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 11:54 PM by Shawn26

[MikeOH]

Umm, your thread title is, shall we say, a bit misleading

[McMooney]

I was sooo scared to click, but i did 8)

 

 

[Brandt]

Well, first of all 1600 seems pretty quick to start.  I suggest some rose petals on the hangar floor and asking the Mooney about its day before going that fast.

[jetdriven]

I would check P-lead grounding to start

[PT20J]

Since the engine runs normally, the magneto must be functioning properly. When you say that a 1500 rpm mag check is normal, I assume you are getting a rpm drop when running on one mag indicating that the p-leads are functioning. Nonetheless, I would turn the ignition switch off at idle and make sure the engine stops running to test the p-lead connection.

I’d check the p-lead shields to make sure they are grounded at the magneto.

I’d check the GEA 24 connections. There should be a wire for each p-lead which connect to the ignition switch p-lead terminals through 400 k-ohm resistors and should use shielded wire grounded at the GEA-24 connector and the ignition switch.

Where are those capacitors connected?

[Fly Boomer]

9 hours ago, Shawn26 said:

Recently, I removed my Bendix D3000

No need for triple-posting -- you will get plenty of air time without it.

[MikeOH]

5 hours ago, Fly Boomer said:

No need for triple-posting -- you will get plenty of air time without it.

Yes, erratic postings will do that

[Shawn26]

9 hours ago, PT20J said:

Since the engine runs normally, the magneto must be functioning properly. When you say that a 1500 rpm mag check is normal, I assume you are getting a rpm drop when running on one mag indicating that the p-leads are functioning. Nonetheless, I would turn the ignition switch off at idle and make sure the engine stops running to test the p-lead connection.

I’d check the p-lead shields to make sure they are grounded at the magneto.

I’d check the GEA 24 connections. There should be a wire for each p-lead which connect to the ignition switch p-lead terminals through 400 k-ohm resistors and should use shielded wire grounded at the GEA-24 connector and the ignition switch.

Where are those capacitors connected?

Yes, the P-lead is connected correctly, and the shielding is grounded to the body of the mag. There is no issue with the grounding, I’ve checked the P-lead wire and the shielding for continuity. Mag is also grounded to the engine. There is no issue there. 

Engine shuts down when I turn the switch ( mag ) off. 

the capacitor is connected to the backplate of the mag. 

Garmin install manual recall for 400 ohm resistor. I don’t know how to look for it or check the connection to the GEA 24. 

prior to my 500 hours inspection, everything was working fine, I don’t know what went wrong! 

would you think it’s better to install a RPM sensor ( attached to the mag ) or diagnosed the problem? 

I was told RPM sensor is a better and accurate installation than a P-lead. I think I should find a shop so they can rewire the installation, hopefully this should do it. 

[PT20J]

1 hour ago, Shawn26 said:

Yes, the P-lead is connected correctly, and the shielding is grounded to the body of the mag. There is no issue with the grounding, I’ve checked the P-lead wire and the shielding for continuity. Mag is also grounded to the engine. There is no issue there. 

There should be two p-leads. Did you check them both? Also, check that the engine is grounded to the airframe. There should be a couple of heavy braided ground straps.

2 hours ago, Shawn26 said:

the capacitor is connected to the backplate of the mag. 

I’m unfamiliar with this. Perhaps it was added due to radio interference or something. I woul try disconnecting them.

2 hours ago, Shawn26 said:

Garmin install manual recall for 400 ohm resistor. I don’t know how to look for it or check the connection to the GEA 24. 

It’s actually a 400 K-ohm resistor. There should be two at the back of the ignition switch where the GEA 24 wires connect to the p-leads on the ignition switch. 

2 hours ago, Shawn26 said:

would you think it’s better to install a RPM sensor ( attached to the mag )

I don’t think that's an option with the dual mag. 

You made two changes at the same time: overhauled mags and replaced capacitors. To troubleshoot, I would disconnect the capacitors since the mag seems to work.

 

[Shawn26]

2 hours ago, PT20J said:

There should be two p-leads. Did you check them both? Also, check that the engine is grounded to the airframe. There should be a couple of heavy braided ground straps.

I’m unfamiliar with this. Perhaps it was added due to radio interference or something. I woul try disconnecting them.

It’s actually a 400 K-ohm resistor. There should be two at the back of the ignition switch where the GEA 24 wires connect to the p-leads on the ignition switch. 

I don’t think that's an option with the dual mag. 

You made two changes at the same time: overhauled mags and replaced capacitors. To troubleshoot, I would disconnect the capacitors since the mag seems to work.

 

 

I checked both P-lead, Center conductor, and the shielding for continuity. No issue there. The shielding is directly to the back of the mag, and the mag housing to the engine airframe is the only source of ground. I checked this triple time, I have solid ground contact when I check it to the aircraft body and engine. I do not see any other spot for ground or see any heavy braided ground strap getting attached to the mag. I don’t recall ever seeing one. 

 

I don’t know if I need to reinforce the ground contact between the magneto and the airframe or not really. 

I came across of this post. The arrow marking ( attached ) I don’t have. 

 

http://mechanicsupport.blogspot.com/2010/02/how-to-properly-ground-dual-magneto.html?m=1

 

I installed both old capacitor. I’d the magneto shop to check them and I kept them for reserve. I thought maybe the new one maybe out of parameter. The result is still the same. There is a distortion when RPM reaches around 1600.

 

I never check under my panel for the connection of GEA24 and the 400ohm resistor to the ignition switch because I didn’t touch anything. And to be honest I don’t know what to look for if I want to inspect it and check the resistor if it may gone bad. 

 

I think they make RPM sensor for the bendix dual mag. D3000. I think this is it. 

 

https://www.enhancedaero.com/parts/p/rpm-sensor-for-bendix-dual-mag?srsltid=AfmBOoqDYhwv725m9h0R5eoMJoO9bTZHuW-cpYeQRDREPwmotyilzWHS

 

[N201MKTurbo]

I would check the point gaps. it is tedious to get it correct. I usually readjust them after I send it out for a 500 hour because nobody does it right. If it is wrong the pulse width can be very short. this might be filtered out if the tach has some glitch suppression filtering.

[PT20J]

The ground straps go between the engine and the firewall.

Check the latest version of the install manual, but I don’t think Garmin lists a compatible mag sensor for the dual mag. The one you posted might work and could be considered a minor alteration to the STC if it works.

@N201MKTurbo  might be onto something. You might also give Cliff Orcutt at Aircraft Magneto Service a call. He used to service the dual mags and is quite knowledgeable.

[Shawn26]

13 minutes ago, PT20J said:

The ground straps go between the engine and the firewall.

Check the latest version of the install manual, but I don’t think Garmin lists a compatible mag sensor for the dual mag. The one you posted might work and could be considered a minor alteration to the STC if it works.

@N201MKTurbo  might be onto something. You might also give Cliff Orcutt at Aircraft Magneto Service a call. He used to service the dual mags and is quite knowledgeable.

Yes, there is a ground straps that goes between the engine and the firewall. 
I finally heard from the Avionic shop, he spoke to Garmin and said they believe there is a interfere or some sort of distortion going on, rather than chasing what is causing it since I did most of the troubleshooting ( beside of checking if the 400ohm resistor gone bad or something in that nature) we should rewire the RPM with a sensor. I hope they are talking about the same sensor. I’ve to double check with them again, I hope this would solve my problem. The airplane has been down for about a month  

I will definitely look him up on Google and contact him. It would be good to know. 
 

Thank you kindly, you’re very knowledgeable. You also helped me to find my #3 CHT issue with the baffle, thank you for that as well. 
 

 

[Pinecone]

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth” - Sherlock Holmes

You have checked everything, EXCEPT the magneto itself.....

[Shawn26]

44 minutes ago, Pinecone said:

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth” - Sherlock Holmes

You have checked everything, EXCEPT the magneto itself.....

You’re correct! lol  Mag could possibly be the issue and I was thinking to send it to another place to verify. My mechanic thinks otherwise. Since everything works, engine starts and run normal, he doesn’t think that’s the case. 

 

[fred hamel]

I just finished G3X install today, doing post installation run-up and getting that SAME issue.....

[PT20J]

27 minutes ago, Shawn26 said:

You’re correct! lol  Mag could possibly be the issue and I was thinking to send it to another place to verify. My mechanic thinks otherwise. Since everything works, engine starts and run normal, he doesn’t think that’s the case. 

 

A way to find out would be to put an oscilloscope on the p-leads and compare the waveform with a known good one. 

There is nothing wrong with the dual mag, but they are notoriously tricky to set up properly. Cliff Orcutt told me that he was the only one at his shop that he would allow to touch them and when he started thinking about retirement he just quit accepting them. The problem is that as folks retire, a lot of institutional knowledge and experience gets lost.

@jetdriven probably knows a good shop for dual mags.

[EricJ]

The JPI engine monitor uses a sensor on the dual magneto that senses the magnet rotation.   It works well with the JPI.

[Shawn26]

3 hours ago, fred hamel said:

I just finished G3X install today, doing post installation run-up and getting that SAME issue.....

If you figure it out, please keep me informed.  

[Shawn26]

3 hours ago, PT20J said:

A way to find out would be to put an oscilloscope on the p-leads and compare the waveform with a known good one. 

There is nothing wrong with the dual mag, but they are notoriously tricky to set up properly. Cliff Orcutt told me that he was the only one at his shop that he would allow to touch them and when he started thinking about retirement he just quit accepting them. The problem is that as folks retire, a lot of institutional knowledge and experience gets lost.

@jetdriven probably knows a good shop for dual mags.

I came a cross Continental service bulletin for the bendix d-3000 ( SB651 ) I got to look in to it. 

[fred hamel]

Had the same issue and finally fixed it!!!!

my mag wire shield was not grounded on the magneto side.

[jetdriven]

On 1/14/2025 at 1:24 PM, EricJ said:

The JPI engine monitor uses a sensor on the dual magneto that senses the magnet rotation.   It works well with the JPI.

It counts double RPM and they’re no way to halve it. You Have to use the P-leads.  The single magnetos the JPI RPM sensor works great 

[EricJ]

5 hours ago, jetdriven said:

It counts double RPM and they’re no way to halve it. You Have to use the P-leads.  The single magnetos the JPI RPM sensor works great 

My JPI900 works fine with the sensor on top of the dual mag at the magnet.   There is no connection from my JPI to the p-leads.

[jetdriven]

Yes, because in the JPI you just said it to eight cylinder engine and it cuts the pulses in half. Same thing with the 830. But the Garmin has no setting and I emailed them trying to get them to do it and they said since it wasn’t TSO they weren’t going to anyway.