820-2915 A1286 no chime

[Alexwfm]

Hello, just another macbook pro standing between a landfill and a future of usefulness.

Laptop history: faulty discrete GPU, problem fixed by software forcing internal graphics. Worked really well for a few months with only a problem here and then not waking up from sleep.

Last week it wouldn't turn on with no chime, opened the back to have a quick look, put it back together and it turned on fine for a few days.

Now the problem came back:
No chime
No image on display and no backlight
Fans spinning at lowest rpm and heatsink building up heat after few mins
Usb mouse lighting up on one port
HDD and CD bay working
Doesn't beep without RAM

S0 voltages tested good:
PP0V7_S0
PP1V05_S0
PP1V2_S0
PP1V5_S0
PP5V_S0
PP3v3_S0

PPVCORE S0 CPU = 1.01V
PPVCORE S0 AXG =0.90V
PPVCORE S0 GPU = 0.89V
ALL_SYS_PWRGD = 3.3V
CPU_VCORE_C = 0V not sure what's this for.
PP5VSUS= 5V
GPUVCORE_EN= 3.3V
PM_ALL_GPU_PGOOD= 3.3V
GMUX_RESET_L=3.3V
PP3V3_SW_LCD = 0V
DP_MUX_EN = 3.3V


I've heard of someone with similar symptoms could get the chime back by removing the GPU resistors R8911/R8910. Not sure if he got the machine actually back to work in the end but I wanted to check more voltages before taking out the soldering iron.

Anyone know of any control voltages that could make the faulty GPU abort the start up process?

Thanks a lot in advance,
Happy new year!
Alex

[Alexwfm]

Quick update:
It seems if all the voltages are right and there's still no chime it's because the GPU state got so bad that it's preventing start up.
Thats what I'm assuming as after I've removed R8911 I got the laptop to run fine again -with the exception that I can't control hardware brightness and now it never wakes up from sleep.

If anyone has any soft solutions to that I'd be very grateful!

Cheers!
Alex

[TheDefpom]

Install the hardware modification instead, you just solder a small PCB over the SPI ROM, you've already removed GPU power, the board also allows you to hook up the brightness controls to get that back if you are on a system newer than 10.12.

I have used this board on a few machines now without issue.

http://www.cmizapper.com/products/tiresias-gpukiller.html

[Alexwfm]

Oh I did read about it, I was just wondering if it's worth the investment for just the brightness control -which I'm not too bothered about.
Does that mod fix any sleep-wake issues?

[TheDefpom]

It basically reconfigures the machine to not even try to use the GPU, without it the machine still hunks there is a GPU present, but then cant find it.

[Alexwfm]

I see.
Anyway what I thought would be the laptop not waking from sleep it's actually just the backlight control not turning on. And this device apparently should fix it.

Thanks for the suggestions!

[CMIzapper]

If the GPU is left powered then it can cause the Mac to crash during the Power On Test. The Power On Test speaks to the GPU before the Mac goes 'boing'. Depending on how sick the GPU is the Mac might hang at that point. A nice test is to remove all the RAM. The long beep for 'no RAM' is given before the Power On Test looks at the GPU.
Removing R8911 turns off the power to the GPU shutting it up quite professionally.

The CMIzapper Tiresias sets the nvram variable for the Mac not to use the GPU. The advantange of this is that if you clear the NVRAM/PRAM the Tiresias will stick the nvram variable back the next time the Mac boots.

In High Sierra and later you would lose the backlight control because Apple fails to recognise their own LCD as a 'legal' LCD. As an added bonus Apple 'forgets' to turn the backlight back on after waking up from sleep.
The CMIzapper Tiresias takes over the backlight control and has it work all the time (even when no OS is loaded).