Released October 2008 / 2.4, 2.53, 2.66, 2.8 or 2.93 GHz Core 2 Duo Processor

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After logic board replacement, no brightness control, stuck at max

I replaced the 2.4 GHz logic board in a late 2008 MBP due to a liquid spill. I replaced it with the 2.53 GHz logic board, due to the part being available and for a low price.

I replaced the board and it powered up just fine (after the spill it wouldn't power up at all.) While the thermal paste was breaking it I got a few sparklies on the screen and a few screen flashes / flickers. I continued to break in the thermal paste with a few more short term usages.

Now it seems to be running fine with one exception. I have no screen brightness control. The ambient light sensor works, in that it dims the keyboard backlight accordingly, but doesn't dim the screen. Further troubleshooting shows that the display won't dim by the F1 & F2 keys or the System Preferences>Display Settings either. The display sleep setting doesn't work either.

So it seems the computer has no control over the screen brightness. Is this a cable I missed (I followed the ifixit guide very closely) or is there some difference between the 2.4 GHz logic board and the 2.53 GHz logic board.

Thanks for your input.

Answered! View the answer I have this problem too

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Actually I fixed it today. It turns out that when I boot it from a remote OS from a server the brightness control began working. That was after trying a new user account to see if it was a profile setting that was stuck.

It turns out when I selected a different motherboard some system file that was in charge of controlling the brightness (whether to dim before sleep or based on input from the ambient light sensor) was no longer correct.

I fixed it by reinstalling Snow Leopard and auto-dim/keyboard brightness controls were restored.

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If the liquid was spilled on the keyboard there's a possibility that the F1 & F2 keys are stuck in full brightness. Could you try unplugging the internal keyboard and connect an external keyboard then check if the brightness control is restored.

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Hi Andrew, Can you tell me how to reinstall SL? are you talking about wiping every thing off my mac and reinstall every thing from the scratch or there is a better way to do it? I really don't want to format my mac and start from zero, Thanks.

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Andrew S will be eternally grateful.
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