Erratic CHT gauge

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nhraracer90

Filing Flight Plan
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Apr 30, 2020
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nhraracer90
Hello! My CHT gauge will jump up when I turn my landing lights on (2 LEDs). I just figured it could be some interference and the landing lights won’t be on much so I wasn’t too worried about it. Now I noticed it also jumps up when the aux fuel pump is turned on (didn’t notice it doing it before). I’ve attached a video of what happens when I turn the landing light on. Sometimes it jumps up much higher than that and comes back down some. There are no wires hooked up to the gauge other than the ones that run directly to the probe. Any way to test what it could be or where to look?

It’s on a Hiller helicopter w/ a Franklin engine if that helps any.

Thanks


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Well it won’t let me add a video now.


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The engine and alternator are poorly grounded to the airframe. Switching in loads causes a voltage drop, so the voltage regulator intensifies the alternator field to get more amps for the new loads. If the engine/alternator grounding has even a little bit of resistance, some of the alternator's increased ground flow finds its way into things like the CHT probe, and through the probe to the instrument. The increased flow shows up as a higher CHT.

If you have an electric oil temp gauge you might see a spike there, too.
 
And just like that: A clear concise answer which more than likely leads the original poster to a solution of cleaning contact areas and a very inexpensive solution. Rarely found in my experience, I always seem to need the costly repairs or extra time chasing gremlins or weeding out possibles causes of my aircraft problems!
Very clear and concise explanation, Thank you Dan from all that may read this. Gene
 
nhraracer90 said:
Any way to test what it could be or where to look?
Click to expand...
Has this been an ongoing problem or something new?
Was there any previous mx or other aircraft changes prior to noticing this issue?
 
Dan Thomas said:
The engine and alternator are poorly grounded to the airframe. Switching in loads causes a voltage drop, so the voltage regulator intensifies the alternator field to get more amps for the new loads. If the engine/alternator grounding has even a little bit of resistance, some of the alternator's increased ground flow finds its way into things like the CHT probe, and through the probe to the instrument. The increased flow shows up as a higher CHT.

If you have an electric oil temp gauge you might see a spike there, too.
Click to expand...

Thanks for the detailed reply! I will check into all the grounds and see how they are. I haven’t noticed anything on the oil temp gauge. This also happens when the engine isn’t running as well.


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Dan Thomas said:
Then it's not likely a grounding issue.
Click to expand...

Hmmm okay. What else could it possibly be or anyway to isolate it? This is a very weird issue lol


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