dead iMac 2010/27", contractictory diagnostic led versus test pin voltage

The machine is back to life. Short story: it was the optical drive or the drive's sata cable.

Long story: I replaced the back battery on the back of the logic board, and before reinstalling the logic board I removed the black mylar tape that covered the diagnostic leds. Upon reinstalling the board and powering the machine diagnostic led #1 was lit. For reference, it takes a second or two after applying power for it to light. There is a chance that the black mylar tape was totally opaque to the led and that's why I didn't see it before, but I have trouble believing Apple would cover the diagnostic LEDs with black out tape then write documentation saying to look at the LEDs through the bottom vent holes. Whatever the case, I do have some uncertainty as to whether the diagnostic led was lit the whole time, but invisible due to the black tape, or if the battery resolved something. Either way, the battery needed to be replaced. Continuing... Diagnostic LED 2 would still not come on when I pressed the power the button and the machine remained totally dead. Considering LED 1 now being lit to be progress I consulted the technicians guide for suggested next steps. It suggested disconnecting the aiport card, hard drive, card reader, and optical drive. I disconnected those one at a time and tested. And low and behold, when I disconnected the optical drive the machine came to life. The optical drive's sata cable has visible damage at both ends, especially bad at the drive end, where it bends to plug into the drive. It looks to be stress cracking in the outer insulation. That appears to be the problem, best guess, but it may be actual drive also. Until I replace them I won't know for sure.

Conclusion: I suggest some old age maintenance on your aging iMacs. First, if your iMac has mylar tape covering the diagnostic LEDS then remove that tape or verify ahead of time that you can easily see the diagnostic leds when they are covered before you need to see them for actual diagnostic purposes, second check all your cables for age related damage, cracking, etc. SATA cables especially since they appear to induce particularly crippling boot problems. Third replace the backup battery battery, and finally do some dusting when you have the machine open. It also wouldn't hurt to dig up the technicians guide, if its available, and keep that on hand just in case.

I studied the logic board schematic (found online, of course). The test pins have to do with panel back light power. They are is an incorrect equivalence connoted between test pin 4 and diagnostic led 1 in the iMac 2007/27 technicians guide. They are not equivalent. The diagnostic led is clearly powered by a 3.3 v supply. Test pin 4 is related to the 12 volt panel backlight supply. There is even a line on the schematic labelled "ITS_PLUGGED_IN", right on the schematic for the 3.3V supplied diagnostic led. The 3.3 V supply powers many many components. No wonder the machine is dead as door nail. I still can't differentiate between power supply and logic board failure, however.

Here is one case of no diagnostic LED where it was the power supply: https://discussions.apple.com/thread/8487611

Here is a case of no diagnostic LED where it was the logic board: https://www.ifixit.com/Answers/View/530012/No+power+on,+No+LED+on+logic+board

... getting interesting.

Yes, I might drop by the Apple store if the diagnostic service is free. However, my machine will not run AHT because it won't even power up! I should mention, I've also removed the memory before doing any of this testing in order to rule that out.

p.s. keep a TimeMachine backup, is the lesson. I have one, up to date, and ready to restore on a new system if necessary. :)

Yes, I'd be surprised if I got free diagnostic service for a 2010 iMac. I also don't think they'd be able to do much more than I've done. Mainly I'm trying to narrow it down to power supply or logic board based on some analysis, or experience, rather than a guess. Then I'll ebay one or the other and diy it ($100 ish each on the used market, although new after market power supplies are available). I'm 50/50 power supply versus logic board. I've never read of this situation of contradictory diagnostic led versus test pin voltages. That's a big open question.

The "always on" power rail if logic board is labelled "GH3" on the schematic. The always on power line is PP12V_GH3_ACDC on the schematic. What the "technicians guide" calls test pin #4 in that document is actually pin #11 on the logic board schematic and it is labelled PP12V_GH3_ACDC. The technicians guild correctly calls this standby power but calling it pin #4 is confusing if one is also referencing the schematic. The power supply has an always on PP12V_GH3_ACDC ouput. (See the link below for picture of the power supply pin out.) All of these voltages (logic board, and power supply) can be easily tested with the screen removed. My power supply is supplying the correct 12 volt always on power. Call: logic board failure, specifically something related to the 3.3v power supply provided by a regulator directly on the logic board.

http://hqlaptoprepair.co.uk/imac-a1312-psu-pinout/

p.s. Apple's technician guide is not very good. I can post the link to the guide if asked, but I guessed that these docs were leaked somehow and Apple probably doesn't want them advertised. Anyways, they are a google search away.

OS Support is gone, yes. I agree the ebay logic boards look suspect.

I'm putting a new backup battery in it to see what happens (a coin cell). The old, original, battery is 2.94 volts. The technicians guide says the limit is 2.7 V, but how knows. A fresh battery is 3.3V. I have removed the logic board and inspected. No obvious visual problems. I'll post what happens when I reassemble.