Chosen Solution
why dont my cooling fans increase in speed to keep my imac cool
apple has them set (i believe) to ramp up at 90degrees celcius. A temp sensor may not be working, or it hasnt reached the 90 degree mark yet.
I would install this app TG Pro so you can monitor your thermals and fans. If you need to you can turbo the fans to pre-cool the system when you are planing on being aggressive in your processing/graphics load.
