The Power Management Unit (PMU) is a micro-controller chip, controlling all power functions for the computer. It is a computer within a computer, with it’s own memory, software, firmware, I/O, two crystals, and a CPU.
The PMU performs the following functions:
- Tell the computer to turn on, turn off, sleep, wake, idle, etc.
- Manage system resets from various commands.
- Maintain parameter RAM (PRAM).
- Manage the real-time clock.
It is VERY IMPORTANT you are careful when handling the main logic board!
Always remember to remove the battery when handling the logic board. This will help prevent damage to the PMU. It is very sensitive. A simple touch of the circuitry on the logic board can cause the PMU to stop working correctly. If the PMU is not working correctly, the battery life can goes from about five years to about two days, if the PMU is not reset.
The PMU chip can be found in the general vicinity of the battery.
Many system problems can be resolved by resetting the PMU chip. Whenever you have a computer that fails to turn on, follow this procedure before replacing any modules.
To Reset the PMU Chip:
- Disconnect the power cord from the affected system. It is also a good idea to disconnect any additional devices which connects to the system such as the Keyboard, Mouse, Printer, Scanner and etc.
- Check the battery in the battery holder (BT1) with a Volt Meter. The battery should register between 3.3 V to 3.7 V. If the battery does not register in this range, it is bad and needs replaced. After you replace a bad battery with a new one, wait ten seconds, and then continue to the next step. If the battery is registers within the correct range, continue on to the next step.
- Press the PMU reset switch (S1) on the logic board, only once! DO NOT PRESS THE PMU RESET SWITCH A SECOND TIME! It could stop the PMU chip from responding.
- After you have pressed the PMU Reset Switch, wait ten seconds before connecting the power cord.
- Power on the computer. If the computer turns on, you can re-connect any additional devices which may have been connected to your system (I.E. Keyboard, Mouse, Printer, Scanner and etc.). If the computer does not turn on, the PMU needing reset, is most likely not the source of your problems. This may mean you have a more severe problem which requires the professional help of Apple Technicians.
The information in this article came from https://macosx.com/tech-support/mac/powermac-g4-mirror-doors-won-t-power-up/14527.html!