Chosen Solution
Hello, I have a 2009 27” iMac that upon pressing the power button, makes the Mac chime, but then displays nothing on the screen and the fans then spin up. I have tried resetting the SMC and checked for other power related issues.
Using a flashlight, place it on the display glass at a sharp angle do you see a faint image of your desktop and the icons? You could also try an external display as well. If you see the faint image and/or external shows a good image. Your internal displays back light needs repairs. A common issue is a burnt LED Driver board iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement and here’s the needed part iMac 27" LED Driver Board (‘09) 661-5311 for reference. Update (03/13/2020) If you look at the far bottom left of the logic board you should see a piece of black electrical tape covering the logic board
You’ll see four onboard diagnostics LED’s
With the display disconnected what do you see when you power the system and hit the power button normally.
This behavior isn’t easily diagnosable with the symptoms described. Have you tried reset the PRAM? See here: https://support.apple.com/en-us/HT204063 If this doesn’t correct the behavior you may have a hardware issue. When you say that the fans “spin up” do you mean that they spin at a higher rate than normal? In 2009 the thermal sensors for component temperatures were replaceable, can go bad and cause disengagement of components and high fan RPMs.
