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Modelo A1419 / EMC 2806 / Finales de 2014 o mediados de 2015. 3.3 o 3.5 GHz Core i5 o 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 finales de 2015 / 3.3 o 3.5 GHz Core i5 o 4.0 GHz Core i7 (iMac17,1) Todo con pantallas Retina 5K

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iMac no Bios beep or display output - 3 LED's light up continuopusly

After pressing the power button on my iMac Intel 27" Retina 5k Display, the following happens:

1. 3 x green LED's light up continuously (I'm looking throught the RAM tray)

2. I hear the Hard Drive spinning up and the main fan sounds like it comes on

3. There is no Bios Beep and no screen output

4. After about 5 seconds, the Mac powers itself down

5. The Mac keeps restarting itself again after 5 seconds or so

I tried swapping out the RAM with a known working set but that hasn't helped. I can however get a Bios Beep if I remove all the RAM. Other than that I don’t think there is anything else I can try without taking the screen out. And even then I wouldn’t know what to look for. :)

Can anyone help me please?

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I’m not sure what you are seeing through the RAM tray. The onboard diagnostic LED’s are on the front side of the logic board behind the display panel (you need to remove the display assembly)

The only thing I can think of is you are seeing LED’s built into the SO-DIMM’s. Some vendors add LED’s to allow you to trick-out your system (clear cases) World's First RGB DDR4 SO-DIMMs!

Personally I think they are a waste in an iMac as you can’t see them!

Here’s the location of the diagnostic LED’s

Block Image

And here’s a deeper look on what they mean

Block Image

LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.

LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.

LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

LED4 won’t light with the display disconnected. Apple uses a frame to hold the dimply to the side and extension cable set. Now with a better idea what you need to checkout

I would first put back the original RAM as the RAM you got is likely the wrong type before you go in.

Locate your system from this listing to get the details of what you need for RAM specs EveryMac - iMac’s to check what you do have if different you’ll need to get the required SO-DIMM’s

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Dan

Thanks for your reply. There's some good info here :)

The LED's I can see are only visible from a certain angle through the RAM housing. I think they are attached to the logic board (not the RAM). They must be the ones in your post. I can only see 3 lit at any one time though.

”I would first put back the original RAM as the RAM you got is likely the wrong type before you go in.”

The Memory module we tried from the other Mac was bought from Crucial. According to their memory finder, the RAM (DDR3L-1600 SODIMM 1.35V CL11 - CT2K8G3S160BM) it is suitable for both iMacs. I was hoping that the Memory was faulty because it would be an easy fix. However I think we can rule out a Memory issue.

I am seeing 3 green LED’s and from your post it looks as though we’ve got power and the video card is communicating. How do I fault find this further? I can hear the HDD (Fusion Drive) spin up briefly. Could the HDD be faulty and cause this type of fault?

I'm really poking about in the dark here because I have no way of knowing which component is failing!

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Without seeing the LED's directly all you could be seeing is the internal reflection of the light. There is no LED's on the backside of the logic board.

Here's the guide you'll need to follow to remove your display assembly Remplazo de la pantalla del iMac Intel 27" Pantalla Retina 5K This is not easy! And if you are not careful you can damage the display!

Maybe it's time to seek out someone with the deeper skills to fix your system.

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Hi Dan

Thought I’d just provide an update.

I bought the iFixit plastic ‘pizza cutter’ tool which enabled me to cut the adhesive and remove the screen. That part was quite easy although I think it is better if you have a helper. The tool makes light work of the screen removal - it’s just perfect.

Anyway I took out the power supply and checked the capacitors both visually and with a multimeter. They were within limits so I discounted it as a likely cause of the fault.

To my surprise, after re-seating the SATA connector on the fusion drive, the iMac booted correctly and has been working perfectly since. I did take a photo once we removed the screen and the photo shows that the connector wasn’t pushed in squarely.

For now I’ve just lightly taped back the screen for now while I test it. It’s been working for a couple of months now so I’ll probably seal it back up with the adhesive strips I also bought from iFixit.

Thanks for your help! Mark

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@hudson64 - Yep! A loose cable can confuse things. Happy it was simple fix.

If you are good, don't forget to accept the answer! Thanks

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Oops. Looks like you solved this one yourself. Excellent!

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Quite a stumper.

My only thought is that I know Macs can be really picky about ram. If it were me, I’d put only one module in at a time, swapping them out to see if any one of them worked at all. And I’d check one more time on which specs and brands are known to work. I’d take these steps just because they’re simple, cheap and possible to do immediately.

Good luck!

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Hello

About the 3 LED indicator lights only on a mac.

I had an issue, and I wanted to post this somewhere, since it appears that the indicator lights have multiple purposes, NOT only the commonly re-quoted meanings presented in this thread. Also, it seems like these questions come up year after year.

Hopefully, this will be of value to someone in the future.

As the original post said, the three indicator lights came on (basically) at the same time, and immediately after power being applied; then would go off and reappear.

This appears to have NOTHING to do with the staged 1-2-3-4 light sequence when the mac boots, showing communication between CPU and video.

In my case, I was playing with a 2007-2008 iMac 7,1/8,1. Everything was working fine. I was making changes, and then, I got the three LED lights on pattern described here.

I also thought it was something to do with video communication.

But, then I started going backward. Removed the video card. Applied power - 3 LED lights.

Removed the CPU. Applied power - 3 LED lights.

And that's when I realized I made a little mistake. Previously (only ONE time of many), when re-seating the CPU, I did not completely turn the CPU socket retention screw (so,. the CPU wasn't correctly attached).

A bit of an oversight. Anyway, I re-seated the CPU, and put it all back together, applied power, and it booted up.

I am leaving this here as information for others.

The mac indicator lights appear to have multiple functions. If you have a situation where 3 lights come on IMMEDIATELY on power application to the motherboard, it has nothing to do with the staged video power up sequence.

Rather, it is some much more elementary problem that is stopping the mac from completing its POST (memory, CPU, whatever).

Thanks

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