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 Monday at 11:56 PM by Shawn26