Haldex malfunction

Hi, i have a europian S3, but it has similar quattro system as audi TT, so i figured to ask you (tt owners) about my problem.
My haldex unit seems to have malfunction, because it's now FULL time awd. I mean that my rear end doesn't turn off at all and it causes that when i'm doing a small speed turn with my wheel turned fully to one side, i end up having big bumps in my rear wheels (because haldex doesn't turn off my rear wheels and when i'm doing a turn, there's difference in speed of front and rear wheels). I don't understand is it mechanical problem in haldex or eletronical. Because the strange thing is that ESP works with no problem, and esp light doesn't show any malfunction. I tried looking for problems with vag-com, but it doesn't show any faults. I just don't know what to do, because than i leave the car to stand with egine off for 10-15min. it seems that haldex turns the rear wheels off, but as soon as i turn on the engine, haldex immediately turns the rear wheels on.