Original post by: Charlie ,
FIXED THE ISSUE.... This took a little time, but it was a simple fix. The LCD sensor that you have to disconnect from before removing the LCD was the issue. After receiving HIGH CPU fan speeds, I began looking at possible issues. First, I had [http://www.macupdate.com/app/mac/12381/temperature-monitor|Temperature Monitor] Installed. I had all my sensors including the ambient air sensor reading. After I upgraded my HD to a SSD I started getting HIGH CPU fan speeds, but the HD and DVD fan speeds were fine. I looked at my Temperature Monitor and I noticed i had several Temperature Monitors not reading including the ambient Air Temperature (bare with me, these are steps to troubleshoot). After reseting the SMC and PRAM, still nothing. I shut down and disassembled my IMAC again. This time after i removed the LCD, I looked at the LCD cable that you must disconnect when removing the LCD and BAM, I saw the issue. The wires that connect to the Motherboard from the LCD Sensor, inherently like to lay across the screw hole the Bezel attaches to. I have a couple of pinched marked that exposed the wires and when I tighten the bezel down, it shorted the wires and caused the issue. I cut small pieces of electrical tape and covered the bare spots. When I reassembled the IMAC, i mad sure that all my BEZEL screw holes were completely clear of any wires. Once back together, I plugged in the IMAC, waited 5 Seconds and turned it on. All is well. I feel this a majority of everyones issues when they take apart their IMACS and put them back together. Hope this helps.