the switch that has been watar dameged.
It works on the dock, but the LCD doesn’t work.
The backlight turns on.
I suspected corrosion and replaced the LCD connector, but it didn’t work.
I measured the LCD connector according to Calvin’s diode map and it was normal.
Is there any other approach?
I had one with same issue.
Remove joycon rails, speakers and see if it help
I had a speaker and one of the rails shorted and it prevented the lcd on. I just started connecting on st a time until I found out which boots and which cause it to short
Also check that the chip “8316” next to the LCD connector is providing +5 and -5v (check on the caps)
Same here but with a Nintendo switch lite, charging at 0.4amps with backlight only when charged power buttons and volume buttons not working I think it’s got to do with the LCD connectors or something.
I’ve been sleeping for a long time. 
Sorry for the late reply.
8316 was confirmed, because the pad had been corroded, it was carried out reballing work.
It was confirmed -5.4v and + 5.4v in the cap of the relatively large size.
But it is not displayed.
The LCD connector has also been replaced twice.
If you have any other ideas, please let me know
OK. I don’t think you said, and wanted to double check, but have you tested it with a known working LCD? I’ve had many water damaged Switches and the LCD itself often burns out…
Sheriff
Happend to me a few times: Bend pin on the Display connector. When looking on the connector from top, you might see a black spot, where a pin should be. Then this pin is pushed inside the connector.
Another sign for this is when the flat cable doesn’t go into the connector easily.
It’s pretty hard to get them back into position, i was able to fix it using a microscope & small a tool / needle.
It worked when I replaced the LCD!!
No corrosion was found, but as you say it seems to be out of order due to water damage.
Thank you Sheriff and Zwif. for your wonderful knowledge.