20" iMac (Aluminum) LOUD FANS Problem!

[Doc]

Hi

I looked at a 20" iMac (Aluminum) recently that has a problem of the internal fans running constantly at high rpm causing a lot of noise. I tried the SMC reset, making no difference.

Anyone know what could be done to sort this out?

 

[Skyhooker]

First thing I look for is dust/lint/hair buildup on the heat sinks.

 

[lebpcmedic]

I had the same problem with a heatsink fan on an HP desktop. Replaced the fan and all was well!!!!

 

[Doc]

First thing I look for is dust/lint/hair buildup on the heat sinks.

I did consider the build up of dust on the heatsinks but the imac is only about 20 months old, there was no dust on the small grill on the back and the room it was in didnt seem particularly dusty. I probably will open it up and check.

 

[anonymous Mac Tech]

There are 3 fans in the 20" iMac. HD fan, CPU fan, and optical drive fan. Download and install istat pro which is a free dashboard widget. It will give you readouts of all three fans along with temps, CPU usage, RAM usage, etc. As long as the CPU usage as pretty low, look for whatever fan is running high RPMs. I'd have to say it sounds like a sensor was left unplugged. Did you swap a HD or optical and not plug in the sensor?

 

[Doc]

There are 3 fans in the 20" iMac. HD fan, CPU fan, and optical drive fan. Download and install istat pro which is a free dashboard widget. It will give you readouts of all three fans along with temps, CPU usage, RAM usage, etc. As long as the CPU usage as pretty low, look for whatever fan is running high RPMs. I'd have to say it sounds like a sensor was left unplugged. Did you swap a HD or optical and not plug in the sensor?

Thanks for the replies so far! The imac has never been opened up as far I know. I did install iStat when I looked at the imac (it was powered on for around 10 mins) and I wrote down the info:

Fan Speeds
CPU 3900rpm
HD 6000rpm
optical drv 5097rpm

Temps
HD 23°C
CPU 25°C
Northbridge 1 35°C
Northbridge 2 30°C
PSU 62°C

 

[anonymous Mac Tech]

Thanks for the replies so far! The imac has never been opened up as far I know. I did install iStat when I looked at the imac (it was powered on for around 10 mins) and I wrote down the info:

Fan Speeds
CPU 3900rpm
HD 6000rpm
optical drv 5097rpm

Temps
HD 23°C
CPU 25°C
Northbridge 1 35°C
Northbridge 2 30°C
PSU 62°C

It looks like the CPU is running at a fairly decent temp along with everything else. Just to be safe I'd see what istat is saying about processor usage, but from the temps I'd imagine the CPU usage is low. My opinion is the HDD sensor is unplugged either from a time when maybe the HDD was replaced, and in turn is ramping up the RPMs on the other fans (maybe a warranty HDD replacement? Failing drives aren't uncommon of these.) Without any load on the machine those fans should be sitting at around 1000RPMs and even under load won't even go even to or over 2000RPM. The temp sensor cables for the HD and optical are literally stuck on with plastic holders that are only held on by a light adhesive and a tape pad. If those sensors aren't making good contact with anything, the fans will go nuts.

 

[Bizybone]

As long as that unit is installed with the OS via the install CD that came with the unit, you should be able to run the Apple Hardware Test diagnostics by holding down "D" after restarting the unit, before the chime. It should take several seconds to get it going.

It does sound like a sensor issue which is usually corrected by resetting the cables. Run the quick test first, if nothing shows up, run the extended. If there is an error it will tell you exactly which sensor to pay attention to.