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Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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Dim LED indicator on iMac logic board

I replaced the logic board on the iMac 27" 5K Late-2015 with a used board from the same model.

Everything went well and before connecting the screen I powered up the machine.

The fans started and the startup chime sounded and the first three LEDs on the logic board illuminated.

The third LED (indicating communication to the GPU) however was very dim compared to the first two.

Indeed, I was unsure whether it was actually on or just reflecting some of the light from the LED beside it.

Hooked up the screen and started up. Nothing on the screen.

Tried again, and this time the screen brightened up. Moved the screen slightly away from the body and saw all four LEDs glowing brightly.

Subsequent startup attempts however were unsuccessful and the dim third LED returned with the fourth off.

What can I deduce from this given that the third LED is not defective (it did shine brightly on at least one startup attempt) but, in general, does not brighten further than a rather sad-looking dim stage?

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You likely have a bad logic board video connector and/or the display cable is damaged.

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Thanks for your answer.

However, the dim third LED is visible WITHOUT the screen being attached meaning that neither the video connector on the board nor the LCD display cable are in use.

So neither of these can be contributing to the "dimness" of the third LED indicator.


@pfxh - I’ve seen that too! The issue is the connector which could have been damaged by the bad cable.

Do a careful inspection of the cables connectors (both sides).


Thanks again for your comment.

OK, that's interesting.

You're saying that a damaged LCD cable connector on the board could be the cause of the dim third LED (even before the screen has been hooked up to the board)?

But then it would be difficult to explain why, on at least one occasion, the third LED (and the fourth) brightened to the expected brightness when the LCD cable was attached.

The second law of thermodynamics (Entropy) suggests that once something is damaged, it will very rarely "undamage" itself.

Nevertheless, I had indeed already examined the LCD connector on the board and it looks in perfect condition (of course, this doesn't guarantee that it works correctly).

As I understand it, the third LED has the following function:

"LED 3 - Indicates that the computer and the video card are communicating."

So, LED 3 "on" means GPU functioning correctly.

LED 3 "off" means GPU not functioning.

But what does it mean when LED 3 is neither on nor off?

Somehow the GPU must be involved?


@pfxh - So a loose light bulb which flickers on and off as your kids are jumping around on the second floor above defies Entropy! 🤔

Here the connectors pins are not making a good contact as the shell got distorted so like the light bulb it makes and breaks contact depending on the tension of the cable.

The dim diagnostic LED is not a worry. That’s just a bit of signal leakage as the cable is not connected. And the last two LEDs won’t light unless the cable is connected and working.


Once again thanks for your thoughts and comments.

While the faintness of the illumination of the third LED may be due to signal leakage as you say, the problem is that when there is "signal leakage" the screen does not activate.

And when there is no leakage, the screen does turn on.

I'm still very puzzled.


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