Monitor is black out! Help!

I opened up my iMac a second time. I replaced thermal paste on the CPU & GPU. In the process I got some paste onto some other parts which I wiped up with alcohol.

After re-assembly my system would not turn on the main display. When I hook up other display via Thunderbolt (DisplayPort) the system is still working.

I went to repair center and told me to change logic board for 700 bucks. It did happen before without opening, but some how it cameback with PRAM draining trick.

Only thing I didn't try is changing the coin battery and changing display cable. It looks like backlight is not working

I can't think of any reason for it to not work. other than thermal paste (Arctic Silver 4) heard it is not conductive. The fact that the computer itself still works makes me wonder even more.

Does anyone has solution to this?

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I have never seen one of these coin batteries go bad on a Mac.


I've seen a video to fix black screen issue with the coin battery he said it has about 10% chance when thr issue occur. I'm ganna try tonight


First the PRAM battery last quite a long time! As @mayer stated. It's only used when the AC power cord is disconnected. I'm just seeing a few 2009/10 iMac's that need a replacement battery, I doubt thats your issue here unless you leave your system unplugged or turn off your power switch on the surge suppressor, then the battery might need replacing.

You talk about going into the system a second time, so the reason you did was for this display issue? And you thought the GPU's thermal paste needed replacing.

- Did you at least run the onboard diagnostics? If you did any errors?

- Did you monitor your thermals & fan TG Pro? I would have started there. Are your running hot?

- What are you running on your iMac that could be pushing your system?

As you imply the display is now dark all of the time lets shine a flashlight into the screen. You'll need to hold the flashlight against the display at a sharp angle. Are you able to see a faint image of your desktop and the icons on it? If you do your issue is the backlight logic and its power. If you don't your issue is the display cable and/or the logic board cable connector.


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