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  #1  
Old 09-11-2017, 06:55 PM
Rupester Rupester is offline
 
Join Date: Dec 2005
Location: Mahomet, Illinois
Posts: 2,161
Default PTT causes EGT displays to change ... ???

The other day I noticed for the first time that when transmitted on the comm frequency, the EGTs all changed. Huh? When I let up on the PTT button, all the EGTs returned to their former temps. I don't know if this has been happening for awhile and I just never noticed before ( I'm nearly always looking outside when I talk to ATC, not at the engine data.), or if this is a new phenomenon.
When running LOP with EGTs in the -40 range, they all go to around -50 if the PTT button is depressed briefly. They go to around -60 and seem to top out there when I hold the PTT down longer.
Upon landing, when my EGTs were all showing about 400 degrees on the rich side, the numbers also get bigger on PTT and still about the same 10 to 20 degrees.
CHTs do not change, nor does any other engine data that I can see.

A search of these Forums didn't produce anything. Anybody got an idea of why this might occur?
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  #2  
Old 09-11-2017, 08:18 PM
Bevan Bevan is offline
 
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It's probably always been happening but you've not noticed it yet. Your eyes have been outside and that's a good thing.

My fuel pressure fluctuates too when I push PTT. It likely has to do with RF getting into the sender or associated wires bringing that signal into the engine monitor or whatever you're using to display the info.

I never noticed it until one day I moved a comm antenna which included extending the feed line. Then the fuel pressure value increased (on PTT) to the point where it tripped my high fuel pressure alarm. That has since been deleted. I suspect the addition of the connectors to lengthen the feed line has caused some leakage or reflection back to the radio causing more RF to be present in the cockpit.

Bevan.
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  #3  
Old 09-11-2017, 08:22 PM
Bevan Bevan is offline
 
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It's probably always been happening but you've not noticed it yet. Your eyes have been outside and that's a good thing.

My fuel pressure fluctuates when I push PTT. It likely has to do with RF getting into the sender or associated wires bringing that signal into the engine monitor or whatever you're using to display the info.

I never noticed it until one day I moved a comm antenna which included extending the feed line. Then the fuel pressure value increased (on PTT) to the point where it tripped my high fuel pressure alarm. That has since been deleted. I suspect the addition of the connectors to lengthen the feed line has caused some leakage or reflection back to the radio causing more RF to be present in the cockpit.

Bevan.
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  #4  
Old 09-11-2017, 08:40 PM
Rupester Rupester is offline
 
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Thanks for the info Bevan .... very interesting. I too have GRT gear - 4000 Engine Monitor and dual Sport displays. For comm I use an SL-30. What you say makes sense (.... well, as much sense as you can make with electrical stuff to a mechanical engineer.
The puzzling part is why the CHTs also wouldn't reflect that interference.... those leads run in a bundle with the EGTs. Hmmmmmm.
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  #5  
Old 09-11-2017, 08:46 PM
Mich48041 Mich48041 is offline
 
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Check all ground wires, especially going to the engine. The engine mount should NOT be used as part of the grounding circuit.
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  #6  
Old 09-11-2017, 08:51 PM
Aluminum Aluminum is offline
 
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Location: San Jose, CA
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Quote:
Originally Posted by Rupester View Post
when transmitted on the comm frequency, the EGTs all changed. Huh?
My Dynon SkyView EMS does this too. I have a FL760 mounted inches away from the EMS module. All the RF wiring is very well shielded so I suspect something to do with grounding, or leakage from the radio body itself. I don't worry about it.
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  #7  
Old 09-11-2017, 09:10 PM
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airguy airguy is offline
 
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Dynon EMS did this on the early units - they came up with a filter to solve the problem but the EMS box had to go back to them to install it, no charge. It's RF power getting into the EGT/CHT sensor loop.
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  #8  
Old 09-11-2017, 09:12 PM
tim2542 tim2542 is offline
 
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These little guys fixed the same problem for me:
https://tinyurl.com/y9cpowho
Both my comms on certain channels caused fuel pressure and egt to move. I'm confident it's RF getting into the senders or wires. Anyway, the chokes fixed it nicely. One small one over the fuel pressure wires, and one large one over the entire CHT/EGT harness just before the D-sub connector.
GRT 4000 EIS.
Tim Andres
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  #9  
Old 09-11-2017, 09:53 PM
esco esco is offline
 
Join Date: Jan 2011
Location: SoCal
Posts: 325
Default "Oh... that happens to all of them"

What I was told when I reported the issue to an "expert"

This with an all steam-gauge panel, years ago...
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  #10  
Old 09-11-2017, 10:10 PM
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bruceh bruceh is offline
 
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Quote:
Originally Posted by airguy View Post
Dynon EMS did this on the early units - they came up with a filter to solve the problem but the EMS box had to go back to them to install it, no charge. It's RF power getting into the EGT/CHT sensor loop.
I had this issue also with the Dynon Skyview EMS. Sent it back and they added the filtering and it has been fixed for good. No rewiring needed.
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