Both upgraded to macOS Mojave 10.14.6 18G103 - It took me a while to notice on the MBP because I use it as a desktop with a wireless keyboard and a dust cover over the built-in keyboard, but I upgraded the MBA, noticed this on the first restart, and then came to see that the MBP has the same issue. Reset PRAM/NVRAM & SMC on the MBA, no change.

Any idea whether it's a known issue or what else I can try?

  • (I'm not on mojave, so I'm not seeing the same system preferences as you) but... is there any setting under keyboard that mentions auto dimming or auto adjusting levels that might be setting the value to low to notice? – dwightk May 6 '20 at 1:22
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    Nothing I can find either under keyboard or accessibility. When I search for "keyboard" in the System Preferences search control there's one item called "illuminate keyboard", but it highlights the keyboard icon, which has no controls for illumination... – Eljuan May 6 '20 at 2:38
  • I forgot that Macbook air 5,2" was a thing... It came out weirdly late too (~2013). Like waaay after the whole mini laptop (netbook) craze of ~2008. I remember looking at that thing at an Apple store with disgust. Anyways, I have no idea how to fix it. It seems like this isn't a very common issue or at least I've not seen one where the "Automatically illuminate keyboard in low light" setting goes missing. My late 2012 MacBook air is running Mojave and the backlight is working fine. Do you see the stop symbol if you try to adjust the backlight manually from the keyboard? – Joonas May 6 '20 at 7:38
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    5,2 is an Apple model designation for a 13" model from mid-2012 (A1466). There is no response on-screen, but I can see this on the console: default 08:57:55.680716 -0400 kernel X86PlatformPlugin result 0 default 08:57:55.680721 -0400 kernel X86PlatformPlugin result 0 – Eljuan May 6 '20 at 12:55
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    Well I found the problem. Both (one started out as a clone of the other) were missing AppleSMCLMU.kext. Bit of a shell-game with booting from optional devices on a healthy machine to grab it & then install it on the troubled ones, but as soon as I loaded that kext both the keyboard backlight key functionality and the options in the preference pane came right back. – Eljuan May 11 '20 at 3:41

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