CPU Fan Full Speed After Disassembly

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iMac Intel 21.5" EMC 2389

Model A1311 / Mid 2010 / 3.06 & 3.2 GHz Core i3 or 3.6 GHz Core i5 Processor

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Samson Thornhill @thrsam005

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发帖于: 2020年4月3日

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CPU Fan Full Speed After Disassembly

I disassembled my parents' old iMac with the intention of cleaning it and repasting the CPU (ten year old thermal paste is certainly more like thermal dust).

After reassembly, however, the CPU fan was running at a constant full speed. After running Apple Hardware Test, the error code returned was 4SNS/1/C0000008 TCOH-- 124. Per the popular CNET article, this is the CPU temperature sensor. According to Macs Fan Control, the sensor was still reading accurately (it seems), with idle temperatures around 33 C and loaded temperatures not getting above 40 C.

After a quick terminal command (I can't remember or find the source of where I found this) showed the CPU speed to be 39 out of 100. I would just use Macs Fan Control to keep the noise down except the CPU is throttled so the machine is much slower now.

Additionally, I later disassembled the iMac again to swap an SSD in, and I also removed the logic board again to ensure I had plugged in the CPU temperature sensor. I had, and after reassembly again the problem persisted. SMC and PRAM resets were fruitless, so I'm wondering if a replacement logic board is the next step?

I'm okay with that since you can readily find a refurbished one for around $100, I just want to be sure that's the problem.

Thanks.

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Dan @danj

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Well, the good news is you don’t need a new logic board! TCOH is a sensor error for the heat sink per the Apple service manual ‘Excessive processor heatsink temperature.’

Review: Check CPU heat sink sensor cable connection to logic board. It is located on back side and requires removal of the logic board.

Symptom: If the sensor is disconnected, the CPU fan will run at full speed.

Here’s the connection point

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Right, so you're saying the part I would need is the CPU heat sink? The sensor cable is definitely plugged in as I checked after second disassembly. I think I just knocked it wrong or something that ended up breaking the sensor.

由 Samson Thornhill 完成的 2020年4月5日

Thats a possibility, what is the resistance of the sensor?

由 Dan 完成的 2020年4月5日

I haven't checked that yet, but I'm assuming infinite/non-measurable resistance implies the sensor is broken while a numerical value implies it should still be working?

由 Samson Thornhill 完成的 2020年4月10日

The sensor is a transistor if I remember correctly a NPN Test a transistor with a multimeter

由 Dan 完成的 2020年4月10日

Okay so using my multimeter, which has a diode/continuity test rolled into one option, I get an audible sound (continuity) and a reading of 0 when probing the two leads that come out of the heat sink and go into the black wire. Reversing polarity of my multimeter leads gives the same result.

When I place my positive lead from the multimeter onto either of the two leads that run into the black wire, and the negative lead from the multimeter on the third lead I get a reading of around 800 +/- 5. Reversing polarity of my multimeter leads, I get a reading of OL (overload).

This is all fine and dandy, except what the heck does it all mean??? According to my multimeter's manual, resistance of less than 30 Ohms will cause the continuity to trip and audibly sound. Forward voltage should give a reading of 400 to 700, and reverse voltage should read "I" and a shorted transistor will read 0.

Any help you can give is appreciated - you have so far been extremely helpful so far, and I thank you for that.

由 Samson Thornhill 完成的 2020年4月10日

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Right, so you're saying the part I would need is the CPU heat sink? The sensor cable is definitely plugged in as I checked after second disassembly. I think I just knocked it wrong or something that ended up breaking the sensor. — Samson Thornhill
I haven't checked that yet, but I'm assuming infinite/non-measurable resistance implies the sensor is broken while a numerical value implies it should still be working? — Samson Thornhill
The sensor is a transistor if I remember correctly a NPN Test a transistor with a multimeter — Dan
Okay so using my multimeter, which has a diode/continuity test rolled into one option, I get an audible sound (continuity) and a reading of 0 when probing the two leads that come out of the heat sink and go into the black wire. Reversing polarity of my multimeter leads gives the same result.

When I place my positive lead from the multimeter onto either of the two leads that run into the black wire, and the negative lead from the multimeter on the third lead I get a reading of around 800 +/- 5. Reversing polarity of my multimeter leads, I get a reading of OL (overload).

This is all fine and dandy, except what the heck does it all mean??? According to my multimeter's manual, resistance of less than 30 Ohms will cause the continuity to trip and audibly sound. Forward voltage should give a reading of 400 to 700, and reverse voltage should read "I" and a shorted transistor will read 0.

Any help you can give is appreciated - you have so far been extremely helpful so far, and I thank you for that. — Samson Thornhill

score 1 · Answer 1 · 2020-04-04T10:10:13-07:00

mayer @mayer

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发帖于: 2020年4月4日

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Apple used a propritory heat sensor on the hard drive for this machine. You will need to install an OWC Thermal sensor or the CPU fan will run at full speed. Here’s what you need:

OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010 Hard Drive Upgrade

https://eshop.macsales.com/item/OWC/DIDI...

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From iFixit's SSS upgrade kit, the HDD thermal sensor was replaced with the inline sensor you linked above, meaning that likely isn't the issue.

由 Samson Thornhill 完成的 2020年4月5日

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iMac Intel 21.5" EMC 2389

CPU Fan Full Speed After Disassembly

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