@ Domenic I have resetted the PRam but it did not work, this might be because of what Thomas says.
@Thomas A Reed, It seems that the machine isn't sleeping, since when I close it the light on the front edge is on constantly (no pulse). I downloaded IstatPro to see if my vantilator is ventilating (is that even a word? English is not my primary language) too much, but it says that it exhaust about 2000 rpm, I'm not sure if this is too much but it sounds louder then it did before.
First of all thank you for the awnsers.
It seems like we (Thomas A Reed) found the problem. Now I just need a solution. My MacBook Pro does not go to sleep when closing the lid, or pressing the apple logo and 'sleepmode'. I've searched the internet and it seems like I'm not the only one. But the solutions I find aren't helping. When pressing sleepmode the screen goes black but the indication light stay on and won't go into pulsing mode, and ofcourse the ventilators stay on. I deleted /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist as part of a toturial I found on this subject but it didn't work. I hope this won't do any harm to my computer.
Can anyone help me out?
Well, it's possible that there's some third-paty program that is somehow preventing the machine from going to sleep. Try rebooting in Safe Mode by holding down the shift key at startup. This disables all third-party software that loads at startup. Does that change the behavior?
Of course, it could also still be a hardware issue, so running Apple Hardware Test would be another good idea.
Okey, so here is the thing. Can a macbook not go into sleepingmode when the powercord is connected? Because if I remove the powercord it will go into sleeping mode. I don't remember that it won't go into sleeping mode when it's plugged in, I thought this was possible. If this is the case I feel really stupid for wasting your time.
After knowing that it only happened when the powercord was connected I did some research (and got in way over my head). But if anyone else has te problem, this solved it for me:
run pmset -g
You'll get something like this:
Battery Power 1
AC Power 2*
Currently in use:
sleep 0 (imposed by 20)
See the line: sleep 0(imposed by 20)? Well that means something appearently
So I did (according to another discussion): ps -ef | grep 20
I have no idea what it means but I got a whole lot of text as outcome, and saw something between the lines about my printer (which the original poster also had, but then with imposed by 18). So I went to preferences, print and scan, and deleted my printer. This solved my problem.
After running another pmset -g
Sleep is now at 10.
The only point is that I can't print anymore. I'll try to install it later again, and hope that it works. For now my problem is fixed. Thanks everyone for helping me. All credits for the solution above go to 'jcarpio' and you can read the discussion here: https://discussions.apple.com/thread/3190738?start=45&tstart=0