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2018 Pro 820-00850 board with dim display output

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sekidata

Member
I have this 2019 Pro that shows a very dim backlight (it's there but about 1/10 the brightness of normal). After a NVRAM reset, the backlight displays at full brightness during the first half of the boot process, then reverts back to dim.

PPVOUT_S0_LCDBKLT is constant around 44 V, before and after dimming.
BKLT_PWM is 2.5 V during full brightness, and 0.01V during dimming.

I don't have another 2018 display but I tested with a 2017 display and it gave the same results. What's next to check on this board?
 
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sekidata

Member
I (accidentally) found out that when I connect one or more of the LED return lines in the backlight flex cable to ground, the brightness (almost full) returns. Is that expected… is that how PWM works in these displays?
 
2informaticos

2informaticos

Administrator
Staff member
Shouldn't be LED return issue, if backlight is correct when machine starts.
Looks more like ALS issue, as its driver is loaded into macOS.

How did you discard an ALS problem located on the MLB?
Check if ALS really works; KB backlight reaction may help.
 
S

sekidata

Member
You're right, it could be an ALS issue on the logic board.

With the original 2018 display, I get constant keyboard backlight and screen brightness doesn't change with ambient light. Camera works.
 
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