intermittent start issue - only HDD spin-up
The iMac was running for years, but since some months, it starts quite unreliably. After pressing the power button I can hear the HDD spins up, then stops and after a moment, spins up again. No video signal, no sound. It can do this for 30min and suddenly the boot process continues. Once the machine is started, it works flawlessly even over a long period. I found out that it's possible to influence and 'force' the start if I press, hold and release the power button while the iMac is in the trying-to-start loop - but I need some luck to gain the proper timing for that to happen.
Checking the SMART values shows nothing special beside an extrem high spin up / power cycle count.
Where should I start to search? Do the symptoms point to the PSU?
Ok, I did some more tests with the iMac opened up:
- 1st Diagnostic LED is constantly on while device connected to power, no flicker
- I press the start button
- 2nd LED turns grren, fans and HDD start up for about 1 sec.
- 2nd LED, fans and HDD stop again for about 3-4 sec
- the machine continues altering between the last two states about 1-4 times, then the boot continues, you hear an sound from the SuperDrive, the start chime, the 3rd LED comes up and everything seems normal.
- if the iMac is hot (was running before) it seems the trouble is worse. It can take some minutes instead the 1-4 attempts till it finally starts
I tried booting with the hard drive removed as well as with it exchanged with a SSD, the error continues. Starting from either internal or external or AHT does work once the first boot phase is passed. I had a look to the power supply, there are no obviously damaged (blown up) capacitors on it.
What to test next? PSU? logic board? can such symptoms be caused by faulty RAM? AHT does report an HD thermal sensor problem with the SSD (have ordered the OWC sensor cable to fix that) but nothing else.
Is this a good question?