Tacho reading fluctuating

IO-360 with dual P Mags

Dynon D 120 EMS

Tacho varies around 150 rpm with no change in manifold pressure or sound - looks like indication problem.

From memory, the pick up for the tacho is from the P Mag - counting pulses. No other changes to set up in last months.

Any suggestions for troubleshooting or anyone had it before ?

New?

If this is a new installation, then I would look to the Pmag documentation for the diode used to change +9 to +5.

I don't know about Dynon, but on my AFS system there were 2 possible pins for the tach signal.

Finally, if the Pmags have been sitting around for a few years, it would be worthwhile sending them in for a refresh.

Thanks - it is an established system, running for a few years without issue. Only thing may be rain encounter - will check terminals.

The RPM display on my D-180 also varies. I consider it normal. The RPM is bound to change with airspeed changes. The airspeed will change with altitude changes. Even if the altitude is held absolutely steady, the aircraft is constantly climbing and descending in updrafts and downdrafts to maintain that stead altitude.

Typically the D120 will get RPM inputs from both left and right. I suggest that you turn off one of the RPM inputs (through settings menu in D180) and observe behavior while only running on the other then reverse the process. At least then you will know if the issue is related to one of the Pmags / wiring or the D120 itself.

Was the D120 installed with Mags, then upgraded to Pmags? If so, be sure that the resistors were removed from RPM feed circuits.

Larry

Coming up to annual so we will look at all the connections etc. The airplane was originally built with P-Mags and install was as per instructions.