Macbook Pro A1398(820-3787-A) Backlight problem

[Ictc]

Got this laptop with little liquid damage spot around ISL that only prevented power up without battery connected and a shorted cap on the flash drive that prevented the drive to be recognized at boot. Fixing those problems, the machine powers up and boot but still without backlight. I of course cansee the image on the display if I put the light on the apple logo.
I did some measurments and got:
PPVOUT_S0_LCDBKLT = 54v
PP5VR3V3_SW_LCD = 5 volt
I than connected the only retina dispaly I have in house wich is a 13" in stead of the 15" just to give a shot.
This 13" model has the same eDP pinout of the 15" and I tought it shoulded work but in stead of having the backlight on as it is with the 13" inch motherboard, I still getting 54v on PPVOUT_S0_LCDBKLT and that's strange because on the 13", this value goes around 40V once backlight is working.
So far I can't really say that the screen backlight is damaged. Measuring in diode mode between ground and PPVOUT_S0_LCDBKLT I get nothing on both the displays and may be this measurement has no real reason to be done.

Is it normal that the display partially working on a known good machine doesn not on this other one? May be as a different inches model it is normal not to work on a different motherboard.

What else should I check just to be sure the problem is only with the screen backlight and not the logicboard?

 

[2informaticos]

54V means no connection with LEDs strip.
Did you measure voltage at the LCD connector pin?
Be sure connector is not damaged and has good contacts.
Test with known good COMPATIBLE LCD assembly.

Just in case, do PRAM reser 2-3 times.

 

[Ictc]

Just digged around and found EDP_IG_BKL_ON =0
Is there a problem with the PCH? In wich situation this signal should be provided by the PCH?

 

[2informaticos]

If you get 54V on backlight output, backlight circuit is enabled and working.
Of course, EG_BKLT_EN is high, true?

 

[Ictc]

Yes EG_BKLT_EN is 3,3V as it should

 

[Ictc]

For your experience do you think the 13" display should have worked with this 15" logicboard as the pinout is the same? Am I assuming this incorrectly?

 

[Ictc]

PRAM reset didn't change anything.

 

[2informaticos]

"For testing, 13 and 15 inch retina screens of the same generation are compatible."
Did you test with 13" screen from same generation?

 

[Ictc]

Yes I did it but what sound strange to me is that the 13" display backlight works on the 13" MBP but not on the 15" one. I tought it should have worked on the 15" as well at least for showing backlight. If I connect the 15" display on the known working 13" MBP I also do not get any backlight. ???

 

[Ictc]

I made some more comparison measurement between the working conditions on the 13" (known good dispaly and board) and the 15" where none of the two display (known good 13" and original 15") gave backlight on the 15" board. I found some discrepancies with voltage values on the eDP connector. The measurment are represented as follows:

Pin N.

PIN N.	13" display connected to 13" MBP	13" display connected to 15" MBP
1	52> 39V (after boot)	54V
2	0,2V	0,02V
3	5V	0,18V
4	5V	0,18V
5	3.3V	0,35V
6	3.3V	2,58V sometimes 4,98V
7	3.3V	2,38V sometimes 4,98V

Could this be a sign of a problem with the PP5VR3V3_SW_LCD?

 

[Ictc]

I made some more comparison measurement between the working conditions on the 13" (known good dispaly and board) and the 15" where none of the two display (known good 13" and original 15") gave backlight on the 15" board. I found some discrepancies with voltage values on the eDP connector. The measurment are represented as follows:

Pin N.

PIN N.	13" display connected to 13" MBP	13" display connected to 15" MBP
1	52> 39V (after boot)	54V
2	0,2V	0,02V
3	5V	0,18V
4	5V	0,18V
5	3.3V	0,35V
6	3.3V	2,58V sometimes 4,98V
7	3.3V	2,38V sometimes 4,98V

Could this be a sign of a problem with the PP5VR3V3_SW_LCD?

 

[Ictc]

I made some more comparison measurement between the working conditions on the 13" (known good dispaly and board) and the 15" where none of the two display (known good 13" and original 15") gave backlight on the 15" board. I found some discrepancies with voltage values on the eDP connector. The measurment are represented as follows:

Pin N.

PIN N.	13" display connected to 13" MBP	13" display connected to 15" MBP
1	52> 39V (after boot)	54V
2	0,2V	0,02V
3	5V	0,18V
4	5V	0,18V
5	3.3V	0,35V
6	3.3V	2,58V sometimes 4,98V
7	3.3V	2,38V sometimes 4,98V

Could this be a sign of a problem with the PP5VR3V3_SW_LCD?