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[SOLVED]820-3588-a iMac boot but no image on screen

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fbarcella

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First: I don't know if I can post an iMac related question.

After have upgraded RAM and SSD to an iMac, 21,5" A1418 (EMC 2638) with board 820-3588-A, I have reconnected the display and the mac booted up, at first I've seen the image on display and was ok, after a while the backlight was gone and I was able to see only the image very dark, after a minutes the display gone black and never worked again.
In meanwhile the fan speed was at maximum level, (probably because the logic board wasn't able to see the display temperature sensor, I assume). Tried another display same issue. Now I have to try another display cable but I would try to understand if the power on the mother board to the display connector is correct.
Thanks in advance and sorry for my bad english and for this basic question.
 
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dukefawks

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1) You cannot just put another HDD/SSD in these machines without modifying the thermal sensor wire. The original HDD has a built in temp sensor.
2) What happened to the LCD I have no idea. Damaged cable/connector is most likely.

First get the LCD working again then we can fix the temp sensor issue with the HDD/SSD.
 
Repairable

Repairable

Active member
iMac has a separate inverter that gets killed easy or the kabel does,
iMac 21,5 inch A1418 Backlight Inverter

high fan speed is probably due to sensor issue from hdd/ssd
 
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fbarcella

New member
dukefawks I have already upgraded with ssd other iMac exactly same model as this and no problem.

This mac doesn't have thermal sensor included in hdd, there was on some old iMac.

In this case the thermal sensor of the display comunicate with the same display cable, but I guess that if the lcd electronic doesn't power on this sensor cannot comunicate with board.

Repairable I can try to substitute the inverter but I have no lcd power on, not only no backlight issue, how could I test I the correct electrical current goes to the lcd?

PS Apple diagnostic tell me I have problem with display and maybe with SMC.
 
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fbarcella

New member
Tested with external display and the system boot very slow, I guest cause the missing thermal sensor make cpu goes slower, could be?
 
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fbarcella

New member
Here is very cold, is it possible that, after have cleaned the mac with an compressed air gun, (I know I shouldn't clean in this way) a bit of water has formed on the board making shot circuit in some place? How could understand if was gone so?
thanks I haven't many skill in electronic repair
 
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dukefawks

Administrator
I have no idea, you have not tried another LCD cable. Also run ASD EFI version if available for this model. Also there IS a temp sensor in the HDD that needs to be bypassed if you install an SSD in the HDD place.
 
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fbarcella

New member
Why did you think there IS thermal in hdd? Could you link me your sources? Maybe we are talking of different models? Other upgraded macs doesn't have issue and work fine, why?

I have to try with the cable. Ordered.
nobody can help me to understand if there is a short?

i will try with ASD
 
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jgraffis

New member
yes this is a hdd temp sensor. If you have one of these you will have to bypass it in some way. I usually just switch it with the dye type sensor off an older iMac. I would try this with a tested working internal screen/cable. You could run both ASD OS and ASD EFI and see what it tells you.
 
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fbarcella

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That was an image from internet, I don't have such cable, the internal 2,5" sata disk is connected only with SATA cable, for that I supposed that the temperature was read from S.M.A.R.T. Running ASD I got an error from SSD Temperature so I preferred to do what dukefawks told me and that error was gone, I will investigate later for this.
After that I've run again the ASD EFI and what you see are the errors with the original disk.

I'm searching to understand what all these (proximity and co.) sensor are.

I'm still running ASD OS (I will read results tomorrow) and still waiting an replacement for the display cable.
 

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dukefawks

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The sensor is on pin 11 of the power connector of the HDD, it needs to be grounded on the SSD. I normally open up the SSD and ground that pin with solder. It is in between to GND pins.
But looking at the ASD result the SSD is not the only issue. The multiple sensor failure indicates a data bus issue. Can you see what temperatures the failing sensors are reading? Best to run the EFI version of ASD for this.
 
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fbarcella

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The posted image was from EFI ASD. But I don't know how to see the temperatures.

In OS ASD I get more errors.
But I don't know if these are useful or not.
 

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dukefawks

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System is not seeing the sensors which usually means there is an issue on the data bus. I bet the data lines are shorted in the LCD connector or cable. Inspect the connector for damage.
 
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fbarcella

New member
I can't see damage on the connector, changed motherboard and all went fine, (same display same cable), so the problem is definitely on board.

I haven't modified the ssd but as I told you I can't see issues working with it, no high fan speed or slowness. Maybe I would get an error on temp sensor if I would run the ASD again.
Could you tell me if there are other consequences running the mac in this way.

Now I have the broken board and I would like to fix it.
Could I try to replace something? Or investigate in some way? Thanks
 
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