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A possible fix for a Mac that won't power up
Authored by: mike666 on Apr 11, '05 11:05:48PM

As well as keep your machine from ever booting again without replacing the logic board...

robg, PLEASE, post the instructions for a proper PMU reset procedure for desktop machines in the displayed text of any hints that get close to this subject. They are:
1. Unlpug the power cord.
2. Using a non-metalic tool (your finger, if it'll reach) push the PMU reset button (location varies by machine) ONCE and for no more than one second.
3. Wait 15-60 seconds.
4. Plug the power cord back in and hit the power button.

Note that a bad logic board, processor or stick of RAM can cause the same symptoms described by the original poster so a PMU reset is no guarantee.

"I'm never quite so stupid as when I'm being smart." -Schultz

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A possible fix for a Mac that won't power up
Authored by: timhaigh on Apr 12, '05 07:16:01AM

I don't know where the myth came from that suggests if you crash your PMU you need to replace your logic board but it's not true.

The most you can do crashing PMU is to expire the battery.

For those that want the official procedure for resetting the PMU here it is.

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.
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.
For the location of the PMU reset button, see "Logic Board Diagram" in the Views
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.
The above procedure resets the computer's PRAM. After resetting the PMU, be
sure to reset the time, date, and other system parameter settings.

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A possible fix for a Mac that won't power up
Authored by: mike666 on Apr 13, '05 12:47:06PM

I have had a number of clients with un-revivable logic boards who've told me that before bringing their machines in for service, someone they knew told them to either hit the PMU reset button several times in a row or even to hold it down for thirty seconds or more. Granted, this is just a correlation and not a proven cause and effect - more likely these folks may have just had an ESD incident while they were dinking around inside their machines or the MLBs had already failed in some other way - but why take chances? If a little fear causes people to be conservative and extra careful when messing around with a part that's US$300-500 to replace, that's less harmful misinformation than the many and varied PMU reset procedures that get propagated here and other places on the web. BTW, thanks for posting the official procedure.

(Note: Apple appears to be dispensing with the PMU - the newest G5s have a SMU (System Management Unit) which handles most of the same tasks but does not have a reset button. It's reset by simply unplugging the power cord for 15 seconds. I would hazard a guess that Apple may be tired of all the warranty logic board replacements that they've done because the user did an improper PMU reset...)

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