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 :)