@dharr 

So it didn't work with RungeKutta? that is the default method for dsolve right?

@tomleslie 

Thank you, I dont know why it doesn't work the other way but I guess I will have to do this with the rest of the system wich are larger.

The computation finally gave me this message:

Length of output exceeds limit of 1000000

What does this mean?

@acer Thank you very much. That was very helpful indeed.

@imparter I still get a negative result after implementing what you have stated.