ignition failure

The Rotax 912 Heavy Maintenance manual has a diagram that shows the various connections in the ignition system. You should definitely be able to measure the resistance of the primary and secondary windings in the coils. I have attached the diagram below.

I usually measure the secondaries by just pulling the plug caps off the plugs and measuring from inside the plug cap to the other plug cap that is connected to that coil. e.g. 1T to 2T, 1B to 2B, 3T to 4T and 3B to 4B (T=top, B=bottom). You will be measuring the resistance of the plug caps as well the secondary winding. This will tell you whether the coil is open circuit or not. The plug caps are about 5k ohms each and the secondary coil is about 6.5k ohms to the total should be approximately 16.5k. This doesn't tell you whether you have a shorted turn or an intermittent connection.

The primary windings can be measured between the white & blue pairs in the 6 way connector. The attached diagram has the connector pin out. The resistance is very low so you should definitely be able to measure it. Again this doesn't tell you whether you have a shorted turn or an intermittent connection.

Unfortunately it is not easy to swap over the coils, if you have a spare pair of 912 coils then you could substitute them but I'm guessing that is not an option. If you have a suitable crimp release tool then you could swap the primary connections to the 3T/4T with the 1B/2B connections. You can leave the plugs out and connected and just see if you have a spark when cranking. If the problem has now moved to the 1B/2B plugs then the coil is your problem.

Here's a link to the manual https://www.rotax-owner.com/manuals/MMH_912-914_Series_ED1_R6.pdf

Thank you, sir.

A. I have the manuals - all FIVE for the 912/914. Have kept them current since 2014. Wouldn't turn a wrench on anything if i didn't. [Need an OH manual for a P&W R-985? Parts manual? How about a Wright 1820?] My usual for these runs more to the hose replacement, carb float check/carb ops check,  carb balance, or pulling the gearbox to send it out.

B. I appreciate the insight into testing across the two plug wires. Hopefully I can get SOME value to show up.

C. I tested across the pin as described - FAIL [discontinuity] ---- which is why i was suspecting a bad coil.  I will re-test, see if I can confirm, then chase for failed connections before we order parts.

And, you are correct - I don't see these often enough to stock parts. Hate to tell owner he needs to buy parts - which will have been defective from factory - on the guess that pull-and-plug will fix it.... especially with those modules at $900+ each. Need to keep people safe, but handing an owner a big bill doesn't help accomplish that.