wish I had found your article weeks ago. Those little wires are very fragile.

Rotax Tech Talk: Troubleshooting the 912/914 Ignition system

It was a straight section of wire, so I believe it failed due to rubbing.

For owners of Rotax 912 ULS engines my recent experience of intermittent rough running on engine run up and once in flight may be of value solving this issue if it ever arises. on a recent flight with a friend there was an intermittent vibration of an otherwise very smooth engine. The Rotax 912 ULS has just over 400 hours flight time and the annual and 5 year rubber replacements had been just completed by an experienced LAME. Runups to 4000 rpm before flight were excellent, drops of 30 or 40 rpm on both L and R. We flew down to the surf coast and the Port Phillip Heads uneventfully. On the return my passenger and I felt a shudder, the passenger thought we hit a bird, I felt otherwise. The shudder returned a number of times intermittently. I though I may need to check if it was a capacitive box problem. So In flight I switched from BOTH to LEFT and the engine wanted to stop. I quickly went back to BOTH. on landing I did some runups and got some rough running on the LEFT switch. Over the next few weeks and flights run ups were smooth , so I flew and sometimes the runups were out of spec. So I tacied back to the hangar and jiggled wires and spark plug leads. This often lead to smooth, problem free runups and flights. the problem escalated and the intermittent rough runups became more common. This issue needed to be solved! i watched a Rotax training doco on YouTube and followed the methodology, change one variable at a time. this involved switching the connectors coming from the stator to the Capacitive boxes A and B. Result, Left switch still causing rough running. return that connector to its original position then swap the connectors from the boxes. Same result, Left switch , rough running. Add starting carb at 4000 rpm on right switch, resulted in more revs, smooth running, but adding start carb on left switch resulted in greater rough running. at this stage I realised that the issue was from the left male connector back to the stator. I tested the resistance of the coils on the stator, all came out at the book 250 ohms, continuity was fine, no breaks. I tested the red leads which power the Capacitive boxes by cranking the engine under load by slipping a few stands of wire into the connectors. They gave good values 7.6 volts ac at 500 rpm. the final test solved the problem. The earth wire…..I set up the multimeter, one probe on the earth wire on Box A, the LEFT, white(the heavy maint. Diag. Shows brown) and the other lead to a ground on the engine. With the ignition switch on OFF, the multimeter set on continuity screamed as it should, on Right it screamed as it should, but…when on LEFT it should be quiet.. it screamed. Box A, the LEFT was earthing! A quick inspection of the wire found the break. There was a failure in the plastic coating of the wire and it was touching on a metal braided loom. An easy fix. I hope my experience is of potential help to somebody else.