When I start the machine the machine boots up but the screen is black. I then reset the PMU and start up the machine and the screen lights up. Now I have replace the internal battery and external battery. Is the a PMU problem? The hinges were replaced a couple of years back. Sometimes if I move the screen the machine will lock up.
If you look real close at the screen when it goes out, can you still make out an image like the brightness is turned down all the way? Another good test is pointing a flashlight into the Apple logo on the back and see if you see the image.
I think my problem is the PMU. I have left my laptop plugged in all of the time and it starts up just fine. When I unplug the laptop from the power source, it starts up with a black screen. And there is no evidence of anything on the display using a flashlight.