Chosen Solution
On my A1502 if i connect to hdmi i can see video and computer works fine. Some times the lcd will come on then shut off within a few seconds. I dissasembled the screen and check the display cable. It looks ok. Anything else i should check?
Well, that clearly points to a connector or logic board issue. Carefully inspect the solder joints of the display connector look for a cold solder joint. If you have the skills I would resolder the connector. You’ll need the get the schematics and boardview drawings and trace out the backlight logic. Update (11/19/2021) Here’s a block diagram to help explain things
Here the MUX logic sends a signal to the PCH to disable the the internal display when the lid is closed via the Hall Sensor.
