WAIT!!! The plot thickens. It seems that the solution isn't necessarily the Intel provided graphics driver vs. the  Dell provided Intel graphics driver.

I applied this fix to one of two machines that I am troubleshooting with the exact same issue and surprise surprise. Out of nowhere, the machine that has the latest Intel provided driver installed has magically fixed itself, in regards to the fn brightness key???

I did just wake the computer up from a hibernation of about 4 hours, so let's see how long its last...It could mean that something didn't fully load out of hibernation? I don't know.... It's all guess work at this point.

Yep buddy, join the club.  I've been trying to get the M4600 and M6600 display driver working for some time.  It worked solidly for about 6 months, and then nada.  Yep, installed newer, and reverted to older Dell Feature Enhancement Packs, nada.  Even installed Microsoft VC++ Redistrobution pack as a hunch (it fixed my E1705 for display brightness).

Recently, I've slammed Windows 8 onto the M4600.  Windows 8 seems to have native support for Fn Key brightness control, with no Dell drivers!  Believe it or not, the Fn key and Arrow up/down worked!  It worked as-is with no driver install!

Of course, I had to go and install the Intel chipset drivers as well as the touchscreen drivers and touchpad.  After those are installed, the brightness stops working again.  ARGH.

I was using the chipset drivers straight from Intel.  I'll try to install Dell's older ones, but the newer Intel ones enabled full SATA3 support under Windows 8 (though SATA3/USB3 is native to Win8).  

I've recently installed WIndows 8 again (be careful, no more "trial" available in Windows 8 - it activates every time now).  After a raw install, the brightness did not work ever, unlike before where it did work after a fresh install before the Intel drivers.  

Who knows... It's so buggy.

Subscribing to this thread, so you can keep us posted in your success.  

Dansheer,

This problem is with the Optimus being enables.  Once to disable Optimus the Fn keys for brightness always work.  That may be a clue for you.  I have a M6600 and a M4600 both having the same problem.  It is not the drivers, its Optimus.  Dell is no help as they do not know of a solution.

Thanks

Paladin

Orisocean,

Disable Optimus in your Bios setup and you should be good.  There is no other solution.  

Thanks

Paladin  

I have M4600 with same problem here, Fn+up or Fn+down is not working

Help me to solve the topic

Thanks

I too have verified it is Optimus enabled in the bios.  The custom Dell bios has a bug in it for when Optimus is enabled, there is little to no Fn key support (oddly enough, it works the first couple of reboots during OS install - then nada).

Personally, I must use Optimus.  I constantly get 3.5 to 4.5 hours out of my 6 cell battery in meetings all day, running two VMware VMs - both with 4 virtual CPUs and 6 GB of ram each.  Without Optimus, I only get 1.5 to maybe 2 hours with the nvidia card powered up.  I do this every day - I constantly run my battery down, every day.  I was one of the first who got this laptop in early May 2011, before the ship dates was announced on the site.  Even technical support didn't have my tag number in their system when I got mine!  

So needless to say, I've been cycling my batteries quite a lot and know the advantages of Optimus.  I also use Optimus to power 4 displays from my M4600.  Two from the Nvidia card (HDMI and Display Port), and two from the Intel 3000HD graphics on the CPU (VGA and laptop screen).  With my advanced port replicator, I always use 3 monitors (two DVI and one VGA).  You cannot do more than 2 displays with Optimus disabled - that is because the Nvidia 500 series cards only support 2 displays (the 600 series supports 4 per GPU!). 

Tip for those that want to get the most out of Optimus: First, make sure  you aren't using any Dell drivers.  Install real Nvidia drivers, and real Intel HD3000 drivers.  Next, make sure to goto the Nvidia control panel and in the menu bar, under Desktop I think, make sure to show the GPU Activity icon in your tooltray.  Then go down into your tooltray, and force it to show at all times.  What this icon does is it changes colors from black (Nvidia GPU is off) to colored rainbow (Nvidia GPU is on) when something is using the GPU.  I constantly scan that icon to make sure my GPU is disabled, for maximum battery life.  If it is in use, you can click it to see exactly what is using the GPU - so you can exit the program (Logitech G930 headphones want to use my GPU? Why in the hell?  Denied it by shutting it down).  Finally, go into your Nvidia Control Panel and into 3D Settings for the Optimus settings.  Makes sure the defaults are all selecting Integrated Graphics (which is the Intel HD3000).  That gets you 99% of all on Optimus.  I had to force a few things to use Optimus, like IE9 (switched to Chrome now) because IE9 thinks it needs the full pwoer of the Nvidia GPU to render flash videos - it doesn't.

Bottomline, there is a bug in the Dell bios for the M4600 for when Optimus is enabled you can't control the brightness.  Tech Support doesn't even know how to use Optimus properly, and advices (on every call I've had with them) to disable Optimus.  Very sad for consumers.

Out of nowhere.  When the PC is put in hibernation and it comes out of it, the brightness keys start to work.

I've got the same problem with two Dell m6600 too for Windows 8. Latest drivers, quickset from Dell, hasn't resolved in a year of troubleshooting this. Any Thoughts?

MW64