How does the laptop know to go to sleep when the lid is closed?
On the PowerBook G4 15″, there is a magnet on the back of the LCD that activates the sleep switch when the lid is closed.
While the position of the magnet and sleep sensors vary on different Mac laptops, the 15″ and 17″ (all models except the 1.67Ghz) PowerBook G4s are unique in that the magnets are just taped to the back on the screen.
It is important to make sure that when installing a new screen, the sleep magnet is transfered to the exact same spot on the back of the new LCD. Even half an inch one way or another and the magnet will not activate the sleep mode when the lid is closed. New kapton tape should also be used to insure that the magnet does not fall off with use.
The first laptop we noticed with this sleep magnet location with was the original PowerBook G4 Titanium. To this day, I would say the TiBook screen replacement is the most difficult of any laptop I’ve ever worked with (the bezel, display house and hinges are all glued together and require expert workmanship for a clean removal and seamless re-assembly.) You wanted to be 100% certain of the location of the sleep magnet before finishing that repair.
Later Mac laptop models have specific places on the bezels where the magnets go making screen replacements much easier in that respect.
Does the magnet being on the screen hurt the display? Not at all. The strength of the magnet is pretty weak and only CRT screens were that susceptible from damage from magnets.