2005 PowerBook G4 Special Report: Oversensitive Ambient Light Sensors, solutions

For some 2005 PowerBook G4s, the ambient light sensor -- responsible for automatic keyboard and screen brightness adjustment -- is apparently oversensitive.

For some users, the screen's auto-brightness is so sensitive that very subtle changes in light are triggering the feature. For example, Eric Jasso writes, "The screen twitches as the shade from the tree near my window moves!" and Fred Brunner notes, "The screen dims almost instantaneously from something like someone leaning over your shoulder to look at the screen and blocking some ambient light." Phil Geller notes that simply moving his hands around the keyboard causes immediate changes in screen brightness; this wasn't the case before the update.

Many readers have become so frustrated by this issue that they've resorted to completely disabling the feature -- this "fixes" the problem, but then forces them to do without what had previously been a useful feature.

Potential Solution

MacFixIt reader Nick Wirz has recently discovered a solution for the constantly adjusting brightness problem which involves using the shareware tool Pacifist to extract and install an older version of a kernel extension that interacts with the ambient light sensor to make adjustments.

The process is as follows:

1. Download Pacifist
2. Search for the file "AppleLMUController.kext"
3. Use Pacifist to open the install package of your original Mac OS X 10.3.x installation CD (the one that shipped with your 2005 PowerBook)
4. Using Pacifist, locate search for the older version of the same file, "AppleLMUController.kext," in the installer package and extract it.
5. Replace the "AppleLMUController.kext" file on your current installation with the version provided on the Mac OS X installation DVD. (You will be asked for your administrator password)
6. Restart the PowerBooks

Nick describes another problem where the ambient light sensor is not recognized at all:

"This has a couple of symptoms: Normally when working in a bright environment and attempting to turn on the keyboard lights, a small 'prohibited' sign appears underneath an on-screen feedback. When the ambient light sensor is not found however, nothing at all happens when you press the control keys for the backlight. Also, the checkboxes "Automatically adjust brightness as ambient light changes" in the 'Displays' preferences as well as 'Illuminate Keyboard in low light conditions' in the 'Keyboard & Mouse' preferences are missing. This can happen, for example, if you clone your OS from your PowerBook to a non-portable Mac and then back, or if you 'upgrade' to a PowerBook from a G4 for example, by just cloning the system."

In many cases, the age-old workaround of re-applying the most recent Mac OS X combo updater (10.3.8) resolves this issue.

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