I've often heard tales involving the removal of a Mac's battery — the little, round, watch-style one that makes sure the machine remembers what time it is, among other things — from the motherboard for a half an hour as a solution to various power issues, but it's never been effective for me. Until recently, that is.
The other day I was vacuuming. The computer was shut off. When I'd finished with my cleaning I noticed that my powered-down Mac Pro was making a faint clicking noise which was coming from near the power switch. The machine would not turn on.
I recalled something similar happening before — New York City has notoriously flaky electrical power — and I remembered that the solution involved resetting the SMC switch, the little module that's in charge of certain aspect of power management for the system. So I unplugged the beast from everything, laid her on her side, opened her up and pressed the SMC switch. Waited a minute. Plugged her back in. Same faint clicking, no power.
What followed is your basic lesson in hardware troubleshooting 101, in a nutshell: Remove and reseat all removable internal components and test. This includes the RAM, the graphics card, whatever fans I could pull and an extra internal hard drive. Nothing helped. After each remove-and-reseat, the same clicking sound, the same inability to boot.
Finally, the last ditch attempt. Before heading all the way down to the Apple Store to request service, I figured I'd pull the little internal watch battery — the PRAM battery as it's called — and wait a half an hour. I mean, what could it hurt, right?
Bingo! It actually worked. After putting the battery back in the Mac, and plugging her in, the clicking was gone and I could power up again. Amazing!
I was pretty disappointed to find literally no such similar problems out there in Internetland. In fact, searching "Mac Pro battery" seems to only yield results pertaining to MacBooks. So consider this my contribution to the great troubleshooting reference in the sky. Hopefully, those having similar problems — if they're even out there — will land here and know what to do.
If none of these steps works for you, you might try replacing the battery altogether. The battery for my Mac Pro (Early 2008) can be found at OWC. Apple keeps a list of systems and their batteries if you have a different machine.
If that doesn't do it I'd highly suggest heading to an Apple Store (or certified repair shop), if possible, or calling up Apple for service if not. My guess is you likely need a new power supply or motherboard.