If you’ve owned an Apple laptop for any period of time, you may have experienced some finicky behavior that seems to be related to power. The PMU (Power Management Unit) controls how the laptop deals with all things electricity. Here are some examples of problems you can have that are related to the PMU:
- Your laptop has trouble literally turning on (as opposed to Mac OS X booting up properly).
- Your laptop doesn’t want to go to sleep or come out of it.
- Batteries that refuse to charge or even be recognized.
- Laptops that do not recognize their power cords.
- LCD backlighting that fails to activate.
- Trackpads that won’t respond or do so erratically.
- Fans spin loudly even though the computer is not hot.
- The sleep light does not function properly.
If you have any of these issues, you might be able to save yourself a phone call or a trip to an Apple Store by following these instructions to reset your PMU.
While resetting your PMU should not do any harm to your computer, it will reset your date and time. If you know of other troubleshooting steps you can take in your situation before resetting your PMU, you should exhaust those first.
Your Laptop Model Determines Your Troubleshooting Steps
Over the years, Apple has changed the procedure for resetting your PMU, so you will need to know what kind of Mac laptop you have in order to reset your PMU properly. Resetting the PMU requires no use of software. Instead, it typically involves removing your battery and/or pressing key combinations while your computer is off. As such, you will not get any feedback from your computer that you have performed this step properly or improperly.
The easiest way to know that your computer’s PMU has been reset is to boot your computer and see if the clock has reset (but note that if you have your time synchronized with a time server, it may get updated before you notice.
If, after resetting your PMU, your technical problem goes away, you can chalk it up to a corrupt PMU setting, and you’ll know how to solve the problem again in the future. You can be fairly confident you do not have a hardware problem, as it is not uncommon to reset your PMU at some point in the time you own your laptop (I’ve found that it’s especially common to have to do so if you’ve left your laptop in storage for several months and decide to “unretire” it).
If resetting your PMU does not solve your problem, you can try again, but it’s likely the issue is not related to the PMU. Thus, it’s an all-or-nothing proposition. Here’s what you need to do, based on the model of your specific laptop. Note: Make sure you turn off your laptop before performing these steps.
MacBook Air & MacBook/MacBook Pro (Late 2008 or newer)
- Connect the laptop to its power adapter.
- On the left side of the keyboard, hold the Shift, Option, and Control keys; while holding them, press the power button, then let go.
- Wait five seconds.
- Power on your laptop.
Older MacBooks and MacBook Pros
- Disconnect the power cord and remove the battery.
- Press and hold the power button for five seconds.
- Reconnect the battery and power cord.
- Power on your computer.
PowerBook G4 15″ & 17″ (Greater than 1.5 Ghz speed)
- Disconnect your power adapter and remove the battery.
- Press and hold the power button for five seconds.
- Reconnect your battery and power adapter.
- Power on your PowerBook.
All Other Aluminum PowerBook G4s
- Hold down the Shift, Option, Control keys and press the Power button. Do not press the function key.
- Let go and wait 5 seconds.
- Now power on your computer.
All White iBooks (G3 and G4)
- Hold down the Shift, Option, Control keys and press the Power button. Do not press the function key.
- Let go and wait 5 seconds.
- Now power on your computer.
Older iBooks and PowerBooks
- These can be more complicated instructions. Read this kBase article on resetting the PMU to learn more.
Did a PMU reset solve your problem? Did you have one of the above symptoms you solved with a different hint? Let others know in the comments.
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{ 3 comments… read them below or add one }
17 inch 1.67 Powerbook, issues are same as PMU corrupt. resetting PMU did not fix issues. Computer goes to sleep in the middle of working, will not wake. Could it be the power adapter? The plug has been dented slightly and is sticky when pulling out. Is it possible that it causes an the power level to freak out the PMU?
My computer goes to sleep, then wakes up for 5 seconds and goes back to sleep until i press the space bar. Even if i move the mouse it goes to sleep…. what do you suggest I do?
I had this problem, last fall, a 17 inch powerbook, PMU did not help in my case. Turn out to be the fan, it quit working on one side. Then the laptop would get to hot and go to sleep. Apple fixed the fan issue, $300 dollars. They would of course fix anything else they thought was bad. So, a another logic board, heat sink, hard drive, and keyboard later. I got it back, with a 90 day warranty. 95 days later, the logic board went out. I only used the laptop 3 times, due to getting a new MacBook Pro while the PB was in the Apple Shop. Anyway Apple would not touch it without another $300 deal. Oh, the MacBook Pro’s hard drive went bad at the same time. Warranty fixed that. Point being, Apple is not free of issues. Hope your travels go better. Also, Apple Genius told me that they have rarely seen laptops as clean as mine. HA!
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