No Rpm on one magneto

HI.. hope someone might know the answer to this question.

I have an rv7 with an io360. Instruments are Dynon Skydeck digital. On the magneto check, when selecting only left magneto the rpm indication drops to zero after about 3 seconds. However the mag is working fine and engine runs as normal for single magneto. 

The mag is not live.. Engine cuts when both mags selected off.

Had it checked by the lame that does its annual. He says just indication problem as mag is working normally. That is fine but I am trying to understand how it could happen. If one mag is not sending rpm signal, then why would the digital reading not be half the true rpm? 

Possibly because the tach sender is only connected to one magneto, and when that mag is off it will show no RPM.

Thanks. It's only recently done this. Does that mean a failure of tach sender somewhere? 

Thanks. It's only recently done this. Does that mean a failure of tach sender somewhere? 

Quite possibly, or cabling/ connections.  Have you tried talking to the Dynon people?

I have found it gets responses  very quickly both from  other users and from the dynon tech staff. Usually get overnight answers.