Dynon Skyview - P-Mags - Erratic tach readings

I have a Dynon Skyview (non touch) and two P-Mags. Feed is from the "tach" connection on the P-Mags.

After 170 hours of steady RPM readings the tach has become unstable, varying by as much as several hundred RPMs. I can get "engine speed" warnings at 2500 rpm, not a distraction I need. The engine itself is not varying in rpm...

I have checked the P-Mag plugs, splitter plugs, etc., nothing seems loose. The only change I made about the same time was the addition of another splitter to install a "Knob Panel".


Dynon suggested experimenting with varying resistors in the feed line...

Someone else on VAF suggested Zener diodes between terminals 1 and 6 on the P-Mags...

Someone else indicated that nothing was needed...


Why after 170 hrs would the readout suddenly become erratic?

Has anyone else experienced this problem? If so how did you solve it?

I'm reluctant to start putzing around until I figger out why this has changed...

What signal are you splitting, the tach?

How about a simple check, move the tach signal wire from one Pmag to the other. You only need the signal out of one Pmag, not both.

How are your tach signals below 700 RPM?

If they are erratic, pull the breaker on your ignitions, one at a time. This will tell you which ignition has the problem. (If your P-[mags are wired per their installation manual, the only way to turn them off completely is to be below the self-power cutoff and then remove power from them.)

I assume you have each P-Mag Tach signal connected to different SkyView EMS pins. In my case the Lt Mag tach signal is connected to pin 32 while the Rt Mag tach signal is connected to pin 33. If so then install the Zener diode as per the P-Mag install instructions. I had the same issue and when I finally installed the supplied Zener diode it fixed the problem.

Thanks for the replies...

The "splitter" referred to is a network splitter for the knob panel. I have several splitters in the system which were used, rather than a hub, based on equipment configuration and the advice of someone at Dynon...

Both P-Mags are connected to separate pins into the Skyview as outlined in the Dynon install manual. I'm not sure if it compares signals, or just uses the other if one goes away...

I did not receive zener diodes with the P-Mags although I noted in the install manual that they are an option. I have no idea what value is appropriate.

I'm still curious why the tach reading would be rock solid for 170 hrs then suddenly become erratic...

I've contacted E-mag air so we'll see what they say...

Words of wisdom are appreciated...

They are just 5.1v Zener diodes you can buy at Radio Shack at about $1.00 each. Mine worked fine for about 30hrs then sporadically would go erratic. The diodes did the trick.



Also, do you have a wire from the P-Mag ground (pin #1) to an EMS ground pin (like SkyView EMS pin #30) in addition to grounding the P-Mag to the engine block? This also helps provide a clean tach signal to the EMS preventing erratic RPM indications.

KISS

When diagnosing electronic faults... I avoid jumping too far ahead. Looking at diodes, and other component additions.... consider the facts. It worked for 170 hours. The plane vibrated during those hours. Rather than a design fault, it probably is as simple as an intermittent connection.
I know on my P mags, I have to pull the D connector on the sensor every 50 hours or so and re-seat it. I use contact cleaner at the same time.
The voltages are TINY. The effects are large.
I would start with the simplest theory... and ring out all your connections.
Keep us posted.

Reset the timing

I have a G3X system and had the same issue. After about 90 hours began having erratic tach readings with dual pmags. Tried swapping wires for the tach inputs and it did not work. Went through the timing set up process and all was back to normal.

Mark

I took the wiring connectors out of the P-Mags today, tightened the wire clamps and sprayed the pins and sockets with contact cleaner.

I ran the engine and the tach readout had improved considerably, from several hundred variation to occasional 10 variation. This is what I am used to seeing and hope it continues.

Brad at P-Mag suggested a direct ground from the P-Mags to the Dynon chassis. I will try this if it happens again...

Thanks to everyone for their suggestions...

Ron RV8 said:

...
Brad at P-Mag suggested a direct ground from the P-Mags to the Dynon chassis. I will try this if it happens again...
...

Click to expand...

That will not hurt but do not remove the ground from the P-mags to independent engine case bolts. (Assuming you have yours wired that way, if not, please ground them to independent case bolts before your next flight.)

Ron - I always use the grounds from an EMS system (& use the 18awg ground to engine block), that's why I don't see any instability & therefore do not have to use the zener diode.

"Someone else indicated that nothing was needed..." that was me, I have installed many P-Mags