Vans instrument issues

[ace123jim]

Hello Group,
Last week while flying my RV-4, I had a burning smell in the cockpit. Checking the panel, both the EGT and CHT instruments weren’t working and were hot to the touch. After returning to the field, and removing them, I disassembled both and found that both circuit boards (nearly identical boards in each) had burned spots around the resistors in the middle. There was also moisture on the inside of the glass on both. Voltage was in the normal range. Running Earth X battery.
Looking for ideas on what caused this. See photos.

Jim

 

[BobTurner]

IMHO to have two separate instruments fail like that suggests an over-voltage event. Since you say buss voltage was normal, it suggests a transient (just a few seconds) event, which means very hard to trouble shoot. To start, I’d look at all connections to the alternator and voltage regulator for any sign of a loose buss voltage sensor wire.

 

[Paul 5r4]

I haven't a clue but agree with Bob. The odds of two separate instruments failing at the same time and in the same failure mode just isn't likely. I would explore the upstream electricals.

 

[Mikeyb]

That took some energy to do that damage. Because it was two instruments connected to the engine another possibility is an overcurrent caused by a poor engine ground where the starter current returned to ground through the thermocouples. Does the starter crank normally?

 

[Jpm757]

Might be a good time to install a full EMS setup if checkbook allows.

 

[rick9mjn]

https://en.wikipedia.org/wiki/Zener_diode#/media/File:WaveClipper1.png

to add my .02 cents to, "I haven't a clue but".
i looked up the spec's of the IC. #cs289 (a tachometer driver IC).
the specs show's the max. supply voltage, of 20.0 volts.
ALSO looking at your photos, i am "thinking",
the larger resistor that is overheated.. is in series with a (shorted) Zener diode, to the input voltage (12 volts)
SO.. explore the upstream electricals "I.E" alternator and voltage regulator and wiring and grounds
...good day
...
..
.

 

[ace123jim]

Instrument issues. Thank you for the responses.

Thanks…..Will check those items.

I still need an oil pressure gauge if anyone has one.

Jim

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