Low light or no backlight on LCD
I have an 27" LED Cinema Display and when i connect it to my Macbook Pro 5,1 and connect the power supply i can only see the display on very very low light, i have tried changing the resolution and the light in the system preferences
The backlight with this model is built into the LCD display assembly and draws power, through a separate backlight cable, directly from the device's power supply and depends on the logic board to provide backlight information.
Diagnosing the failure in this would require you to use a digital multimeter, set to DC, to check for 23.3-25.7 V between the chassis ground and the 24 V test point on the logic board. If no voltage is present then the power supply is likely bad. If proper voltage is present then you should inspect the LED backlight sync cable, which is attached to the back of the LCD panel, for damage and then reseat it. If the issue persists then the next step would be to replace the LCD panel or logic board, in either order since they are both costly components.
Hopefully that gives you something to go on.
Here is a teardown for the 27" Thunderbolt display, the backlight portions of the internals are very similar to your model.
I did as "LikeARabbit " suggested above, and it worked for me.
I was having an issue of a very dim to complete shutdown of my Apple LED Cinema Display 27".
I opened the cinema display as per the instruction from the "Apple Thunderbolt Display Teardown".
The inside are almost the same as I have both and can attest to this.
I tested the power on the logic board that "LikeARabbit" suggested and mine was running a consistent 24.3 V on my digital multimeter. As to where to test this, please see attached photos. sorry for the bad quality photos, but I had very little time.
Since my power was within in range I made sure all the cable were seated properly by unplugging and replugging them one at a time. Do this with the power cord unplugged so you do not fry the boards, LCD, or yourself.
One plug I noticed right away was the "Function" plug which was very loose. I believe this is the LCD background lighting system as it istantly increased my brightness after reseating this cable.
After powering the LCD up, I moved this one cable around a bit, and sure enough, it was the one causing my issues. It is now properly seated and I have not had any problems as before. The LCD is working as it did when new.
I did create new monitor profile after this as I noticed the colors were off at this point. I am a photographer and need these LCD to match. with my iMac 27.
Below are the photos of the cable that was loose inside my Apple LED Cinema Display 27 and the two connections where I tested my power to the LCD backlight system.
I hope this helps.