Replaced hard drive, Fans spin high

[Xeep]

Computer is an early 2009 iMac, 2.93 GHz Core Duo 2. I just upgraded the hard drive to a 2 TB drive and when I started back up, the fans were spinning out of control, and the computer would go to sleep after about 30 seconds. I put the old drive back in, but the problem persisted, so I concluded it didn't have anything to do with the new drive, and I reinstalled it.
I found that the temperature reading on the ambient air and display sensor were reading off-scale high, so that explains the fans. This is the sensor that is normally mounted on the top left corner of the display. I also found that the sensor would read off-scale high when the wire was bent too much, so straightening it out seemed to fix this particular problem. However, now the temperature seems to read correctly, but when I mount the sensor where it's supposed to go, the CPU fan will spin up to max when the temperature hits 25 degrees C. It obviously didn't do that before the surgery. I currently have the case removed and the sensor dangling out so that it reads ambient air temperature (18 degrees C) and the fans are behaving normally.
I would like to put the computer back together, but as soon as I mount the sensor, the CPU fan maxes out. I tried shorting the sensor so that it reads -128 degrees C but this also causes the fans to max out for some reason. I'm all out of ideas now. Any suggestions?

 

[Brian Y]

Computer is an early 2009 iMac, 2.93 GHz Core Duo 2. I just upgraded the hard drive to a 2 TB drive and when I started back up, the fans were spinning out of control, and the computer would go to sleep after about 30 seconds. I put the old drive back in, but the problem persisted, so I concluded it didn't have anything to do with the new drive, and I reinstalled it.
I found that the temperature reading on the ambient air and display sensor were reading off-scale high, so that explains the fans. This is the sensor that is normally mounted on the top left corner of the display. I also found that the sensor would read off-scale high when the wire was bent too much, so straightening it out seemed to fix this particular problem. However, now the temperature seems to read correctly, but when I mount the sensor where it's supposed to go, the CPU fan will spin up to max when the temperature hits 25 degrees C. It obviously didn't do that before the surgery. I currently have the case removed and the sensor dangling out so that it reads ambient air temperature (18 degrees C) and the fans are behaving normally.
I would like to put the computer back together, but as soon as I mount the sensor, the CPU fan maxes out. I tried shorting the sensor so that it reads -128 degrees C but this also causes the fans to max out for some reason. I'm all out of ideas now. Any suggestions?

The SMC reads -128 as "sensor unavailable', so it will automatically ramp the fans up.

Have you tried resetting the PRAM and SMC on the machine?

 

[Kanunu]

Software fix

I had the same problem when I installed an SSD. I think there is a way you can jumper the sensor but this is a lot easier.

http://exirion.net/ssdfanctrl/

The software will run your fan off the SMART sensors instead of the Apple sensors. The only quirky think is that it does not go to the menu bar as I would expect and it will not minimize. After it loads, you need to go to the dock and click Hide.

I have had it for a month now and it seems to work perfectly, responding to changes in both load and ambient temps.