Chosen Solution

Hi, the CPU fan of my iMac is running at full speed. iStat says it’s not running (0rpm). The temperatur sensor is working, my ASP says, that’s a problem with the logicboard. Any resets (SMC, PRAM) are done but this problem is still there. I just bought an external fan control but first I want to ask if somebody know the pin assignment of the CPU fan plug.

Something is not right here with iStat. Do you have the older widget or are you use the current version? Give Temperature Gauge a try, does it show the samething? Lets see what the second tool tells us. The fan speed sensor is part of the fan unit so you may need to replace it. I don’t see the logic board as the issue here, pending what new info you can tell us.

I had done the same thing when dismantling the iMac 27" Mid 2010 Model, the internal heat sinks where clogged with dust making the iMac run really hot (but strangely all the FANS remained at normal speeds), so as I have done countless times with notebooks I dismantled the unit and removed the main logic board, but in my dismantling when i pulled out the power button cable connector, i must have inadvertently ripped off the SMC resister just above the connector there and this made the CPU FAN report 0rpm which causes the system to run the CPU FAN at maximum speed, I do not know what the correct resistor was but i tested the resister directly above it and it reported as 10k ohm, so i took a guess and soldered on a new SMC 10K Resistor and prayed it would work :) lucky enough it did and now the CPU fan is reporting 950rpm at idle. So I hope this helps, but if anyone knows if the resistor marked (R5797 on the logic board) is 10K or not, I would like to know :)

Hey scallejam, seems to me your L5701 is missing. L5701 is s labelled on the spec as FERR-220-OHM 0402 . I uploaded image for reference of its continuity points, hope it helps!

I can confirm that the fix works. Got help at my office to mount a “47K 1/16W 5% in a 0402 package” at the place specified in this page. They said at my office it looked like there where something mounted there before but is now missing. After mounting the resistor the fan speed return to normal.

R7460 looks like an empy position, If rpm = 0 then control the fan connector , could be a fake contact of the rotation sensor , slightly pulls the wire into the connector , I have solved this way the problem

Could somebody tell me how to connected the three wire of a Zalman Fan Mate 2 potentiometer with the four wire of the fan?

I had done this once, i pulled the wires out the fan plug by mistake, and put them back in the wrong way so the fan span out of control. However i followed the wires round with my thumb from top of fan and managed to get them in the right order. Fan was as quiet as a mouse after that. Don’t know if that is your problem but so easy done.

Anybody know what the value of L5701 is? I accidentally ripped the %#*@ pad of the side that goes towards R5797…currently testing (jumping) with kynar and a couple resistors. Thanks.

i don’t understand, i have change apple hd with another and the fan goes max, after i have installed macs fun control and all ok! after i have installed another hd (SSD) with ifix kit and change i3 cpu with i7 cpu and the cpu fan go max speed!! i don’t know if the r5797 solved my problem or not, how to do? i think that r5797 never installed on my logic board. i have try to sconnect ssd, but nothing!! i have change i7 to i3, but nothing what is happened? please help me,thanks

Looks like I need help with a ripped off part too. Fans are blowing max RPM for CPU fan and I seem to be missing R7460. Any good ideas on what to do to replace it and whether it will fix my CPU fan maxing out? image attached

Update (03/14/2019) I re-seated the wires and last night the fans behaved with the CPU and power supply temp going all the way to 50+ degrees Celsius. The fans stayed at a rock steady 1000 rpm the whole time. This morning I woke up my iMac and as soon as the CPU temp hit 44 the fan CPU fan went full blast. All the other temps seem normal with ambient in the 20s. I’m really stumped on this one. Should I try reinstalling the OS?

ok that’s excellent - thank you so much. I’ll give that a try.

I have same problem here (iMac 27 late 2009 Core i7 2.8 GHz), CPU fan at full speed Long story short, SATA HDD connector at motherboard was broken, I managed to fix it but once it was assembled again, noticed fan problem I’ve checked resistor R5797 is still in place and it meassures 47k (46.73 aprox) My first thought was about skin sensor, as pins went out of plastic when removed, but iStats says all temperature sensors are fine. So… what would be the problem? I was really wishing it was a resistor problem, at least there would be something to start up, but now I’m clueless

I have the some issue with my iMac, the cpu fan report 0rpm which causes the system to rum the cpu fan at full speed. I have checked the logic board and the resistor R5797 is missing. Probably I have ripped it off when I have unplugged al the connectors to upgrade the hardware. Can you tell me the precise attributes of the resistor R5797?