Chosen Solution
My 27” iMac doesn’t want to turn on. Did the usual check of outlet, cord etc. I read somewhere to try unplugging the power for at least 30 seconds. That didn’t work, but I did find that unplugging for several hours did. When on, it seems fine. But if I turn it off, it absolutely will not power back up unless unplugged for an extended time. Does this make any sense? My inclination is to replace the power supply, but am puzzled by why it works okay if I can coax it on. Any feedback appreciated. Thanks. Update (03/15/2019) I did replace the pram battery (Apple didn’t make it easy), but this did not correct the problem. As before, will only start up after being unplugged for several hours. When it does turn on, it seems completely normal. It’s as though something has to cool down before restarting. Is there a component in the power supply that would act this way? Any additional suggestions appreciated. Thanks.
This sounds to me like a cold solder joint on the power supply. Personally I would pull the board and go over the solder joints with a soldering iron and some lead based solder and use a multimeter to test for continuity. Here’s a video on how to test for a cold joint: https://www.youtube.com/watch?v=JrcMQb1g… Here’s one on how to fix it: https://www.youtube.com/watch?v=VLubdi6a… iMac Intel 27" EMC 2309 and 2374 Power Supply Replacement
Well, it does sound like your PRAM battery needs replacing as the settings are getting lost. We don’t have a guide to get to it directly but this one will get you there iMac 27" Late 2009 Intel Processor Replacement (EMC 2374) on the back side of the logic board you’ll find the PRAM battery
