G3X Erratic RPM Indication

[Auburntsts]

I have recently developed erratic RPM indications on my legacy G3X. This developed on a long cross-country flight over Thanksgiving and was duplicated on a local flight this weekend. The sensor is mounted on my right mag (Slick 6350) and has 202 hrs on it. Here's the symptoms:

-- At full power RPM rapidly swings from the mid 2300's all the way to 3000 regardless of prop gov setting.

-- The prop gov (PCU5000) is functioning properly and the RPM isn't actually changing, just the indication.

-- After startup and all the way thru run-up, the RPM indications are normal, then on application of full power on takeoff the erratic behavior begins immediately as the RPMs come up to/near redline.

-- Indications return to normal at idle once on the ground, but will go wonky if power is reapplied.

-- The right mag itself seems to be functioning normally, although I have not attempted a high powered mag check.

-- Externally, nothing is loose or frayed.

This behavior happened once before on a very bumpy local flight back in April (about 50 hrs TT ago) but disappeared on all flights until now. I thought back then that the turbulence might have jarred something in the wiring from the sensor to the ADAHRS, but the indications didn't reappear until now. When it did I was in cruise and the air was smooth
(8000ft, ~21MP, 2350 RPM, highest CHT was ~368 (normal for me), and oil temp ~186).

I have a blast tube on that mag although the sensor doesn't really benefit directly due to the location of the sensor on the mag which places it under my backup alternator on the vacuum pad.

Does this sound like the sensor is dying or something else?

[maus92]

Could be a loose connection, a failing wire, or a failing sensor. I removed my right mag for its 500hr check, and discovered that the RPM Hall sensor was loose / not tight. I sometimes experienced a mildly fluctuating (10-20) RPM indication, which I wrote off to instrumentation error, but maybe it had to do with the loose pickup sensor. I'll find out when I'm done with my MLU.

[Auburntsts]

Originally Posted by maus92

Could be a loose connection, a failing wire, or a failing sensor. I removed my right mag for its 500 check, and discovered that the RPM Hall sensor was loose / not tight. I sometimes experience a mildly fluctuating (10-20) RPM indication, which I wrote off to instrumentation error, but maybe it had to do with the loose pickup sensor. I'll find out when I'm done with my MLU.

The sensor is tight and nothing is loose FWF. I have not looked at the ADAHRS connection but if that's loose it would have to be the individual pins in the D-sub and not the whole connector. I'll definitely check that before swapping out the sensor.

[Jimd]

I occasionally have the same problem with my g3x. It seems to only occur when I use full power in the Climb and keep the power up as I level out. It seems like if the rpm is in the 2500 rom range for a while as I level out it starts this behavior as described in the original post. I don't know for sure, but if I recall it does return to normal operation if I power way back and then stay below 2400rpm. Also, if I am staying in the pattern or not "going someplace " meaning that I don't push the throttle as hard the problem will not happen.

I think this started to occur on my install shortly after installing the system.

I'm not sure where to look, seems like some sort of electrical crosstalk to me.

My rpm signal comes from my slick/LASAR mag unit.

[vanduyn]

I had the same erratic indications as you. Tracked the problem to a lose connection on one of the sub-D pins. Problem fixed.