My laptop screen is black, it does not mean that the laptop has stopped displaying image, what has happened is that image displayed by the laptop cannot be clearly seen because the laptop LCD screen backlight is not functioning due to one reason or the other. The common cause of this type of fault is a dead backlight tube or a faulty LCD inverter.

To troubleshoot effectively, you need to open the laptop screen, before doing this please check http://learnlaptoprepair..com/2013/08/how-to-replace-broken-lcd-screen-on.html for instructions on how to replace an LCD screen. During dis-assembly of the screen, take note of the points where each screw is before it was removed, so that you won’t have problem assembling the laptop screen back.

Possible Faults

1. Burnt or dead backlight tube
2. Faulty LCD inverter
3. The laptop motherboard does not supply the LCD inverter with the 5V needed to power the screen backlight
4. The laptop LCD lid close switch is stuck in the close position

Solutions

1.After opening the laptop LCD screen panel, bring out the backlight tube and check for clear sign of burns on the tube, if sign of burns is visible on the tube then it means that the backlight is burnt and needs to be replaced but if not, you move to the next stage of troubleshooting, get another working LCD inverter, remove the one that was there before and replace it with the new one that has been confirmed to be working, then just power the laptop with all the component plugged in but still disassembled.

2.If the backlight lights up, then there is nothing wrong with it and with some degree of certainty we can also say that the LCD inverter that was in the laptop before is faulty, but in a situation where the backlight does not still light up, it means that we just have to take the troubleshooting process deeper .

3.The next step is to confirm if the LCD inverter is receiving the 5V required from the motherboard to power the backlight. This is very important because the function of the LCD inverter is to transfer power to the backlight from the motherboard.

4.To confirm if the LCD inverter is getting the required voltage from the motherboard, turn on the laptop (remember it is still disassembled) take your multimeter, switch it to D.C voltage and let it be within a range that is above 20V, use the multimeter to read the voltage “+” lead of the multimeter to the pin 1 on the connector and the “-” lead to ground or any metal. .

5.If you get a voltage value of 5V and above then it means that the LCD inverter is being supplied with the required voltage from the motherboard, which implies that the 5V supply from the motherboard is not the problem, it simply means that the backlight tube is bad and needs to be replaced. But if it turns out that there is no supply of 5V from the motherboard to the LCD inverter, then it simply means that you have to jump or transfer 5V from the motherboard to the LCD inverter.

6.To do this, look for a light wire, locate a component with 5V DC supply on the motherboard, then use the wire to tap or jump voltage from the motherboard to the inverter (the positive terminal e.g. pin 1 on the inverter connector). With this done, provided the LCD inverter itself is OK, the backlight should start functioning again and image displayed on the laptop LCD screen will become bright and visible again.

7. In some rare situation, after all the above possible cause of fault have been ruled out. The LCD lid close switch might just be the cause of the problem, this is possible if it is stuck in the close position. In this mode the backlight stays off, even when the laptop is ON. This is to save or conserve power, to release the lid which is often times located near the hinges of the laptop screen from the close position, just press it so that it can come up or use something to raise it up. You can also check the brightness and contrast setting of the laptop just to be sure.

Source: http://www.learnlaptoprepair..com/2013/09/my-laptop-screen-is-black.html