Ugh. I turned off my computer around 12 to do a little housecleaning with it. Clean the fans, lint filter, rearrange the wires. Put everything back together, and push the power button. And push the power button. Nothing’s happening. So begins a four-hour saga.
Part 1. Switches
Maybe the power-on switch stopped working. Set the multimeter to continuity. Nope, the switch works fine. The switch plugs into the motherboard on a two-pin header. When you plug the computer in, it doesn’t come on immediately. You must push the power button (a momentary switch) to turn on the computer. The switch closes a circuit on the motherboard to power on. Maybe I can short the motherboard switch header… no go.
Part 2. Bad connections
Maybe there’s a bad connection on something plugging into the motherboard. Remove every cable and reseat. Remove every card, well the only card for me, and reseat. Remove all the memory and reseat. No go.
Part 3. Bent pins
Maybe there’s a bent pin on the motherboard. Now a mini-saga of removing the motherboard from the computer, and inspecting both sides to find bent pins. No shorted or bent pins on either side of the board.
Part 4. Out of power
Maybe it’s the power supply itself. Fortunately I have an ATX power supply tester. Removing only the ATX connector from the motherboard (leaving the 12v connector) test the power supply. All green on the power supply, and all motherboard connected fans (video card, northbridge, fan headers) start turning. Hard drive spins up. The motherboard isn’t totally bad if it can supply power to the slots and various fan connectors.
Part 5. Hotwire it
Since I’m not an electrical engineer, there’s only one more thing I can try before giving up the motherboard for good. (It could have been the CPU itself, but I find that unlikely.) Dig out the motherboard manual and inspect the ATX connector. Somehow the power-on switch tells the ATX to turn itself on… Here it is: a pin labeled PS_ON#. What happens when I short PS_ON# with a common? Success! Post starts!
My motherboard, an Abit AN7, has an on-board POST readout. It went from 00 to eventually 0A. 0A means “where’s the keyboard?” The only thing plugged in was the power cord.
Part 6. Success and aftermath
So, let’s cut the end of a paper clip, it’s the perfect size to short PS_ON# to an adjacent common pin.
Here’s where it gets weird. With PS_ON# and common shorted, flip the hard switch on the power supply. The system immediately powers on, but doesn’t begin POST until I press the power button on the front of the case. Even more weird: the power button now acts like the reset button after the system is booted. When I momentarily connected PS_ON# and common, the system booted just as if I pressed the power button. While sitting on post code 0A, I could press the power button and the system powered itself off normally (post status goes to 99).
I don’t understand why the system wouldn’t power on with the button but will reset and power off with the button. Now running with a hard-wired PS_ON# and common short, the computer appears to be working normally.