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Erratic RPM on IO-390 (dual PMags and G3X)

samgaddis

Member
I’m getting erratic RPM throughout the flight. I looked through the PMag troubleshooting manual but didn’t get clarity on where I should start first. I’m not even clear which wire I should be looking at (I didn’t install these). Any ideas where to start?
 
The RPM comes from a wire on the pmag to your EFIS. Do you have both pmags connected to your EFIS? Are they both showing RPM fluctuations? Did it just start becoming erratic?

There have been cases of wires in the pmag connector coming loose - what seems to help is to put a ferrule on the wire before screwing it into the pmag.

Some additional discussion:

https://vansairforce.net/community/showthread.php?t=101091
 
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Erradic RPM

I’m getting erratic RPM throughout the flight. I looked through the PMag troubleshooting manual but didn’t get clarity on where I should start first. I’m not even clear which wire I should be looking at (I didn’t install these). Any ideas where to start?

Are you HEARING erratic RPM? Or is it just the RPM indicator? Look at the simple things first. Wiring, timing, etc...
 
Normally this combo is rock solid. Start looking for a lose wire or connector.

The wiring diagrams for the Pmag are available on their website.

If it is a dual setup, only one of the Pmag's will have the tach signal connected to the G3X system.
 
It’s definitely electrical as I’m not hearing any change in the RPM

See attached pics -- Check the wire at position #6 on each PMag (green connector). Is it securely seated, can you pull on it and does it come loose?

From there, check the wiring to the GEA 24, J243 (37 Pin "D-Sub") pins 6 (RPM2) and 8 (RPM1); is the wire securely attached to the socket? is the socket securely installed into the connector/backshell?
 

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See attached pics -- Check the wire at position #6 on each PMag (green connector). Is it securely seated, can you pull on it and does it come loose?

From there, check the wiring to the GEA 24, J243 (37 Pin "D-Sub") pins 6 (RPM2) and 8 (RPM1); is the wire securely attached to the socket? is the socket securely installed into the connector/backshell?

Sure enough, the wire going to the mag was loose and popped right out when I pulled it. It looked as though is was secure before pulling on it.

Thanks so much. You saved us from a trip cancelation!
 
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