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Inaccurate RPM readings usingf Lightspeed Plasma II+ digital

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vshimsl(at)live.com
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Posted: Sat Jul 11, 2009 4:36 pm Post subject: Inaccurate RPM readings usingf Lightspeed Plasma II+ digital

I have the digital pulse information from my Lightspeed plasma II+ electronic ignition feeding one input to my Dynon D-180 and a magneto P-lead tap through 30 k resistor (per Dynon manual) to the other RPM input of my Dynon D-180.



Problem is that with the P-lead Pulse per revolution set to 1 C and the Plasm II+ Pulse / rev set to 2 C my Dynon reads 2 to 3 hundred rpm low at 900 rpm (verified by Tru-Tach) and 4 to 6 hundred low at the high end (2100 to 2700 rpm).



I removed the Dynon and using a square wave generator providing 12 volt pk-pk referenced to ground tested both inputs of the Dynon. It is right on. I also dropped the voltage down to as little as 8 Volts Pk-Pk and the Dynon remained accurate.



Suspecting that the Dynon averages both inputs C I tested each input by removing the other. Both the Plasma II+ and the P-lead were reading low rpm's compared to a tru-tac (which can be calibrated by aiming it at flourescent lighting...pretty slick).



By Resetting the pulse rate of the Dynon to 1.5 for the Plasma II+ C I get readings that are consistently 60 rpm low.  The P-lead with the recommended Dynon resistance of 30K reads 2 to 300 low.



My temporary fix at the moment is to remove the P-lead input to the Dynon from the Magneto (Slick brand via aerosport) and keep the Plasma II+ input set to 1.5 pulses / rev even though the Lightspeed manual says it should be 2 pulses.



Has anyone else been down this road? If so would appreciate hearing the solution before I haul a scope to the airport.



Regards C

Vince_Himsl

RV8 N8432 Flying! (10 hours)

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