iMac 5K 2017 display won't turn on after replacement. iMac boots with external display but fan at full speed.

Steve,

You either have a display that was not intended for your iMac, a damaged display or finally a connection that was not connected correctly.

It is pretty common for do-it-yourself project to not be done correctly and what ends up happening is they cost more than if you did it right in the first place in order to "save" money.

Sorry for your troubles but if if were me, I'd cut bait and replace the computer rather than spending another 1400 Euro on it. At the end of the day you will still have a 7 year old computer rather than a new one.

Sometimes lessons are just difficult to learn.

All of the cable connectors both on the cables and on the Logic Board are very delicate and easily damaged even if extreme care is taken. The LVDS cable can easily have the pins on the cable connector break, maybe without you noticing if it broke off completely. Plus there are two other cables with black plastic connectors which can be difficult to remove which weakens their plastic locking mechanism which can cause them not to fully lock into place.

Or maybe, the issue was not actually the LCD Panel, but the Logic Board which may have broken the good display. While rare, I have seen cases where installing a good part into a broken computer can damage the formerly good part. And if that now newly broken part is installed back into the old working system, there is a possibility it damaged the previously working computer. Unfortunately there is no easy way to confirm since swapping any parts at that point will just expand the area of destruction. Like I said, this scenario is fortunately very rare, but I have encountered it several times over the years so it is a legitimate possibility if you can eliminate any damage to any of the parts.

Alienhearts wrote:

Thanks,

Maybe I should have mentionned in my post that I'm used to handle electronics and to repair computers. All the displays are compatible and are the exact same serial number. I was more looking for advanced or unusual technical solutions or insights, but thanks for your concerns and sobering warnings.

The fact that the replacement LCD no longer works on the original Mac says something got damaged on that LCD Panel. Either its cable/connector, or its electronics. Just make sure the LVDS Cable connector has the black plastic all the way to both edges (the plastic behind the contacts). Many times when this plastic breaks, it can break clean so you may not notice a sliver is missing unless you see gold contacts in mid air or bent contacts.

I don't see the point of spending any more money since you may end up with yet another broken LCD Panel if your first broken iMac damaged the electronics on the donor LCD Panel. I have seen that more than enough times with various parts. The worry then becomes, is the damage to the donor Panel now able to cause damage to the formerly working Logic Board on the donor iMac. Unless I knew for sure, I will usually stop at this point and clearly mark both computers & parts as being damaged so no one will even try to use either one for parts since I would not want a recycler or refurbisher to destroy more components & systems. You need to draw the line somewhere if you know there is no accidental damage to the donor part....what other explanation is there than its electronics were damaged by your first iMac? While not common, it does happen. Of course you could purchase another LCD Panel and test it on the donor iMac first hoping the donor iMac's Logic Board has not been damaged now which in turn could damage your donor replacement LCD. It gets expensive & complicated very quickly.

A tech guy I know who encountered a very similar issue with the same iMac model said to me that even though the iMac boots with an external display, the processor might have suffered and suggested that changing it could resolve the issue. Does that make sense to you?

No, it does not make sense. It doesn't explain why the donor replacement LCD Panel doesn't work on the donor iMac when it worked fine before the swap.

The fans running at high speed is because a temperature sensor is not being recognized on the LCD Panel. That is normal behavior for a Mac under those conditions.

FYI, I have been repairing hardware for my organization for over 20 years. I have seen a lot of very strange things most people will never encounter & probably would never even believe.

Hi buddy!!

Exactly i the same boat as you!! Same LCD change and same result... It can't be just shared bad luck!!

I also has electronic repairs background, used digital microscope and even replaced the flex cable too... Nothing.. iMac boots using an external display but fans are at hi speed and CPU load is 250%!!

Have you had any new finding?

Not yet... I recently found the schematics and board view, but I guess i would have to invest a lot of time to dive into that 96-page PDF!!! Even if I do it, I don't have at hand any donor board to take components from, so I believe is wasted time and effort.

I guess I'll go for getting a whole new logic board, the can be found on eBay for less than USD300. They don't have the CPU (there are some with CPU if you want them) but I guess I could move my 3.8GHz i5 to the new board. I still have to remove the board from the case (a looooot of screws!!) to confirm the exact board part number (it seems there are at least 2 for the EMC3070, A1419 model) before ordering it.

What make me think the board is bad is that even when reconnecting the old display (it was cracked but functioning) I still don't get video. Some guy on some forum told to another guy with a similar problem that powering the board with the display flex connector not plugged in (it happens to me once while trying to assemble the mac) could cause a GPU burn. Who knows?

I'll keep you posted on my findings!

Alberto

on rkaufmann87's recommendation.