iMac 2010 HD Thermal Sensor

Having lab full of 2010 and 2011 iMacs has been pretty uneventful… until last month when their 1TB Seagate hard drives started failing. One after another, a few the machines would start acting weird and freeze up. AST showed some of the drives were bad and the others I figured just hadn’t hit the error threshold yet, so I ordered 7 1TB Western Digital drives. I choose WD drives because I am in the middle of replacing 35 3TB Seagate drives under an Apple replacement program, and I have had to replace a bunch of Seagate drives in our server storage nodes, so I don’t trust Seagate to make a quality product anymore.

I went to replace two of the older machines’ hard drives, only to find instead of an external temperature sensor as the the past, Apple was attaching a single pair of wire to specific jumpers of the drive. Come to find out, the Apple hard drives have their own special firmware that even if I replaced them with the exact same model hard drives they would not be able to read the temperature and the system fans would be running full blast and AST tests would fail.

Transistor wired to existing iMac header

So I hit google up and found out that I could put a simple 2N3904 transistor in place of the jumper. I ordered some heat shrink and a dozen 2N3904 transistors. Looking at the technical docs for the transistor, I wired the gray wire from the iMac to the emitter and the black wire to both the base and collector pins on the transistor.


Transistor taped to the hard driveAST screen showing green check boxes

With the sensor in place (under the red tape above), the iMacs boot up without problems, the fans run normally, and AST shows a nice green check next to the sensor test.

  • Work with 2N2222 transistor

  • Len

    Nice to know but how about posting a picture of the other end of the cable and where it plugs in?

  • John

    2N4401 worked perfectly for me - the indicated temperature matched the Optical Drive sensor exactly. Thanks for the write up. BTW it's shame you didn't put SSDs in the iMacs - they absolutely transform the performance.

  • William

    Tested with an 1N4148 diode on an iMac 10.1 (late 2009), it works fine too :-). Any small signal diode should works too. No heat shrink, no more wires since I reused small connector only (I discarded wires), soldering that diode directly in connector with a very thin iron (0.4mm round, JBC advanced). HDD is now a Samsung 850 Pro 256Gb - funny to put Samsung parts in Apple devices :-D

  • randombyter

    i've seen this in some forums and i hope Apple has sent you guys reasonable compensation. i've also read that the optical drive sensor is a good replacement.

  • InfiniteNovice

    Thanks for the tip, it worked great, and I learned something new about behavior of transistors.

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