In this post I will discuss the importance of the Power_Good(Pin 8 Gray color wire on your ATX PS unit) signal in a PC system. The Power_Good signal is a +5v signal generated in the power supply section when it has passed its internal self tests and the output stabilized.This normally takes anywhere from 0.1 to 0.5 seconds after you turn on the power supply switch.This signal is sent to the motherboard, where it is received by the processor timer chip, which controls the reset line to the processor.
In the absence of Power_Good, the timer chip contiouosuly resets the processor, which prevents the system from running under bad or unstalble power conditions. When the timer chip sees Power_Good, it stops resetting the processor and the processor begins to run BIOS(you can now see a display on your monitor).
In short Power_Good signal is supply voltage that is to be supplied to your processor for your PC to start, if Power_Good signal is absent upon turning on your PC, no way that it will initialize.On the opposite, cheaper PS units may also supply Power_Good signal to your processor even in unstable voltage ratings thereby damaging your peripherals or motherboard later on.
So next time when you plan to buy a new PC or replace your PS unit, consider buying a known good brand, it will difinitely give less trouble to your PC.
Note: A defective Power Supply unit can be one reason of a Blackout(No Display) CPU.