Specifications

Power Mac G5 Troubleshooting - 5
General Information
Resetting the PMU on the Logic Board
The PMU (Power Management Unit) is a microcontroller chip that controls all power
functions for this computer. The PMU is a computer within a computer. Its function is to:
• 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.
Important: 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 goes from about five years to about two days.
Note: For the location of the PMU reset button, see “Logic Board Diagram” in the Views
chapter.
Many system problems can be resolved by resetting the PMU chip. When you have a
computer that fails to power up, follow this procedure before replacing any modules:
1. Disconnect the power cord and check the battery in the battery holder. The battery
should read 3.3 to 3.7 volts. If the battery is bad, replace it, wait ten seconds, and then
proceed to step 2. If the battery is good, go directly to step 2.
2. Press the PMU reset button once and then proceed to step 3. Do not press the PMU
reset button a second time because it could crash the PMU chip.
3. Wait ten seconds before connecting the power cord and powering on the computer. If
the computer does not power on, there is something else wrong with it; refer to the
“System” section of “Symptom Charts” in this chapter.
Note: The above procedure resets the computer’s PRAM. After resetting the PMU, be
sure to reset the time, date, and other system parameter settings.
RAM and Processor Verification: Power-On Self Test
A power-on self test in the computer’s ROM automatically runs whenever the computer is
started up after being fully shut down (the test does not run if the computer is only
restarted). If the test detects a problem, the status LED located above the power button on
the front of the computer will flash in the following ways:
• 2 Flashes: No RAM is installed or detected.
• 3 Flashes: Incompatible RAM types are installed.
• 4 Flashes: No RAM banks passed memory testing.
• 5 Flashes: No good boot images are detected in the boot ROM (and/or there is a bad
sys config block).
• 6 Flashes: The processor is not usable.