A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

[Stephen]

Hello, got a weird MacBook Air here
board looks fresh and no water damage, backlight works and turns on and stays on when installing OS etc. However, when the os is installed and when the system files are loaded before the login page the backlight goes out. I have wiped it, NVRAM reset, etc and even took some other instances and this blows my mind, never seen this before. Backlight just turns off once the OS is loaded. Anyone come across this and if so what would be the cause? I am certain its a board issue since I tested the LCD on another board and it stays on. Again the backlight works when you turn the computer on, but Safe Mode or normal OS loading it turns off completely. You can see the image still when you shine a light through it. Thanks

[reformatt]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

What is the backlight voltage on D7701 during fault condition? 8.6V or 0V? Also check BKL_EN and BKL_PWM during fault condition.

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Will do thanks! I’ll post my readings

[dellxps15]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

check also lid sensor not being the problem.

if u can renable backlight by moving mouse, power botton and it ask for login while u were on desktop before screen goes off, then it's lid sensor.

[reformatt]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Lid sensor is in the DC IN board, so just swap it out to test.

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Not the lid sensor already swapped that out with DC board. Ruled that out very quickly. Will be posting readings here later today

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

What is the backlight voltage on D7701 during fault condition? 8.6V or 0V? Also check BKL_EN and BKL_PWM during fault condition.

0V for D7701

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

No reading on BKL EN

[mon2]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

What is the 820-**** number of your logic board?

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

820-00165-a

[mon2]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

What is the voltage to ground of the following?

EDP_BKLT_EN
BKLT_PLT_RST_L

Each of these 2 mosfets must be ON, to create the voltage divider @ LCDBKLT_EN_DIV_L (gate) and then Q7706 will enable to enable the backlight.

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

EDP_BKLT_EN
0 VOLTS

BKLT_PLT_RST_L
3.3Volts

[reformatt]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

So it is definitely being disabled then. Do you see this problem in Recovery mode? Thinking it's a hardware issue triggered when the kext is loaded in the OS. Backlight enable is from the CPU, won't come on if a display is not detected. Have you tried another display per chance? Might be something screwy with the SMBUS connections perhaps.

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Tried another board with this display it is using so not the display. It is definitely the board

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

So it is definitely being disabled then. Do you see this problem in Recovery mode? Thinking it's a hardware issue triggered when the kext is loaded in the OS. Backlight enable is from the CPU, won't come on if a display is not detected. Have you tried another display per chance? Might be something screwy with the SMBUS connections perhaps.

Recovery mode BACKLIGHT is on.

[reformatt]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Seems to point to a graphics issue within the CPU (or bad solder ball). Is it a an Apple FRU board by chance? Have seen a few weird issues with those, especially if the CPU has been reworked.

Do you still have the PWM present when the backlight is disabled?

[kevingill]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

Might be worth wiring EDP_BKLT_EN to 3.3v then?
Bit of a bodge, but would get it working.

[piernov]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

No SMC_LID measurements?

[Stephen]

Re: A1466 MacBook Air 13" 2017 Backlight issue works but then turns off

I’ll get this looked at Monday. Simple fix I’m sure