I've asked on here before for ideas about a vibration/roughness problem with my 912ULS and Airmaster prop between 2500 and 3000rpm, worst at 2750. Only done ca 100 hours total but well out of warranty.

I've cleaned and synched carbs, checked the gearbox splines, checked prop balance and changed the engine mounts to no avail.

At idle and above 3000rpm the engine is smooth and mag drops are slight and equal with no particular roughness. At 2750rpm, one mag drop is significantly bigger and rougher than the other.

So yesterday I swapped the ignition modules, swapping both the 6 pin and 4 pin plugs so not also swapping the coils and triggers, and the drop and roughness moved to the other mag switch.

So this strongly suggests that it is the ignition module but how on earth can it show this fault behaviour? Good except between 2500 and 3000. It doesn't make sense!

I'm trying to borrow a module to test more definitively but has anyone encountered similar behaviour or have an explanation please?

If I have to replace it I shall join the move to Ignitech.

  • Re: Ignition module failure modes

    by » 2 years ago

    I must admit that I haven't come across those symptoms being tied to an individual module but there is no argument with your conclusive test results.

    There are 3 distinct sections to the ignition module circuit:

    1. The input trigger circuit.

    2. The coil drive circuit.

    3. The power supply which includes the charge store and the kill circuit.

    My gut feeling is that it is possibly in the input trigger circuit. There is some filtering effect that results in it missing some pulses at particular RPM and so the coil drive circuit doesn't fire. The trigger circuit has 2 paths, one that detects the pulse from the leading edge of the flywheel notches, the other path detects of the trailing edge. The angular length of the notch determines the different timing between cranking and running. This in itself is determined by the amplitude of the pulses from the pick up coils which is lower at cranking speeds but increases at normal running speeds. We know that this amplitude threshold can be a problem in some modules which is why they won't start and engine but will happlily sustain it when it is running.

    There may be a problem in this area of the circuit that gives the symptoms that you describe. If you are in the UK, there are some Rotax modules for sale on afors.com or alternative just replace them with Ignitec and have done with it.

    Thank you said by: Mike Wylde

  • Re: Ignition module failure modes

    by » 2 years ago

    Watch the 2nd half in particular.


  • Re: Ignition module failure modes

    by » 2 years ago

    Yes I've previously seen and taken note of that helpful video thanks.

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