iMac 27 year 2011 boot problem

[zeratul75]

Hi all. I sometimes receive various iMac from a friend of mine company, which dismiss them.
I now got this one, which is a 2011 model, 27 inch, now with 8gb of ram and ssd.
It is pretty fast for my need and "almost" perfect but.....there is a but.....
Basically this is the boot sequence: power on, chime, apple logo, and loading of operating system. With this imac it should take around 30 seconds. With this one power on, almost immediately chime, and then 1 minute of grey screen, no apple logo, nothing, just grey screen......after 1 minute the apple logo appears and 30 seconds for loading O.S.

Any idea? After loading masOS everything work perfectly, no problem at all.....but I am sure there is a problem somewhere, because it is really the first iMac I have seen with this strange behaviour.
Tried changing ram, hard disk or ssd, installed first El capitan than upgrade to High Sierra....nothing changes......the boot disk is already selected as macos, so the O.S. shouldn't really search it in other devices.

What is an iMac supposed to do between the chime and the apple logo?
thanks a lot for your help....

 

[r6mile]

Have you tried a PRAM or an SMC reset?
It could also be that the delay is due to your computer trying to scan for a drive that is no longer there. Try going to startup disk (under preferences), select the disk with the OS, and restart. See if it makes any faster.

 

[zeratul75]

Forgot to write First.....already done reset smc and pram.....nothing changes.
I also changed inside small battery on the Logic board......
Start up disk Is correctly assigned to the only One disk and o.s. MacOS.
It Is really strange. For example, reset pram, takes the same amount of time, I have to press cmd,alt,p,r for One minute, before i hear the chime of reset.
The same happens of i Press alt for avaolable boot devices. I restart, keep pressing alt, but i dont get the boot devices immediately. Need to stay with alt pressed for One minute, during this time grey screen only.

 

[USB3foriMac]

Possible that your internal drive is dying.
You could try to install a fresh system to a thumbdrive and boot from there, and if the behaviour is still the same, disconnect the SATA connector at the internal HDD. That way you are sure that the drive is not checked at boot. The fans will spin to the max, so don't worry this is normal consequence of having no drive (it's due to missing temperature sensor which is internal to the drive).
Once confirmed, you can replace the drive (e.g. with SSD), and you can either use an external temperature sensor (OWC), software (e.g. Macsfancontrol), or bridge the pins 11+12 of the new SATA drive (pictures available).

Another possibility is the GPU; maybe there is something wrng with the GPU BIOS. To check that, replace the GPU with a known good unit. For this you need to remove the logic board, so I would suggest check for the HDD issue first as above. It is anywat the more likely cause.