Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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CPU fan runs full speed

My iMac CPU fan runs on high all the time.

I have done the usual SMC. PRAM resets with no luck. I have tested the fan and the control circuit an I believe they operate OK. I think it may be the LCD temperature sensor that is at fault but would like to prove this before I buy another.

Any ideas how to do this? I will measure the resistances between all pins and post them in the hope someone can test their system and compare measurements.

It's a 4 wire device... is it the same as the Ambient sensor??

One last note, I booted the system w/out RAM and noted that the CPU fan runs at normal low speed. Re doing this with the LCD temp sensor disconnected the CPU fan runs on high....

Answered! View the answer I have this problem too

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I have now replaced the LCD sensor, run AHT and there are no faults detected however y fan still runs on HIGH... tried SMC and PRAM reset too to no avail. Hoping ASD may help...

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So I have run both OSX and EFI versions of the ASD 3S123 and neither report any errors and both are able to drop the CPU fan speed down to the normal idle speed during tests so that confirms (IMO) the h/w is working fine!!

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So the restore disks finally arrived and I did a clean install of the 10.5.4 and apps to put the system "back to factory" in the hope it may help this intractable issue I have with the CPU fan running.. Lo and behold whiel I am installiang the fans run full speed but once I complete the install and get to decktop they are nice and quiet. I have a moment of joy while I install iStat and then watch... over the next few minutes the fan creeps up to full speed again... ARGH!

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When the fan creeps back up to full speed, what are the temps on your sensors as indicated by iStat? Specifically what are the CPU and GPU core temps?

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CPU is 26 and GPU 34, CPU HS is 27 and GPU HS is 31, PSU is 65

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When you tried booting the system with no RAM and the fan ran at low speed, then with the LCD sensor disconnected and still no RAM, the fan ran at high speed, that would indicate to me the SMC is properly detecting the LCD sensor and there is no issue with the LCD sensor. But I could be wrong. I just went through all this myself last week on my iMac which I bought used and had no LCD sensor. When I finally installed my sensor, the fans went quiet. If your computer is working just fine other than the loud fan, another sensor may be the problem.

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very intuitive +

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"I booted the system w/out RAM and noted that the CPU fan runs at normal low speed." Sounds to me like a RAM problem not a sensor problem. What kind and where did the RAM come from?

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There should be two RAM slots. Try booting with just one stick and then try just the other. We are trying to test both the RAM and the slots. iMac Intel 20" EMC 2133 and 2210 RAM Replacement

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You can't boot without RAM, the iMac will beep to tell you there is a RAM problem. Maybe you switched the iMac on without RAM. In that case it should beep.

When there's only a problem with a sensor or a fan, you should run apple hardware test (original snow leopard DVD, or leopard 10.5.x CD) which will tell you which sensor or fan is defective.

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Thanks for replies...

mayer - the RAM was the original and known working RAM. I had taken it out as part of the removal of the logic board. It works fine.

Cees - I powered up without RAM by mistake and noticed the change in fan speed. I dont have the AHT DVD yet - on order from Apple.

Eric - Yes agree the SMC does appear to detect the sensor. I have discovered that the PSU temp sensor can cause this issue along with the CPU and CPU HS sensors. The CPU sensors appear OK and I have ordered a new LCD sensor. In the mean time I will hopefully be able to run the AHT in the next day or so and get some conclusive answers...

?? are you DR Fosgate on the apple support forum?

FWIW the Ambient and LCD sensors operate on a serial bus and the little resistors on the board set the address. It does not matter if you swap the plugs around, they will still work...

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Yes I am Dr. Fosgate on that forum. I found out about the resistors setting the address this weekend. I moved one resistor from a spot to the other to change a sensor from an optical drive sensor to a hard drive sensor in my older iMac C2D. Its fans no longer spun fast once I pulled off the resistor swap.

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he he small world. Thanks for your tip on the LCD sensor being the cause of my fan issues. Still working thru it and will post outcome. Some of the sensors are single bit - only 2 resistor positions, and some are 2 bit - 4 resistor positions eg LCD and AMBIENT sensors. Also worth noting that the spec on the IC's used state that the address lines should be pulled to V+ or GND as required to set the desired address so I am fairly certain the 1K resistors are not really required... How the heck did you manage to move the resistor? I tried but could not even melt the solder! Do you have a SMD rework station handy?

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I used a temp controlled Weller soldering station. Not really all that special. I think I used a 1/32" conical tip. Apply flux, add more solder, and the resistor comes off stuck to the side of the iron. Then tap the iron on the edge of a block of wood and off flies the resistor. Clean off extra solder on the sensor with wick and you are ready to move it over. It took me 3 or so times to put the resisor back successfully. I don't have a magnifying lens, so it was very hard to actually see the resistor to then solder on it.

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OK. I have a Weller too but I think I was not getting enough heat... I'll try the add solder trick

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Found this in Apple support forums. It's only temporary - fan will go back to high speed after sleep or shut down. But only takes a sec to paste into Terminal:

Install smcFanControl

Go to the terminal and use these commands to regain control of your noisy iMac:

/Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 12c0

This will set the CPU Fan to max at 1200 rpm: (Oh yes!)

To revert to standard settings use:

/Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 3cf0

This will set CPU Fan to max at the default 3900 rpms

You might have noticed that the last 4 digits are the fan speed (in hexidecimal)

Other fan speeds you can try:

1100rpm = 1130

1500rpm = 1770

3900rpm = 3cf0

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Do you know a similar command to slow down optical fan. Tis worked for me for CPU, but optical fan is still at 5000rpm. I don't have anything in the DVD. Drive.

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Cool, looking elsewhere I found for optical I changed "F2Mx" to "F0Mx". Worked like a charm. I have temperature monitor so I can keep an eye on things, but this computer gets pretty light usage. For now i need to type this after sleep. Hope to dine script somewhere to automate these, I guess maybe an easy job for Automator.

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Hello, hope someone can help cos im getting nuts here

The script runs well for hdd fan and optical fan

But not for the cpu fan....any ideas? This noise is driving me breaking-bad crazy

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Richdave will be eternally grateful.
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