Sleepy boxes (Ducati ignition modules)

Not knowing the circuit or the components within the modules, I can only make a broad assumption based on my experience in the electronics industry.

The one electronic component in particular that does not like being unused and without power is the electrolytic capacitor.

A bad analogy to the capacitor is a battery.  The capacitors died, or at least, went into hybernation.

If the next lines don't make sense to you, thats OK.

If left without voltage across the terminals, some capacitors may loose their form and become leaky.

When voltage is reapplied, the dielectric will re-form and the current will stop leaking.  This can take a short while.

A capacitor should be able to remain unpowered for years however.  So the prognosis is not great.

What Bill said and it's possible that one of the ignition modules stating circuit is failed or failing. You may need to replace both ignition modules. Out of curiosity what year are those modules or engine mfg year?

Thanks Bill.

A Rotax engineer told me that often one box out of a pair will begin to fail at engine start. The engine starts up on the working box alone and when rpm is increased the faulty box is re-energised and starts working again. By the time the pilot gets to performing his ignitions checks both boxes are seemingly working normally so the problem goes unseen. It is only when the second box begins to fail that suddenly the engine won’t start at all. It seems as if both boxes have failed at the same time but actually one of the pair has been disguising the fault of the other for some time. This I can understand now as I thought it far too much of a coincidence that two boxes would fail at the same time. 

Is there a life on the Rotax boxes? I have just tested two boxes off an engine with only 300hrs on it and both were showing faults. Of course in terms of years the engine and boxes are much older at about 10years.

Do the boxes have a shelf life and what factors if any may shorten the life of them (eg heat, vibration etc)?

Funnily enough just a week later after testing this pair I attempted to start a 912ULS on a flexwing trike that had been sat idle for some time having maintenance work done and this one would not start either. I went through everything else to trouble shoot and then thought it couldn’t hurt just to swop out one of the boxes and low and behold it started up first time! 

In fact both owners had mentioned their engines had been proving difficult to start for a while. I’m guessing that if one box has been deteriorating and the engine has been starting on just one box this would result in less clean starts. Of course they would never assume the boxes were at fault because by the time they got to checking them they would both be working! 

Hmmm, I can’t help but think that a sleepy box is a broken box in waiting. It must only be a matter of time? 

Hi Roger

The flexwing trike was built in 2006. The Eurostar whose modules failed was built in 2011 but has had a second hand engine installed which I suspect would have been older.

I would say that regardless of the engine hours run the majority of the sleepy modules where both fail to start are on aircraft of 10years old or more. 

A friend also purchased a pair of second hand modules as spares. These sat on a shelf in the workshop for some time. When he installed them in an aircraft as replacements for a faulty pair they would not start the engine either. He was upset that he had been sold a duff pair but when we swopped in a known working module both of them started working again. 

It is becoming such a regular thing now that I am planning on building a test rig so I can test modules and run them up to speed to wake them up on the bench rather than faffing around swopping them out on an aircraft. 

In a normal year the double module faults would only really occur during winter lay offs but this year with the Covid lockdowns I’ve encountered quite a few during the warmer months.

In thinking more about the circuit in general...

The Generator consists of 8 ALternator Coils and 2 Charging Coils.

The Charging Coils are the Power supply to the Electronic modules.

As they will be producing AC current it would be reasonable to assume that they connect to a rectifier and a relatively Large Storage/Filter Capacitor.

If this capacitor is electrically leaking across the diaelectric it could intially lose power faster than the Charging coil can supply it.

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If you were to measure the resistance from the Charging Coil lead (Oragne Wire) on the Unplugged Module connector to ground, of a recently operating module and compare it to a known intermittent module that has been unused for a significant period,  I suspect the the Good module will display significantly higher resistance than the Bad modules, And the bad module will improve its measurement after a period of operation.

The leaking Caps will show a lower resistance than the fresh capacitors.

Note: Leaking iin this case refers to current leaning through the dialectric, NOT liquid dialectric leaking out of the capacitor.

If you choose to try this meassurement, you may have to leave the ohmmeter connected for a minute, more or less, for the measurement to stabilize as the ohmmeter charges the capacitor.

Expect to immediately see a near short that slowly become a high resistance that eventually settles to a stable number.

The value of signifcance is the final reading.