PRAM RESET! Low boost can be the brightness being down.
Check voltage after backlight diode.
If backlight is 45V or more there is no load on the backlight circuit. Use a known good screen and check the connector. If backlight is 25V-40V the boost circuit is working. Check the connector, cable, and LCD. If backlight is PPBus voltage your boost circuit isn't working. Check the feedback trace, LCD driver, and enables. If backlight is 0 check for short then work your way back toward PPBus. The LCDBKLT_FET, fuse, and enable transistors are all suspect at this point. Also check the voltage dividers.
checking SMC_LID (it shouldn't give an image either if it's missing)
To have boost on this circuit, the LCD must communicate. Be sure to use a known good LCD when troubleshooting.
Measuring before the inductor you should have PPBus, and after it you have some random rounded-square-ish (or triangle-ish) signal that a cheap multimeter won't know what to do with. Measure after the diode unless you are using an oscilloscope.
Missing LCD_BKLT_PWM: Inject voltage or jumper from PP3V3_S0 or similar.
Some great info from Louis Rossmann (User name - zzz) ifixit forum
Most "common" problem with no backlight is ball A5 on LP8550 or LCD connector, but guessing is BS and should not be done here.
put multimeter in diode mode, red probe on ground, black probe on pins 3/4 of LCD connector.
- 0.0 or 0.007 means direct short to ground, usually inside LCD connector or LCD cable.
- 0.200-0.300 is bad LED driver.
- 0.459-0.511 is bad feedback via or corroded away feedback ball under LP8550
- 0.565 is corroded solder ball or bad switch trace inside LP8550
Measure voltage along each point in the circuit. Before F9700, after F9700. At output.
At backlight output, measured on pin 3 or 4 of the LCD connector,
- 0v means short to ground, blown fuse, or no LCD connected. Check that you see an image on the LCD, check backlight fuse, check board via from FET after fuse to boost coil.
- 8v means no short to ground, good fuse, but no boosting. Check BKL_EN is 2.7 to 3v at voltage divider going to enable pin of WLED driver, check BKL_PWM signal exists.
- 27v or higher means all good, but your LCD cable or LCD backlight itself is blown.
If you see power along the line, where is it? Where does it stop?
If you replace a fuse with a short to ground anywhere in the line you have wasted your time, and a precious little 0603 package. :(
Lastly if it worked for a few months and then died most likely your problem is on bad feedback via/ball on LP8550, pins 3/4 coming corroded on LCD connector, or BKL_EN voltage divider resistors in that order.
Have fun! Louis Rossmann
820-00138 BKLT_EN high but LCDBKLT_EN_L is not being pulled down
this is a pretty tricky one, i found 3-4 deadend posts on badcaps / Rossmann's with no solution for this issue .
The problem is that there is no real 820-00138 boardview available, in most cases there is 820-3662 boardview or similar. however PDF seem to be the right one. The difference between commonly distributed boardview and real one is that there is no U7750 on board, and this is the source for BKLT_SCL + BKLT_SDA. To inspect backlight circuit use 820-00163 or 00462(this is DG board version) which is totally fine and has absolutely the same backlight circuit design.
These two are critical for backlight to turn on, this is somehow similar to iMac design, where display is the one who really turns backlight and only once image output is stabilized. This might seem overenginered, but it is kind of bugfix for possible graphical artifacts on system poweron.
On 820-00138 I2C_BKLT_SCL + I2C_BKLT_SDA are coming from eDP connector J8300, pins 3 and 4, they measure 5v unlike most other I2C lines (3.3v).
Otherwise LP8548B1SQ(used on both 13" and 15" Macbook Pro Retina 2012..2015) troubleshooting is very similar to LP8550, but the difference is that on 2015 15" IG model (at least) backlight is controlled by display itself. It renders all 2012-2014 displays not compatible even for a backlight check (however, there will be enable signal from PCH)