Dim laptop LED screen:
There can be several reasons for a replacement screen not to backlight and stay very dark, these include of course the possibility that you have received a faulty or incompatible screen, but, what do you do if the replacement screen is not faulty? Usually a quick test would be to put the old screen back into the laptop again, even if it is cracked it should still light up.
If the old screen is now refusing to backlight and the new screen is refusing to backlight, you need to perform some tests to determine where the issue is.
Testing the laptop:
Firstly we need to ascertain whether or not the laptop is working properly regardless of the condition of the internal screen. Connect an external screen to the laptop or tablet device and confirm the display works when connected to, say, your TV for example.
Assuming the laptop itself is working we now need to ascertain whether or not the replacement screen you have received has a fault. There are professional ways to test an LCD screen, such as with an LCD tester; however in this case we can perform a simple test without any expensive equipment. Refit the old screen, even if it is damaged it should still light up as it did before. If your old screen does not light up now you need to read on:
No screens backlighting anymore:
Now that we've proved your new screen and old screen both will not light up, we need to shift our attention away from the new screen being faulty and start to look at what else could have happened.
You may notice that although black, any screens fitted to this computer do in fact display a faint (very dark) image on the display but the image is not "lit-up". Looking at the screen at an extreme angle or with direct light onto the display can often show up a very faint image of the operating system.
Basically at this point we can conclude that the screen is getting a video signal, but it is not receiving power for the LED backlights.
How does the screen get power for the LED backlight?
The LED backlight is powered by the motherboard which is the main logic board (or brain) of the computer; and sits under the keyboard (usually). The signal cable for the screen (the flat grey cable you connect to the back of the screen known as an LVDS or EDP cable), runs down under the keyboard and also connects to the laptop's motherboard.
Here around the connector socket on the motherboard there will be an array of fuses. If you have replaced your laptop screen without removing the battery or power supply, or you have worked on the machine whilst it is on, there is a good chance you will have blown one of these fuses on the motherboard. Sony laptops are especially sensitive to this and DELL laptops can exhibit these symptoms as well as all other brands and models, however these two brands are kind of known for this behaviour.
Guess why we keep saying remove the battery and power before you begin? ;)
What does the fuse look like?
The fuses (surface mount fuses) are tiny "blocks" on the motherboard, the fuse related to powering the display will usually be around the are of the screen cable connector. Here's an example from a Sony Vaio laptop:
To find the fuse that has blown you will need to use an Ohm Meter. We should mention that the Ohm Meter's electrical contacts must be clean, the motherboard must be clean and free from dust (clean it with rubbing alcohol). There must be a good contact between the Ohm Meter and the fuse pins. We could even go so far as to say to file the contacts of the Ohm Meter a little before performing the test. Any dust or dirt between the contacts can give a false reading.
Wipe each fuse clean in turn and test the impedance. Good fuses should read zero Ohm.
Once you have identified the bad fuse you are ready to remove it and replace it.
The fuse will probably be numbered and usually with Sony and DELL machines the fuse for the screen power is labelled as F and then a number. F1, F7 etc are common numbers you may see the fuse labelled as.
Here is an example of the fuse on a DELL laptop motherboard, this is an Inspiron model.
And another one from a Sony VPC E series machine: