This portion of the AHT will repair no brightness control on a repaired monitor and actually is required when repairs to the internal logic board are performed.
It is not noted or emphasized that for this to work properly OR for brightness control to work properly, the monitor USB cable MUST be attached.
There is no brightness control without the monitor USB cable attached and there is not supposed to be.
In short, make certain the monitor USB cable is attached befire running this repair tool or trying to control brightness.
It is also worth noting that the 27" LED Cinema DisplayPort equipped monitor HAS problems with monitor USB connections.
This is due to the anodization of the chassis. (anodization produces non conductivity)
Make certain the USB connection is made before trying anything else.
Do this by using a keyboard or mouse attached to the monitor USB ports.
If there is no connection (as silly as this sounds) try wiggling the USB cable at the rear of the monitor until a connection is made.
This makes and breaks ground momentarily and typically re-establishes USB connection.
It should also be noted that another setting should also be attempted.
Go to system preferences Sound and choose an Output option other than monitor USB (Display Audio).
For those who use wireless keyboard and or mouse the monitor USB connection must be verified by some means. Sadly, this may mean that your only verification is use of Display Audio as the Sound output.
In any case, make certain that the monitor USB connection is made through one means or another. (try a thumbdrive)
In some cases selecting Sound output as something other than Display Audio can help clear up a USB conflict.
(don't forget to try a restart after changing settings)
The no brightness control condition varies from computer to computer.
For example, this problem on a Mac Pro appears as no Brightness control slider at all and no Auto brightness adjust check box at all.
For others this may appear as grayed out.
If you have followed ALL these simple steps, there is a possibility that your monitor has a logic board problem that can only be verified through use of an AHT** if you are lucky enough to have one or.... Apple ASD which is only available at a service center.
(**NOTE: Apple has discontinued use of the AHT due to limitations that failed to diagnose problems.... don't quote me please!.... but I believe the AHT was discontinued in 2010 for most models)
It’s possible that the issue might simply be resolved when applying both a Software and a Firmware update for the LED Cinema Display. If you haven’t already, please download and install the following to see if these updates resolve the issue:
Secondly, if you are using Mac OS X 10.8 (Mountain Lion) or Mac OS X 10.9 (Mavericks), please try this additional Software Update for the LED Cinema Display if you haven’t already installed it:
Additionally, my research into other customers’ cases with these issues (brightness controls and USB functions not behaving correctly) regarding the LED Cinema Display shows that the following method might fix your issue: When connecting the affected LED Cinema Display into your Mac Pro, first plug the USB connector into the Mac, then plug the Mini-Display connector into the Mac, and finally re-open System Preferences to see if the brightness controls have returned.
That mentioned just previously is worthless for those with an updated operating system but is mentioned regardless.
As it works out those installers will not allow installation and merely state that you don't need this software in updated systems.
They require that you make a brand new OS installation to be of any effect and are included in Software Updates.
They are worth a try if all else fails.
You should also try Disabling your Bluetooth "Discoverable" setting.
The Discoverable Bluetooth setting seeks to establish a wireless USB connection and depending on your RF environment, can become very busy if not conflicted. (can override a wired USB connection)
Last but certainly not least.
Do NOT connect your monitor USB through a USB hub.
Internally your monitor has a USB hub.
Connecting a USB hub TO a USB hub is just asking, (if not blatantly begging), for problems.
I went through three months of discussion with Apple top level technicians.
The Final result was admittance that this is an SMC problem.
(I was given an Apple Extreme Base Station free for my trouble)
The end result was an alteration to SMC but only for the newest Mac Pro and not previous.
(see Apple updates for the 2013 Mac Pro http://support.apple.com/kb/DL1724 )
In few easy to grasp words....
The internal bus speed of the newer Mac Pros occurs so fast that low speed USB connections (your keyboard) are skipped. The System Machine Code (SMC) does not receive a connection handshake within a certain time period and determines that no connection exists. (stops trying to connect)
Oddly enough this does not occur in all models due to slight grounding differences present in cabling and or computer or monitor. (those well grounded work just fine)
ALL do not have this problem and it is somewhat rare but certainly very real.
For those of us who have this problem and do not own the newest Mac Pro with SMC update, there is currently no solution except to disconnect / reconnect keyboard upon startup and restart.
(the Mac Pro will maintain USB connection if put to sleep and woke from sleep)
Somewhat humorously.... if you connect a wired mouse to your keyboard, it will make USB connection with the mouse but not the keyboard it is attached to.
If your monitor / display System Preferences do not show a Brightness Control Slider, you have a secondary problem that is fixed with the EDID_Reset_Tool.
This problem is typically associated with a repair of the monitor. (it is part of the Apple Hardware Test AHT that is now obsolete)
An internet search for your monitor model will give you this tool. (Must be for the correct Monitor type)
For the 27" Display Port Apple monitor.... http://download.info.apple.com/Apple_Hardware_Test/EDID_Reset_Tool.dmg
The only other option is to run an online hardware diagnostic. http://support.apple.com/kb/ht1509
My monitors have always been too bright, not too dark. (creates false photo print adjustments)
My settings are one notch above zero brightness.
If you monitor is too dark, odds are.... your room is too bright. (in energy physics law and engineering, this condition is called a Bias created by the photoelectric effect)