Chosen Solution
Working on a MBP 820-3115-B without keyboard backlight. The board has another issue, it will produce a 3 beep RAM error if the bottom RAM slot is populated. It boots and works fine with a single RAM module installed in the top slot. Even though a faulty bottom RAM slot is a fairly common issue on this model (the legs of the RAM slot detach from the board, but I checked them on my board and they’re solid), I believe the 2 issues might be related. After checking the schematics, I noticed that R5852 is completely missing from the board. There are some very faint signs of corrosion in the area, but from the look of it, it seems like this component was never installed (for the record, it’s a 10k size 402 5% resistor). According to the schematics this component connects to both SMC_SYS_KBDLED and to pin 174 (MEM_B_DQ<55) of the bottom RAM slot. I probed around and all values of components on the KB backlight circuit seem perfectly fine, no shorts or anything. I’m planning to source a replacement and (re-)install the resistor, I’m just trying to confirm that this has the potential to fix the issue. I also noticed that several other components that should be there according to the schematics are missing and it looks like there was never anything soldered to the pads. Why is that?
Apple sometimes takes out unneeded parts. This is the case here. I’m away in VT so I don’t have access to the schematics so I can’t point you to what you need to check physically. What you need to probe the power rails to trace out the keyboard backlight power. I suspect you likely have a damaged keyboard backlight unit or the cable connection. Your memory slot issue is likely a warped logic board. Loosen the mount screws slightly to see if that corrects things.
