This topic comes up from time to time and my belief is that if Rotax and Ducati
felt that there was a problem, then they would have put out a written warning
years ago. Yet there are many who swear that these ACS key switch units cause
ignition and rough running problems. What's the proof of this?
Here's an opinion which someone passed on to me, because he's about to change to
separate toggle switches for grounding his Ducati ignition modules:
"My local airport is KGVT, Greenville, Texas. It is home to the defense
contractor
L3 Mission Integration Systems. L3 employs thousands of engineers. I am
fortunate
to rub shoulders with many of these engineers both active and retired.
I spent much of the day with a pilot/electrical engineer who has over 35 years
designing and specifying aircraft electrical systems. We discussed the magneto
switch when used with the Rotax Ducati ignition system. He agreed the notion
of a magneto switch causing a failure of the solid state module with some type
of "feedback" was poppycock. He also agreed the AC voltage on the Ducati kill
circuit was less damaging to a switch than the P lead circuit on a magneto
ignition.
Lets look at the facts, how the system operates, and possible failure modes.
The Rotax Ducati ignition uses a shorting wire to kill the ignition. It is only
grounded on engine shutdown and during ignition checks. The shorting wire is
grounded through the mag switch. If the shorting wire is disconnected or breaks,
the engine continues to run. The only electrical things that could damage the
ignition module are overvoltage and improper or shorted wiring.
The Rotax engine dynamo supplies power to the modules. Voltage is limited by
engine
rpm. Current is physically limited by the size of the dynamo coils. It is
impossible for the dynamo to supply excessive voltage or current to the modules.
The vast majority of Ducati ignition module failures are wires breaking inside
the insulation close to the module due to vibration.
The premise that AC voltage present on the grounding circuit is too high for a
magneto switch is unfounded. The voltage involved when grounding a Ducati module
is similar to the voltage present on the P lead of most magnetos. The internal
arcing with AC voltage is less than what would be present with a pulsed DC
signal from a magneto. There is no reason to assume one or more toggle switches
will last any longer than a standard magneto switch.
While we have no doubt some standard type magneto switches have failed while in
service on the Rotax engines, we do not believe they are the cause of ignition
module failures. If the contacts on the magneto switch were to corrode or burn
to the point where they no longer make continuity, the respective module would
simply not shut down with the switch in the off position. I dont think LEAF,
Lockwood or anyone else can explain how an open kill circuit due to a bad switch
is the cause of ignition module failure.
Advising aircraft owners to change the magneto switch to toggle switches is
questionable.
The magneto switches are aircraft quality devices with large contacts
capable of handling high voltage and current loads either AC or DC. Magneto
switches are used successfully on many Rotax 912 equipped aircraft without
incident.
The replacement toggle switches are largely unspecified. A 250 volt toggle
switch can be a totally inadequate miniature device capable of only 0.5 amps.
with quality or origin unknown.
In summary, the Rotax Ducati ignition module kill circuit is not likely to
damage
a magneto switch or a replacement toggle switch. Nor is any switch, magneto
or toggle likely to cause a failure of the ignition module.
John Pitkin"
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http://forums.matronics.com/viewtopic.php?p=476797#476797
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