2008 iMac Endless Reboot and Graphical Glitches

[cky880]

Hello, I was hoping someone might be able to help me narrow this problem down. My 3.06GHz 4GB 24" 2008 iMac w/ Nvidia 8800GS has stopped booting normally. This issue started gradually. Over the course of about a month, I'd have maybe a 50% chance making it through the boot process with the ability to use my computer, but the other 50% of times, I couldn't get past the login screen, at which point, the screen became filled with graphical artifacts, and stopped refreshing. This now happens 100% of the time, and will restart itself many of the times, endlessly.

I tried each stick of ram, one at a time, tried booting OS X installed on a flash drive, resetting PRAM, all with no luck, and passed the Apple Hardware Test with flying colors. I'm inclined to think its a video card issue, but when booting from the install disk, and running disk utility, or the Apple Hardware Test, the problem doesn't occur at all. Is it possible that these modes do not use the Nvidia card?

To reiterate, the graphical issue occurs immediately upon seeing the login screen (Mountain Lion).

I replaced the (dead) hard drive with an SSD about 6 months ago, though, it appears to be working just fine, so I don't think that is the issue.

Thanks for any help or suggestions!

 

[iSee]

The power supply is a possibility... is it a brick in this iMac? If so, it might at least be easy to test. (I would put my money on the graphics card or something on the logic board though.)

 

[cky880]

The power supply is a possibility... is it a brick in this iMac? If so, it might at least be easy to test. (I would put my money on the graphics card or something on the logic board though.)

Thank you for the reply. I'm not sure what you mean by "is it a brick in this iMac". The power cord doesn't have a brick in the middle of it, like a Mac Mini power cable, or a Macbook Pro charger, if thats what you're referring to. Standard 3 prong socket on the end that plugs into the outlet (US), and similar 3 prong female socket on the other end that plugs into the computer.

Do you know how I might be able to further diagnose whether it is a logic board or GPU issue? My guess is a professional repair wouldn't be worth the money, so I'd like to determine the issue before I purchase any replacement parts to fix it myself.

Thanks again!

 

[cheebster]

Yeah, sure sounds like the video card is the culprit. They fail on these... BGA balls are at fault, these years nvidia started with the leadfree and the balls under the gpu can't stand the heat and they eventually fail..

 

[cky880]

Yeah, sure sounds like the video card is the culprit. They fail on these... BGA balls are at fault, these years nvidia started with the leadfree and the balls under the gpu can't stand the heat and they eventually fail..

Thanks. So if it is the graphics card, is it the case that the discrete card doesn't get used until the OS actually loads? I'm trying to piece together why I can run the hardware test and boot from a DVD to get to Disk Utility with no graphical artifacts.

 

[iSee]

Thank you for the reply. I'm not sure what you mean by "is it a brick in this iMac". The power cord doesn't have a brick in the middle of it, like a Mac Mini power cable, or a Macbook Pro charger, if thats what you're referring to. Standard 3 prong socket on the end that plugs into the outlet (US), and similar 3 prong female socket on the other end that plugs into the computer.

Do you know how I might be able to further diagnose whether it is a logic board or GPU issue? My guess is a professional repair wouldn't be worth the money, so I'd like to determine the issue before I purchase any replacement parts to fix it myself.

Thanks again!

Sorry, yes, I meant to ask if the power supply was external (and therefore, easy to test if you could lay your hands on another one). But that's no good. I mentioned it because I've had a system with video card glitches (particularly at boot though they could also occur at other times) where the fix was to replace the power supply.

I don't have any great ideas for you...
- Maybe nvidia has a GPU diagnostic tool you can download and run?
- If the system has an integrated GPU maybe you can find a way to disable the discrete GPU (or physically remove)?