Chosen Solution
I recently bought a second hand MacBook Pro 15" mid 2012. I thought I could upgrade to 2 SSDs. Upon checking the F1/f2 brightness o roll were not responding. I could see the indicator on the screen where it’ll show the brightness going up and down but actually the LCD doesn’t becomes brighter or dimmer. The seller said its due to a software bug. The system was at El Captain. It was stupid of me to believe him. I bought it and updated to high sierra thinking it would fix the problem But no. Second problem I found out was that once the system goes to sleep, the backlight won’t come back on until I open up the back cover and unplug and replug the battery again. I went to the Apple Store to get it check and they found a unauthorized modification made to the logic board. Here’s are the images. You can see a copper wire connected to 2 fuse I suppose
Sorry to say, but looks like you’ve been had… So, what going on here is- some reason, your LCD Backlight driver (little square chip above the bottom of the wire) was not receiving its correct signal from the GMUX (larger chip a little below with the silicone stuff around it) If I’m correct, that GMUX would send the signal strength to where the bottom component (resistor) your cable is attached to. That works in tune with your f1/f2 keys and tells the LCD Backlight driver how much to let through. What they did was connect it to a component that should have 3.3V when the screen is open (basically slamming down the gas pedal for backlight brightness) BUT that signal is also dependent on the GMUX… so it seems like your GMUX is having issues an they made a bandaid fix. Sorry about the trickery someone pulled on you and how you had to find out about it… That upsets me because it causes doubt for reputable repair shops alike… Best of luck! If you want it fixed, there are some local repair shops out there that can do it!
