The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for the server. The PMU is essentially a computer within a computer. Its function is to:
- tell the server to turn on, turn off, sleep, wake, idle, and so forth.
- manage system resets from various commands.
How to reset the PMU
Warning: Be very careful when handling the logic board. The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes and is not reset, the battery life is significantly diminished. Many system issues can be resolved by resetting the PMU chip. When you have a server that fails to power up and software issues have been eliminated as a possible cause, follow these steps before replacing any modules.
- Disconnect the power cord and check the battery in the battery holder. The battery should read between 3.3 and 3.7 Volts. If the battery measures outside of this range: replace it, wait ten seconds, and then proceed to step 2. If the battery voltage measures within this range: go directly to step 2.
- Press and release the PMU reset button once and then proceed to step 3. Do not press the PMU reset button a second time, as it could crash the PMU chip.
- Wait ten seconds before connecting the power cord and powering on the server. If the server powers on, go to step 4. If the server does not power on, there is something else wrong with the server. Verify the voltage on the power supply.
- Run Xserve Remote Diagnostics to verify the system.
The procedure outlined above resets the server's real-time clock (RTC). After resetting the PMU, be sure to reset the server's time and date.