80 HP 912UL will not start

by David Wallis in 912 / 914 Technical Questions 10 years ago

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Re: 80 HP 912UL will not start

by Mark Kyle » 9 years ago

Mark Kyle

1 29 15

By the way I am still trying to chase one of the new style ignition units that have the soft start option in them that have failed so I can have a look at them. There certainly seems to be no failures here in OZ that I can track down and very few if any world wide. The new Motorini version certainly seems to be bullet proof so far. I dont want to pull one from my aircraft and pull it apart to look at it...they are way too expensive here.

So again if anyone knows of the later motorini versions that someone has that is no good please contact me. kylecom@tpg.com.au

Mark

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Re: 80 HP 912UL will not start

by Andy Fell » 9 years ago

Andy Fell

0 17 0

The 2uF large cap seems to die. I have found them as low as .5uf and regularly down at 1 and 1.3uF. This is the first place I would start as you cant get enough energy for the firing.

Yeah I wondered that too. though if you wanted to add another one on the back and re-pot it to a smooth/flush finish (as they are doing at Carmo) it's tricky to see how you could get another, say 1uF@400V, cap to fit in there.

It might just be worth a try though, just to see if it works.. then worry about being able to fit it in later :-)

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Re: 80 HP 912UL will not start

by Andy Fell » 9 years ago

Andy Fell

0 17 0

How about stick one of these on the back in parallel with the energy storage cap?

http://www.digikey.co.uk/product-detail/en/C5750X7T2W105M250KE/445-6841-2-ND/2616427

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Re: 80 HP 912UL will not start

by Mark Kyle » 9 years ago

Mark Kyle

1 29 15

You could do that but the original will keep dying and will continue to lose its capacity. Not sure how those ceramics would hold up to the circuit design. The version I am looking at making will have that larger 2uF changeable so it can be easily replaced

Mark

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Re: 80 HP 912UL will not start

by Andy Fell » 9 years ago

Andy Fell

0 17 0

Yep.. If the original is 2uF though, actually you'd only need a couple of those ceramics there. They can crack of course, but since we then know what to change, it shouldn't be such a big deal? It would certainly be a very low cost fix and get us going again.

As long as the original doesn't go short circuit, it shouldn't be an issue?
Would there be another issue with using a ceramic on the back in parallel with the big storage cap??

I would imagine if putting too much capacitance in parallel then there wouldn't be enough time to charge it up to enough volts before the trigger coil trips it (it's all working quite fast as far as I can work out from the coil arrangement in the rotax documentation)... so there'll be a sweet spot.

Boosting this storage cap makes sense, based on evidence so far of how these die in the field and the measurements you've made down to 0.5uF. Sounds like a good bet to me.

You could do that but the original will keep dying and will continue to lose its capacity. Not sure how those ceramics would hold up to the circuit design. The version I am looking at making will have that larger 2uF changeable so it can be easily replaced

Mark

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80 HP 912UL will not start

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