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EGT rise when I turn on the landing light!

A

andrewbutler

Member
Hello,

I have an odd problem. Last summer I had an EGT issue, or so I thought. Pulled the cowling off half a dozen times, suspecting an air leak or something. Eventually, after a successful run up, I put the cowl on and ran through my pre-flight checks and the blasted EGT shot up again during my power checks. I eventually realised the only difference was that I was turning on more electrics, in particular the lights. Without any real action, the electrical Gremlin diminished all of its own. This was EGT3.

For the rest of the season, all readings were rock solid.

Now, all of a sudden, EGT4 is rising a good 200 degrees when I turn on the landing light with strobe and navs engaged, about 100 without strobes and nav engaged. All other remain unchanged.

I recall that I grounded my Landing light on a rib inside the wing. The strobe wires are shielded, and the shielding grounded. I think I have the navs grounded on the last rib of the wing also.

My fuel flows are very even, and all EGTs peak more or less at the same time with a nominal EGT spread of< 30 degrees and CHT spread of < 15 degrees during normal ops. I have an AFS3500 EFIS and use SavvyAnalysis to examine the engine logs. I followed the Aeroelectric methodology in my electrical system, including grounding where possible.

I don't think the EGT sensor wires share a path with any of the light power sources. They come directly from the firewall and into the back of the EFIS, the light wires being lower down in the panel below the EFIS.

Any advice on how to track this down would be greatly appreciated.

Andrew Butler,
RV7 EI-EEO
EITT
Dunshaughlin,
Co Meath,
Ireland.
 
The following has been plagiarized from a competitor's EFIS Installation Manual and modified:
Even very small resistances between battery ground and instrument ground can cause voltage differences which adversely affect engine sensor readings. An easy way to test the quality of the instrument?s ground is to measure voltage between the instrument ground and the ground lead at your aircraft?s battery. With all electrical loads powered on, connect one lead of your voltmeter to the AFS3500 ground. Connect the other lead of your voltmeter to the ground terminal of your battery. The voltage between these two points should measure very close to 0 mV (within 5 mV). If it does not, you must improve the ground connection between the ground of your battery and that of your avionics bus.
Click to expand...
I would also measure the voltage difference between the engine case and the AFS3500.
 
Mich48041 said:
The following has been plagiarized from a competitor's EFIS Installation Manual and modified:

I would also measure the voltage difference between the engine case and the AFS3500.
Click to expand...
Measured the voltage difference between the Battery cathode and the firewall mounted spade avionics bus - 1mV. Then between the EFIS case and the firewall mounted avionics bus - 9mV. Between the EFIS case and the engine block was 8mV. Will put a new fat wire between the case and the avionics bus and see what happens.

Thanks for the help.
Andrew.
 
andrewbutler said:
Measured the voltage difference between the Battery cathode and the firewall mounted spade avionics bus - 1mV. Then between the EFIS case and the firewall mounted avionics bus - 9mV. Between the EFIS case and the engine block was 8mV. Will put a new fat wire between the case and the avionics bus and see what happens.

Thanks for the help.
Andrew.
Click to expand...

To put things into perspective... 100F change on an EGT thermocouple is about 2.5 millivolts.
 
How bizarre, we are getting an EGT rise during aerobatics on No4.

I wonder if it is a g loading event altering the connection. Never realised it was so small a voltage.
 
Bad grounds can do really strange things...things that sometimes don't seem to make much sense. I've see a poor ground connection cause unimaginable issues on electronic drives at work. My suggestion would be to check, yank, wiggle, and re-seat any affected grounds first. If that has no effect, then try running independent grounds back to your ground bus for the lights--even if they're just temporary. Having a chassis ground can sometimes do funky things, too.

Now, keep in mind that this is a mechanical engineer's klunky way of troubleshooting. Someone smarter than me may have a more "elegant" approach.:rolleyes:
 
D-Dubya said:
Bad grounds can do really strange things...things that sometimes don't seem to make much sense. I've see a poor ground connection cause unimaginable issues on electronic drives at work. My suggestion would be to check, yank, wiggle, and re-seat any affected grounds first. If that has no effect, then try running independent grounds back to your ground bus for the lights--even if they're just temporary. Having a chassis ground can sometimes do funky things, too.

Now, keep in mind that this is a mechanical engineer's klunky way of troubleshooting. Someone smarter than me may have a more "elegant" approach.:rolleyes:
Click to expand...
Okay, so I have run a new ground from the Firewall Mounted avionics ground last night. Turned on the EFIS and was getting a reading of 78/79 from EGT 1-3 (warm day in Meath yesterday) and tested the reaction of EGT4 when loading the BUS. Meanwhile EGT 4 read slightly higher to around 85. The more load, NAVs, STROBES, and LL, the higher the reading, up to about 99. All others didn't budge. Then when I turned off the master and ran the EFIS on internal battery EGT4 dropped to 72/73 flicking between the two. The others remained unchanged.

So I am pretty sure at this stage that the EGT probe has a dodgy connection. Will start with the crimps and work my way back to the EFIS connection.

Thoughts?

Andrew.
 
How is the battery grounded? Is the negative battery terminal connected directly to the firewall avionics grounding bus? What is the millivolt difference now between the EFIS case and battery negative terminal with all electrical loads turned on? If not zero, then check all connections and increase wire size if necessary.
 
Thanks Joe. It is 1mV. Checked out the EGT connections. All good. On a run up, all was good until I engaged the ALT. EGT reading went up. I do have some ALT whine. What are my options with regards eliminating the ALT from the picture?

It is the 60AMP version from B&C with an external regulator.

Thanks again.

Andrew.
 
Wire bundle?

How are your EGT leads routed? Are they isolated, or are they part of a bundle of other wires - particularly going thru the firewall. If so, are any of those other wires high current - alternator output or battery cable?
 
How are the alternator wires routed?
Try connecting a heavy ground wire from the alternator case to the firewall grounding block.
 
Okay, so I checked the connectors and all okay. I then pull the cowl (for access) and moved the probe to #1 exhaust. The erroneous high reading traveled with the probe to #1 and #4 read correctly.

Now I know the probe is the problem and not my electrical system. Can anyone shed some light on what the failure mode might be?

Thanks,

Andrew.
RV7 EI-EEO
EITT, Meath, Ireland.
 
I had a very similar problem just a week ago but with CHT readings. I eventually found a wire shorting intermittently to one of the probes. We also cleaned the probes where they ground to the fittings really well. Totally fixed it. It does sound like a grounding issue to me.
 
Everything that you ever wanted to know about THERMOCOUPLES. Thermocouples are basically two dissimilar metal wires welded together at one end. Obviously a thermocouple will not work if a wire breaks. Another failure mode is degradation of the welded junction which could occur in the hostile environment of an exhaust pipe.
 
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