Spurious high RPM warnings

[N2843R]

I have HDX installed in my 182 with O-470-R engine. I am getting high RPM warnings ("Engine Speed") but there is no overspeed. The RPM display is steady and, I believe, accurate, but I notice periodic glitches (fraction of a second) showing overspeed. I was told by a Dynon rep (booth at an air fair) that the fix is to install a resistor in the sensor line. Has anyone experienced this and what would be the exact fix?

 

[benapilot]

In order to properly troubleshoot the issue, we need to know what signal it's reading. Is this a new problem, or since installation?
If off the P-lead, here is the install manual verbiage:

12.2.9.2 P-lead pickoff (Lycoming and Continental)

If you do not have a dedicated tachometer pickoff, you

must follow the instructions below.

Use the two (2) included 30 kΩ resistors (color bands:

orange, black, brown, red, brown; connect in either

direction) to attach left and right P-leads to the standard

RPM Left and RPM Right inputs on the SV-EMS-220.

Connect them as shown in the figure to the right. It is

important to connect each resistor as close as possible

to the spot where you tap into the P-lead. This minimizes the length of cable carrying high

voltage spikes.

Six (6)-cylinder Lycoming engines sometimes need more inline resistance

to prevent false readings by the SV-EMS-220. If you supply your own resistors, they need

not be exactly 30 kΩ. Additionally, both 1/4W and 1/2W resistors are acceptable to use.

If, after setting the PULS/REV R and L values as described in the Engine

Information Section, you see unstable RPM, you may need to increase the

series resistance. We recommend increasing the resistance in increments of

approximately 30 kΩ – approx. 30 kΩ, approx. 60 kΩ, approx. 90 kΩ, approx.

120 kΩ, and approx. 150 kΩ. Try each value in sequence. A higher resistance

value is not necessarily "better", as too high a resistance can reduce the

amplitude of the RPM signal to the point where you may see no, or low, RPM.

I highlighted the section that may be the solution to your problem.

 

[vlittle]

If this is a new problem, spurious RPM can be caused by a fouled plug or a failing plug wire.