G3X fluctuating EGT/CHT temperatures

We had an A&P wired a GDL 39D to our G3X few months ago. All G3X displays seemed fine initially but we noticed some fluctuations in the EGT/CHT temperature readouts with all the indicators went up and down at the same time.

Few weeks ago, the problem got worse with the EGT/CHT temperatures dropped way down to the abnormally low range (100's for CHTs and 500's for EGTs).

My initial thought was that some of the wires or plugs were disrupted when the A&P added new wires. I removed the J732 plug to check all the pins, continuity etc and could not find any broken wires or pins.

Since all the temperature readings move up and down together, I suspect the problem might be due a faulty reference temperature. The G3X P732 connector diagram shows pin 9 for Thermocouple Ref In Lo and 10 for Thermocouple Ref In Hi. However, there were no pins inserted to these positions (ie no thermocouple wires!).


The only thing I could was that the screws to hold the ground wires to the Jackscrew backshell seemed to be not very tight. I tightened the screws then made sure the J732 plug was in tight. This 'fix' seemed to work initially with all the EGT/CHT temperatures back to the normal range for the first flight. The problem came back in the second flight with the fluctuations started shortly after the flight and got worse with time.

I repeated the process one more time and found the same pattern. The 'fix' only lasted for one flight. The fact that all the readings were normal on the ground but fluctuated then dropped to the mud as the flight progress seemed to indicate something was loose and got worse with vibration (and may be heat?).

This is quite puzzling. I'd appreciate all suggestions on how to figure out the problem and/or where to find an avionic shop in NY/CT area. The avionic shop on the field in KPOU does not work on experimental stuffs. The A&P who wired our GDL39D is a very nice guy but he does not seem to be very experience with the G3X (I helped him a bit with the diagram and wiring suggestions based on what I learned from this forum).

Thank you in advance for your comments/suggestions.

The CHT#2 (orange line) is showing an erratic amplitude fluctuation when read using savvy analysis. Looking back on earlier graphs, the fluctuation started a few flights ago, and has been increasing slightly with each flight. I'm thinking that there is an electrical problem with the connection between the sensor and the lead that connects to the EIS module, but I'm not sure. I'm hoping that this is not an indication of an engine issue. Graph attached below for comments. Thanks.

hpl said:

We had an A&P wired a GDL 39D to our G3X few months ago. All G3X displays seemed fine initially but we noticed some fluctuations in the EGT/CHT temperature readouts with all the indicators went up and down at the same time.

Few weeks ago, the problem got worse with the EGT/CHT temperatures dropped way down to the abnormally low range (100's for CHTs and 500's for EGTs).

My initial thought was that some of the wires or plugs were disrupted when the A&P added new wires. I removed the J732 plug to check all the pins, continuity etc and could not find any broken wires or pins.

Since all the temperature readings move up and down together, I suspect the problem might be due a faulty reference temperature. The G3X P732 connector diagram shows pin 9 for Thermocouple Ref In Lo and 10 for Thermocouple Ref In Hi. However, there were no pins inserted to these positions (ie no thermocouple wires!).


The only thing I could was that the screws to hold the ground wires to the Jackscrew backshell seemed to be not very tight. I tightened the screws then made sure the J732 plug was in tight. This 'fix' seemed to work initially with all the EGT/CHT temperatures back to the normal range for the first flight. The problem came back in the second flight with the fluctuations started shortly after the flight and got worse with time.

I repeated the process one more time and found the same pattern. The 'fix' only lasted for one flight. The fact that all the readings were normal on the ground but fluctuated then dropped to the mud as the flight progress seemed to indicate something was loose and got worse with vibration (and may be heat?).

This is quite puzzling. I'd appreciate all suggestions on how to figure out the problem and/or where to find an avionic shop in NY/CT area. The avionic shop on the field in KPOU does not work on experimental stuffs. The A&P who wired our GDL39D is a very nice guy but he does not seem to be very experience with the G3X (I helped him a bit with the diagram and wiring suggestions based on what I learned from this forum).

Thank you in advance for your comments/suggestions.

Click to expand...

Hello hpl,

Your GSU 73 engine monitor does indeed rely upon the type K reference thermocouple that should be mounted in the backshell of the P732 connector and connected to pins 9,10.

Properly installing that reference thermocouple should certainly be the first thing you try.

Thanks,
Steve

Steve,
Thank you so much for your suggestion.

The avionic harness was put together by a major avionic supplier so it is quite puzzling that the installer did not wire the reference thermocouple. According to the manual, the thermocouple kit is included in the G3X installation kit.

I also do not know why the engine monitor worked fine for 5 years up until after the GDL39D installation. Another interesting observation is that the fluctuations reduce significantly with warm weather. In the last few flights, the temp readings fluctuate a little bit on the ground (due to cold hangar?) then become rock solid for the rest of the flight.

I will order the thermocouple kit, wire it in to see if the problem goes away.

Also, check the connections between the probes and the wires to the engine monitor at the engine. Poor connections here often cause problems with the small millivolt EGT signals...

Yes, we checked all the connections several times. The fact that all the readings (EGTs and CHTs) moves up and down at the same time seems to indicate a common source (reference point).

After seeing Steve's post last night (I did not check the forum after not seeing a response after few weeks) we called Garmin this morning. We were told that older G3X units like ours definitely need the reference T/C and newer ones do not. It was likely that the installer assumed that it was not needed and did not use the supposedly included T/C kit.

The Garmin rep was very helpful, he had the kit shipped to us today with no charges.

I do not know whether this forum has the notification feature when there is a reply to your post or new post in a thread which you participate. I will check around to see if such options exist. I wished that I had checked back into this thread earlier.