912 ignition problem logic

by Mike Goodrich in 912 / 914 Technical Questions 11 years ago

I am trying to help a friend with his 100 hp 912.

He has an 800 rpm drop on the ignition switch that controls the A module.
He has carried out some tests changing over the 6 pin plug/receptacles between the trigger coils and the modules and between the modules and the ignition coils.

My friend is convinced that the problem is with the trigger coils and I'm convinced that the problem is with one of the modules. Attached is a simplified wiring schematic showing my logic that I hope is easier to follow than pages of text. The switch is shown schematically to show that we are switching that circuit off, I know that in reality you close the contacts to switch the circuit off.

Note also that this is with the modules with 6 pin plugs and that the ignition switches go into the trigger coils side of the plug.
Also note that this assumes that the problem is in the ignition system and not one of the many other possible causes that have been given in the recent thread about a similar problem.

Can anybody with experience confirm if one of us is correct.

Mike G

9126pinplugignition.jpg (You do not have access to download this file.)

912ignitionlogic.JPG (You do not have access to download this file.)

Re: 912 ignition problem logic

by Rob Seaton » 11 years ago

Rob Seaton

3 1.32k 251

nice thorough troubleshooting Mike. I would say you are correct.
Can you send your friends modules for a bench test? Your profile says you are in Europe, I know Franz Aircraft in Germany has a bench tester.
Rob

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Re: 912 ignition problem logic

by Mike Goodrich » 11 years ago

Mike Goodrich

8 123 0

Thanks Rob and thanks for your reply in the other thread.
My friend has already had both modules tested by the Rotax guy in France. The test consisted of putting the modules on another engine and they came out OK. So that is confusing for me because with my logic I'm pretty certain the problem is the B module.
However he did check the trigger coil gaps and the gap for trigger coil N°3 (the one at 10 o'clock on Fig 74-20 above) was too large. He closed the gap and it made no difference to the problem but he says the idle is now much smoother than before.
Any other suggestions would be most welcome.
Mike G

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Re: 912 ignition problem logic

by Rob Seaton » 11 years ago

Rob Seaton

3 1.32k 251

I know this sounds silly but run the swap tests again but this time de-pin the p-leads and keep them with the same module.
another simple test is to rig up a temporary p-lead and have some brave guy touch them, one at a time, to ground at the RPM that you have the 800 drop. Caution! The bare wire you will get a good shock! Also do not use a very long wire (I used a 3 meter wire once to be safe around an amphib pusher and the engine would not shut down!) then touch both to ground to kill the engine. this test eliminates the switch circuit.
Just like you have been doing its good to divide and conquer each separate circuit:
-Replace spark plugs.
-Check p-lead/switch circuit (eliminate it with test leads as previously described)
-Stator: de-pin and swap red wires. no change? the stator is good. If bad check the ground wires (accessible through flywheel holes)before removing stator. Ohm out each red wire separately with the ground ring terminal disconnected to get a true reading.
-Triggers: de-pin p-lead (do not swap) so any change will be obvious (for my simple brain this needs to be done)
-Modules: swap coil connectors.
-Coils: (Very unlikely to be the problem)ohm out primary and secondary wires. at the same time clip off 5mm of the secondary wire to get a good connection to the spiral prong in the spark plug cap. Clean the primary ground wires (at the 1/3 intake manifold)
-In the end carb problems are much more likely to be the cause of a bad mag drop but a 800rpm drop is also likely to be an ignition problem.
Why does a carb problem show up as a high mag drop on one ignition? Because each ignition fires 4 top plugs and 4 bottom plugs and the bottom plugs are "less optimal" (they are not positioned in the combustion chamber as well as the top plugs and they tend to get more fouling). Combine this with 2 separate carbs and you can have confusing diagnostics. Also the induction system is not perfect; the fuel/air mixture has a longer/straighter path to the #1 and #2 combustion chambers so you get rich/black plugs in the front cylinders at idle and low rpm. At higher rpm they work fine so if your mag drop is fine at high rpm and bad at low rpm then consider the carbs to the problem.
Rob

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Re: 912 ignition problem logic

by Roger Lee » 11 years ago

Roger Lee

128 5.28k 4.98k

It could even be a wire that is broken in the connection where it is crimped and only the insulation is holding it in. It will look in goods shape. Give them all some good tugging.

Just remember most problems and fixes aren't any thing exotic, but simple things.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: 912 ignition problem logic

by Mike Goodrich » 11 years ago

Mike Goodrich

8 123 0

Roger and Rob
Thanks for the feedback, I shall be traveling and off the air for a week now so will look at it all again when I get back if my friend hasn't solved the problem himself by then.
Mike G

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912 ignition problem logic

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