A1418 no GPU_Good signal, not turning on.

[pcsrepair]

Hey guys,

I've got an iMac A1418 over here with boardnumber 820-3588-A
No liquid damage but doesnt turn on. The fan spins for about 5 seconds and then turns off, No chime or backlight to see.
If i do a SMC reset it keeps spinning till i pull the powerplug.
The diagnostic leds are telling me that the GPU is faulty. Only the first 2 are lit.
I've tried an known working LCD screen, and connected it to an external monitor. No luck so far.

Since i'm not that experienced in troubleshooting GPU issues, I'm hoping someone can give me a good point to start with.

Thanks in advantage!

 

[dukefawks]

The LED only tells you when the GPU is successfully initialized. If it doesn't light ANYTHING before that point can have failed, so I doubt it is really the GPU. First pull the RAM and see if it beeps. If not then you can heat the GPU and see if it works again

 

[pcsrepair]

Thanks Duke!
It didn't beep without ram and all the voltage rails were coming into the GPU.
I tried heating it, and after that reflowing it but the problem is still there. i guess it's just a dead GPU.

 

[dukefawks]

No, as I said it can be ANYTHING before GPU initialization. If it was just the GPU there would be a beep without RAM

 

[skynetwarmax]

I have the same problem and I have some doubts, are you saying that to know if it is gpu really try without ram and see if it beeps? on the other hand if it does not give pythios you should check first. For example, I have been checking and the ISOLATE CPU MEM L signal is not activated, it is in a low state and it comes from the smc.

 

[skynetwarmax]

What do you advise me to check first if it does not beep?

 

[2informaticos]

The end "_L" means signal active low; so it is ENABLED with 0V.

Please, open new thread with your board.