iMac with Snow Leopard crash without second display connected

Hi all,

I've a iMac 24" and I would like to resell it urgently.

But I've a big problem, if I disconnect my second display (belinea 20"), the iMac screen become grey or blue after many second.

If I boot system, after the grey clock : nothing, grey or black screen.

The Os is snow leopard disc, update version (10.6)
The iMac version is iMac8,1
The EFI bios is : IM81.00c1.b00

I tried :
- Reinstall 10.6 with format (normal and zero write format system)

- Utility disc on boot cd (scan disk & rights repair)

- Try to boot cd without second screen
> I've white screen (after a update to 10.6.4)

- Try PRAM reset
> no working

- Try a 10.5.2 install (delivered with the iMac)
> I've directly a grey screen just after boot with cd : no install menu appears

You can see my post on apple forum here

Help me please

I just read thru your post and replies on the Apple forum. Not much was helpful.

The external monitor may have nothing to do with the problem - what we need to do is test the system. Boot the iMac with your Snow Leopard DVD and run Disk Utility from the top menu "Utilities". Do a verify and repair for the hard drive. Also repair permissions. (Will take awhile.)

Next, try the Apple Hardware Test: See this LINK.

Post back results of the two tests above.

* verify and repair hard drive & repair permissions : no change
* AHT (quick test 2mn) : no hardware problem detected

Next step : try a advanced AHT ? (30 mn test)

Question1 : the AHT start well with my "native" OS install CD (10.5.2) but not the installation program, I MUST install Snow Leopard 10.6 CD, no choice, normal?

Question2 : The Snow Leopard Install Disk, install the OS in the disk0s2, is it normal ?

Run the Apple Hardware Tests with your Snow Leopard DVD. The AHT runs from the install DVD # 1. Like I said above, we need to try to isolate the problem because there should be no reason why the machine needs an external monitor in order to work properly.

If you've done a complete clean reinstall of the operating system and the problem still persists... that's hard to understand. A clean reinstall would have removed all the old preference files and settings.

Hi chscag & thanks for your answers,

I tried :
* AHT from SL disk (the 3 mn test) : no problem
* AHT from Leopard Test : (1 hour test) : no problem

Others tests :
* A safe mode start without the external display : no problem

Next test to do :
- try the AHT long test from SL disk

Note :
* When I start with no external display the top of the iMac become very hot in few minutes (+15°C in few minutes)

OK. Doing some more research and looking over what you've done (also of note is the overheating you mentioned) to try resolving this leads me to think....

That the graphics adapter on the logic board may be defective. It seems like an odd ball glitch but it's possible that when you hook up the external monitor the video load on the GPU levels out and it operates normally. Without the external the GPU is loaded, gets hot, and causes the shutdown. Starting the machine in SAFE mode precludes loading some things. Take a look here at what happens when you boot to SAFE mode.

All the above is just my opinion of course and it may be wrong. But to be honest, I can't think of another reason why this should happen. And by the way, the graphics adapter is surface mounted (specially soldered) to the logic board of your iMac. Changing the GPU means changing out the entire logic board which could be a very expensive repair. ($700 or more)

If your machine were under warranty, I would recommend taking it to Apple and let them repair it. However, having to do the repair by yourself would probably not be worth it. Perhaps you can sell the machine as is?