Perhaps, if the problem is not caused by a hardware issue or failure,
you may be able to open up the bottom and use the Power Management
Unit Reset (button) that is hidden next to the airport wireless card location.
There are a few expensive parts that could also be to blame for this issue;
and a trip to the repair shop for a diagnostic may be warranted.
The PMU reset is a small button; there are some instructions in the service
manual I happen to have a copy of... that won't help you much! I saw in
another post in Discussions, where someone had linked to an XLR8 take-
apart page for the iMacG4, maybe it has this button clearly marked. The
button is rather hard to see once the metal plate is removed from the iMac
and it is small, too. I've not yet had to reset mine; but remember a few
discussions at MFIF referring to that as a cure for some issues.
The PDF service manual page is #108+, for details on PMU location/reset.
The image is most helpful; and the orientation is not mentioned in this
manual I have. Looks like the button has a little picture on or above it
that looks like a tiny battery; you press this ONCE. Twice will do bad.
The little picture is part of a row of small details under a plastic cover,
near the Airport Card slot where the contacts are for that to connect.
Here is a quote from the procedure page of the manual; please note
there are other tests and things not mentioned that a service tech could
perform to better diagnose that machine's problem. [Pg 108-109]
"The PMU Chip"
The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for the computer. The PMU (location shown below) is a computer within a computer. It has memory, software, firmware, I/O, two crystals, and a CPU. 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: The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes, the battery life goes from about five years to about two days if the PMU is not reset.
•Refer to the next topic, “Resetting the PMU on the Logic Board” for the procedure. Many system problems can be resolved by resetting the PMU chip. The PMU reset button is located on the right side of the board, refer to the picture (not here.)
"Resetting the PMU on the Logic Board Resetting the PMU"
(Power Management Unit) on the logic board can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.
1. Disconnect the power cord.
2. Remove the user access panel from the bottom of the computer. 3. Press the PMU reset switch (shown below) once on the bottom side of the logic board and then proceed to step
3. Do NOT press the PMU reset switch a second time because it could crash the PMU chip.
4. WAIT ten seconds before connecting the power cord and powering the computer on. If the computer powers on, go to the next step.
If the computer does not power on, there is something else wrong with the computer, refer to the symptom/cure, “No Power” in this chapter.
There could be a hardware failure, and if you cannot boot from the
Apple Hardware Test (on supplied CD or boot DVD) you should take
the computer to an authorized Apple service providor for testing, etc.
Best of luck in this; & thanks to Apple for the information posted above.
