Garmin G3X RPM erratic issue after Bendix Dual Magneto D4LN-3000 / 10-682555-11- 500hrs inspection.

[Shawn26]

Recently, I removed my Bendix D3000 dual magneto for a 500 hour inspection. During the process, the gears were replaced, and I purchased two new capacitors (KA10-400574). After reinstalling the magneto, I noticed that whenever I exceed approximately 1600 RPM—whether on the Left, Right, or Both magnetos—the RPM reading becomes erratic. Despite this, the engine shows no issues with timing or starting, and it runs perfectly fine even with the erratic RPM readings. When performing a mag check around 1500 RPM, everything appears normal.

My Garmin G3X with EIS reads the RPM through the magneto P-leads. I don’t have a separate sensor on the magneto or a mechanical RPM sensor. Initially, I suspected the issue might be related to a grounding connection. However, after checking continuity, I confirmed the magneto is grounded, and all connections appear correct. 

To troubleshoot further, I reverted to the old, functional capacitor and even pulled the alternator fuse to rule out interference from the alternator, but the erratic readings persist. 

I contacted Garmin support, but they were unable to assist and suggested I reach out to the avionics shop that installed the system. Unfortunately, the shop is also unsure of the cause. The magneto shop assured me the unit was properly bench-tested and functioning correctly, so I don’t believe the issue originates there.

I’m wondering if anyone has experienced a similar issue or could help diagnose the problem. The aircraft is currently located at Van Nuys Airport, and since a functioning RPM gauge is mandatory, I can’t fly it to another shop. Obtaining a ferry permit has also proven to be quite difficult.

If anyone has insights or is available to help troubleshoot this issue, I would greatly appreciate it and willing to compensate for their time. I've also uploaded a video of the issue on YouTube for reference.

Thank you! 

https://youtube.com/shorts/umHdKcL2wxQ?si=nLp6FBlf1nmhfDjC

Edited January 13 by Shawn26

[bigmo]

I don’t know your setup, but do you have a GEA24 as the EIS interface? My previous a/c had twin G3X and anytime I had G3X reading anomalies, it was either a Molex connection that needed to be reseated (just crappy Molex connectors), or the connections at the GEA24 needed to be pulled out, reseated, and securely fastened. The Molex I cleaned with contact spray, then dielectric paste. For the GEA24, I just reseated them. That fixed it every time.

[skykrawler]

Something else seems to have changed.    On airplanes with two mags I have seen the connection for the Garmin EIS connected right at the magneto screw with the P-lead.

The dual mags have a knurled nut that connects the P-lead directly to the capacitor.  So the EIS must be connected to the mag switch or some other thing (read bigmo above).  So what he says makes sense.   I think these systems just sense the induction from the capacitor since there is no real voltage involved when the mag is not grounded by the switch.

 

[jetdriven]

There is also this 

Tom sb651 dual magneto capacitors..pdf

[Shawn26]

2 hours ago, bigmo said:

I don’t know your setup, but do you have a GEA24 as the EIS interface? My previous a/c had twin G3X and anytime I had G3X reading anomalies, it was either a Molex connection that needed to be reseated (just crappy Molex connectors), or the connections at the GEA24 needed to be pulled out, reseated, and securely fastened. The Molex I cleaned with contact spray, then dielectric paste. For the GEA24, I just reseated them. That fixed it every time.

Thank you. I’ve GEA24. I don’t know what to look for when I’m under the panel. Should I find a shop to do it for me? I don’t want to mess up something. 

[bigmo]

Each plug should look a bit like a computer plug with a knurled screw on each side. I found them prone to backing out. I just loosened them, unplugged and replugged the plug. Then securely closed each screw by hand with a little extra torque by a small flat blade screwdriver.

 Check ALL of your connections on the engine side too (those feeding the EIS). Open, spray with contact cleaner, put a little dielectic on them and close them. 

I found the G3X didn’t know how to deal with little fluctuations in signal, so it would be whacky for a while and then go red-x or out of range.

[Aaviationist]

What you need to know is where and how the mag is interfaced. Is it straight from the P Lead (based on the symptoms this is what I suspect) is there a Hall effect sensor?  Other?

 

in the case of direct p lead connection the newly rebuilt mag may be outputting a higher voltage and needs a resistor inline to compensate. 
 

alternatively, what fuel are you using?

Edited January 15 by Aaviationist