display backlight dark after PRAM reset

Discussion in 'OS X' started by rikers_mailbox, Jan 16, 2013.

  1. macrumors 6502a

    Sep 27, 2003
    In the process of troubleshooting a 20" iMac (Late 2006) I reset the PRAM. This somehow resulted in some strange LCD display behavior.

    Now when I boot the machine the screen initially lights up and displays the grey screen, I hear the usual startup chime, but then after a few seconds the screen goes dark (just about the time the Apple logo appears). The machine otherwise starts up normally and I can see the boot finishing if I hold a flashlight up close to the screen.

    The screen appears to work fine, the backlight comes on at startup but then shuts off. And it worked fine up to the point when I reset PRAM.

    What happened when I reset PRAM that would cause the backlight to stop working? How can I restore PRAM display settings so the display will function normally again? What's wrong with my boot.efi?

    Thanks in advance for any help.
  2. macrumors 603


    Reset the PRAM again, hold the keys until you've heard the third chime them release, see what happens.
  3. thread starter macrumors 6502a

    Sep 27, 2003
    I have tried resetting PRAM again. No change.

    Reset PRAM instructions say to hold until the 2nd chime. Why do you suggest the 3rd?
  4. macrumors 603


    Because that's what we are used to, but I think 1 should already do the trick.

    What about a SMC reset, did you try that one, or try Apple Hardware Test and see if you get errors.

    An SMC reset is more or less this, shut down the mac, unplug from power for minimum 10 seconds, plug it in again and wait for minimum 5 seconds(From Apple) and startup again.

    I think though there is something going on with backlight, might be Hardware.
  5. thread starter macrumors 6502a

    Sep 27, 2003
    Tried SMC reset already. No luck. Another odd thing, the machine boots itself when plugged back in without pressing the power button.

    If it's a hardware issue, why does the screen turn on initially at boot?
  6. macrumors 603


    There are preferences to set the startup to be automatic when the power has been cut, in my System it's under the Tab Options in Energy saver but I am on an older Mac.

    Do you have a bootable backup, if so try to boot from it and look if it happens there.
    Or, hold the Command-R to get into recovery, not to recover but just to test if it happens there as well.

    And, do the Apple hardware Test, hold D at startup.

    Edit: Might be a loose connecter.
  7. macrumors member

    Dec 17, 2006
    You left out a key step: When the unit is unplugged, push the power button and hold it for 5 seconds, then plug it back in and power it up. Without pushing the power button unplugged there is no SMC reset.
  8. macrumors 603


    Since it's an iMac this goes, from Apple:

    Intel-based Macs: Resetting the System Management Controller (SMC)


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