On Mar 2, 9:05*pm, Andy wrote:

Anyone that designed a system to transmit a critical parameter as a
fraction of a reference voltage but did not also provide the reference
voltage *to the receiving system would be better employed flipping
burgers. *In other words, a properly designed engine speed monitoring
system would be independent of the battery voltage.

RFI could have strange results though. *Maybe the incorrectly
installed capacitor allowed RF on the radio power line and that
coupled into the engine monitor.

I'd generally fully agree with you on this one, but there is no signal
outside of the electronics box (case mostly metal and made by Ilec
BTW). Except that you can connect an external voltmeter to act as RPM
indicator on older units and on new ones there is perhaps an internal
DVM module to do the same.

The box senses the RPM as ignition pulses via the earthing lines from
the two ignition modules (coils) and does the conversion internally.
Depending on said signal three LEDs are lit to indicate in which PRM
range (green, yellow, red) the engine is turning and the ignition is
switched off (earthed) when a threshold of ~7000 RPM is exceeded.

The above does contain some informal reverse engineering concerning
the internal workings of the box on my side to be honest but i'd
happily buy you all a round if i'm wrong.

(IMHO it's much better to try to contain the RF in the radio and
antenna cable than try to harden everything in the glider against it.
Otherwise i'd end up with tinfoil around my b**** :-)