I have a problem with an iMac.

Digital Micro

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I don't do much with Apple products so this issue has me stumped. I will do my best to describe the whole problem. Hopefully someone who works on Mac's knows what is going on.

This iMac was taken to the Apple store because the screen would go black and the fans would crank up. Sometimes you could click the mouse after 10 minutes and it would come back up for a few minutes. The people at the Apple store said it didn't have an Apple drive in it, that the drive was going bad and because they didn't know what the other person did they refused to repair it. So I was called to replace the drive.

I opened the iMac and it had a replacement Seagate drive and their was no censor cable connected to the drive like there should've been. I replaced the drive and installed the censor cable. Powered the computer on and it instantly went to black screen, loud fan. I turned it off and back on. This time I got to select the wireless network and click on the usb icon with Sierra on it. It went to a gray screen with the Apple logo and stayed there for 10 minutes, then black screen loud fans. I then reset the pram not knowing if that would do anything. After that I made it all the way through the install process up to where you set it up as the end user. I restarted with no problems. I restarted again and did a full hardware test. It pasted with no problems. I restarted and let it sit at the user setup screen for 2 hours to see if it would black screen again, it never did. This morning I power it up. I get the gray screen, the progress bar starts to move and black screen loud fan.

I'm hoping an Apple person here will laugh and say do this, but I am not holding my breathe.

Oh this is also the 21" not the 27" model. I read where the 27" had recalls for this.
 
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What year? Did you leave the iMac unplugged from AC for more than 15 minutes after you replaced the drive? That will reset the SMC. What happens when you boot from a different source? Such as OS X or linux on a usb drive/stick or CD/DVD. But it does sound like a probable chip set issue. Not having the HD temp sensor connected in of itself will not cause a problem. Just that the fans will run at max after a while.
 
What year? Did you leave the iMac unplugged from AC for more than 15 minutes after you replaced the drive? That will reset the SMC. What happens when you boot from a different source? Such as OS X or linux on a usb drive/stick or CD/DVD. But it does sound like a probable chip set issue. Not having the HD temp sensor connected in of itself will not cause a problem. Just that the fans will run at max after a while.
I think I found the issue. I haven't tried the repair yet.

I realized after doing a pram reset it boots up to the end user set fine. I can reboot and let it sit their for hours no problem. Then turned it off for an hour, it booted again fine. If I unplug the power cord for some time I have to do another reset and it works fine again. So I am leaning toward a bad pram battery. I could be wrong.
 
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