Low EGT readings

[Audibabba]

I have a D120 with low EGT readings. I am trying to determine if they are real.

This started a few months ago where the EGT readings started reading close in tandem and reading low. At times, they would go from 1150F to 1250F to 950F, all in the same flight, same altitude, same throttle setting. I have attached a photo showing the low readings. What you cannot see is the rapid temperature changes that appear to occur in tandem.

With a cold engine, the EGTs read ambient and close to CHT and oil temp. I checked the carburetors for float issues and pulled the spark plugs, expecting to find black plugs. I found very clean plugs. I also had the EGT probes replaced, woth the same readings, which indicated it may be a A-D issue.

I shot a signal from Precision Signal source for 1300°F in type K on each channel at the multi-pin connector on the back of the dynon and I have attached the photos showing a 50°F higher reading. There is also a question of the mid-range temperature on the side not connected to anything.

Is this a cold junction issue or something you can repair if I send the unit?

Any other suggestions?

Thank you.

 

[airguy]

Could very easily be a ground issue on the engine. If there is any slight resistance on the ground path between the engine and the airframe (where the EMS is grounded) then as your alternator loads up and down responding to various electrical loads, the ground path from the engine to the airframe will experience a voltage drop across it. This voltage differential is also present on the thermocouples, and will cause skewed readings on the CHT/EGT.

The difference between 50 degrees C reading of a Type K thermocouple is less than two millivolts. According to Ohms law, if your alternator is pulling, say for easy math, 20 amps to power your current electrical needs, then a resistance of only 0.1 milli-ohms will produce that much voltage differential between the engine and the airframe.

 

[Audibabba]

I wish that were the case. Thermocouples are self-powered and the alternator should have nothing to do with it. Any single wire RTD (CHT, oil temp on Rotax) with reference to a ground would be affected as you suggest. In this case, the engine was not running when I shot the signal, so there should be no alternator influence. Thank you for the suggestion.

 

[Raymo]

Upload the engine data to the Savvy service and post a link to review.

 

[Audibabba]

I have no clue how to do that. Savvy does not handle light sports to my knowledge. I will have to educate myself in downloading.

 

[Raymo]

I have no clue how to do that. Savvy does not handle light sports to my knowledge. I will have to educate myself in downloading.

They support most engine monitors, which is what matters. You'll download the user data logs from the Dynon screen. The files to upload are relatively small CSV files.

 

[Audibabba]

The only way I can see to download the D120 is by serial connector. Is this how you do it?

 

[Raymo]

The only way I can see to download the D120 is by serial connector. Is this how you do it?

Sorry, not familiar with the D120.