Electrical interruption and RPM gauge going haywire

Did you put the recommended resistor inline with the tach signal wire as recommended by Rotax?

I thinkthat the problem is due to the supply not the signal, the OP reported supply failures of other pieces of equipment, notably the GPS reporting a loss of supply.

I am curious about this resistor that is meant to be fitted inline with the tacho. I've just checked the heavy maintenance manual and the diagram there shows a direct connection to what they have labelled as the 'rev counter'. What is the purpose of the resistor? I don't seem to have a resistor fitted in my aircraft. Perhaps it is only required when using specific types of tacho?

My comment about the inline resistor recommendation was not entirely accurate; the recommendation came from the Dynon installation manual for installing a Skyview system with a Rotax 912 engine. Erratic tach readings with this system can frequently be solved with the addition of a 69k resistor inline with the tach wire. After about 500 hours of trouble free tach readings on my Skyview/Rotax 912 system, I started to develop erratic tach readings. Installing the resistor solved the problem completely and I am now at 950 hours.

Ah that's interesting. I think that the Dynon input circuit must be more sensitive to the pulses generated by the tacho sender. Does the erratic behaviour only occur at higher RPM? The additional resistor probably creates a potential divider to reduce the amplitude of the pulses. I think that I recall a friend who had to insert a resistor to his Dynon too.

Another correction to my sloppy typing; its a 60k resistor, not 69k.

Yes Kevin, the erratic readings were usually at higher rpms, but could also occur randomly at any rpm. What has always been a puzzle to me was how could it work perfectly for 500 hrs, the start going bad. Before I added the resistor I checked and cleaned all connections thoroughly several times with no improvement. I have heard of others with similar stories. I would recommend to anyone installing the Dynon Skyview/Rotax system to just go ahead and install the resistor right away. It apparently does no harm if it is not needed, but can save a lot of headaches later on.