Spurious tach indications with engine OFF


Apr 26, 2013
Fort Mill, SC
I'm finalizing my Dynon Skyview in preparation for my first engine start. I've used a VP-X Pro, and have dual P-Mags. The Dynon manual says both tach sensor wires can be used, since the SV will use the first value it sees.
The P-Mag manual says "Collect your tack (sp?) signal from only one, not both, ignitions unless your
instrument has provisions for two (separate) inputs
I started with both sensors connected, but saw many spurious spikes in the tachometer ...WITHOUT THE ENGINE RUNNING.
The spikes were occurring about every 5-7 seconds, more or less, and ranged from <100 to >1600 rpm.
Today I disconnected the right tach sensor wire, and the spikes are much less prevalent, but still occur.
Any ideas about whats causing this? :-/


Dynon Staff
Staff member
Jan 14, 2013
Woodinville, WA
So SkyView does have provisions for two separate inputs, and we take both so that we can still give you RPM in the case that one of your ignition devices fails. If it's a modern p-mag, it should have a 12V output, and so you want to make sure you're using pins 32/33, not pins 34/35.

Beyond that, the EMS is counting the amount of times that the voltage is getting above its counting threshold, and turns that into RPM. So something is tickling those wires with voltage, and SkyView is counting it in the form of RPM. You might disconnect the other side just to make sure that it's coming from outside of SkyView and the EMS module, for starters. Beyond that, though, I don't have a hunch on where that voltage might be coming from.