Angier,

 

There is a document produced by VM that describes a diode or diode-resistor combination (depending on which Plasma system) to isolate the tach signal.  Next time I am out at the hangar I will get that doc and send it to you.  The point is that you may consider a diode on the tach to MVP line to stop the backfeed.  Remember, it is 2 pulses per revolution.

 

I never installed a switch to select which LSE would provide the tach signal since the primary (also connected to the LCD display), when switched off during engine preflight, provides no tach display or data to the engine instruments for the RPM.  Thus, should I lose the primary LSE in flight, I have no RPM information.  How did you resolve this?

 

Scott Krueger

7 years on dual Plasma III

 

N4ZQ has dual Lightspeed Plasma III ignition.
Each module is grounded directly to its respective Odyssey battery.
Each module gets power through its respective switch by way of a 5A breaker, directly from its dedicated Odyssey battery.
Both Plasma III modules are factory wired to the panel LCD display which shows RPM, MP, and ignition timing degrees for each unit.
I couldn't figure out why the LCD display showed 12-14 RPM when the Plasma III's were energized but the motor wasn't running. Through process of elimination, each system was powered down without effect until I pulled the MVP-50 breaker. At this point, the Lightspeed LCD RPM display dropped to zero. I should point out that each Plasma III module outputs a 5v square wave for RPM and these outputs are received on pins 25 & 26 of the MVP-50 bottom EDC connector. So, the Lightspeed LCD display was receiving a miniscule voltage from the MVP-50. Whether any of this matters remains to be seen when I light off the motor for the first time...

If anyone on this list has a 4cyl Lycoming, dual Lightspeed Plasma III with Simpson LCD display AND an MVP-50, please contact me directly.

Thanks,

Angier Ames
N4ZQ
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