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No backlight but fuse Ok, LCD or Logic Board?

Hi iFixit,

The backlight on my MacBook Pro 15'' Late 2013 went down.

Following the instructions from Backlight fuse and wled driver location on logical board a1398 2014post, I located the fuse on the logic board and checked it with a multimeter - it is OK and not blown. No spills or mechanical damage anytime in past.

I read that the LED backlight failure in LCD assembly is extremely unlikely, can anybody confirm that?

The other point of failure is the logic board with backlight driver IC. Is there any diagnostics for it? I cannot even find a part number for it. The one from the other post seems to be Apple's proprietary.

The cost of LCD assembly is about same price as logic board. I don't want to buy both to see which one failed. Is there any other way to figure out the source of failure?

Any thoughts are greatly appreciated

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Remove the logic board and check for liquid on the back of the board. Even though you say liquid has not been spilt on this, I have had many MacBooks come in where the customer swears this is the case, yet there is liquid damage...

The backlight IC is very robust in the Retina models and very rarely fails, it is not like the BGA LP8550 on the Airs. I have only ever had to replace 2-5, and only due to being visually burnt or liquid damaged.

Backlight issues with the screen can happen, but only if liquid is spilt over the screen in most cases, to get on the screen's PCB. Check pin 1 of the LCD connector to see what voltage you are getting for backlight (whilst the screen is plugged in). This will give an indication of the issue.

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Thank you very much, Reece, for the prompt response. I measured 0V on pin 1. I wasn't sure on which side pin 1 is so checked both. I don't know where the ground is so put the black probe on LCD connector housing.

I also measured the voltage on the fuse, it is 12.12V on both sides.

I will check pin 1 again with finer probes tomorrow, and if it is zero, I'll inspect the other side for damage

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Update: I checked the voltage with thin probes, it is 5V between pin 1 (closer to the board edge) and the housing.

I also inspected the top of the logic board, seems to be OK.

Is it an expected voltage level? If yes, then, I guess, it is the LCD issue

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Pin 1 for backlight is on the other side, not by the board edge. 5V on the pin nearest the board edge is normal

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Checked again with oscilloscope - no voltage on pin 1 then. Is there anything else I can do to diagnose it before i buy a new logic board?

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Check for a short to ground on this pin (with LCD cable plugged in and unplugged), check the LCD cable too to make sure the plastic hasn't melted around the pins.

If there is no short, check the backlight fuse to make sure this hasn't blown, since this means the main voltage is not reaching the backlight chip to be boosted most likely.

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