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MacBook Pro 15" A1398 black screen after battery replacement

Hi everyone, a friend asked for help with his MacBook that was stuck in a boot loop, displaying only the Apple logo and a loading bar. After trying SMC and NVRAM resets, I reached the user menu, but the loop resumed after entering the password.

I noticed a bloated battery before opening the device, and inside, I found a very defective battery. Disconnecting it didn't stop the loop. I found a video suggesting disconnecting the display and trackpad, which worked—albeit slowly—using external peripherals.

Research indicated the boot loop could be due to the battery design. I ordered a new battery, backed up the data using the same workaround, and cleaned the dusty internals. With the new battery installed, I only heard the fans without any display. An SMC and NVRAM reset brought back the boot chime, but still no image.

Shining a light revealed an image without a backlight. External monitors worked fine, suggesting a backlight fuse issue. Despite checking continuity with a multimeter and cleaning connections, the issue persists.

Any ideas on what might be causing this? I've handled similar Apple device repairs but never encountered this problem before.

Answered! View the answer I have this problem too

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FIXED: after a very long investigation of what was causing the display failure and checking most components were working perfectly (might check the backlight capacitors, not my issue tho, the voltages were good on mine), also checked for a connector problem on the board (as Alexis mentioned) however it was clean and no external things in there, so I decided to check the display LDVS flex cable had some damage on the first pin from the right (that pin is the one in charge of the backlight voltage). Changed the flex and problem solved, I wish I could show the video used as guide to check (was great) but can't find it anymore.

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Try to disconnect the signal cable, use a brush to clean both the cable and its connector on the motherboard then make sure the signal cable is well position from the hinges and connect it back. Then test it again.

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Thanks for your help! I did clean the connector, but never paid attention to the flex itself (where the damage really was)

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Alejandro Peñafiel will be eternally grateful.
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