right, had another thought. the limit mechanism on tube motors consists of 2 long , usually nylon, threads with nuts that run up and down them. the nuts operate two microswitches which provide the top and bottom limit. When the motor is running, it also drives these threads, and when the nut reaches the switch, the door stops. the adjustments turn the threads manually to change the position of the nuts on the threads, thus setting the limits of movement. Again, it sounds complex, and the mechanisms are very fiddly to deal with, but I have managed to repair them, so it can be done. I dont know how big your door is, but my largest door is about 3m wide by 2.5m high, and the replacement unit was about £90 which is motor and limit sw itches all in one unit. The only way forward is to remove the old unit and strip it, you can usually get to the switch mechanism wothout much trouble, You need the door right down, then unbolt the door from the roller, disconnect the wiring, and unbolt the ends of the roller, and lift it to the floor. motor is usually held in the tube with a few self tapping screws visible on the outside of the roller, and the motor will then slide out. Fronm there on, you are on your own, as I do not know this type of tube motor, but you have nothing to lose by having a go! You can usually remove the limit switches from the tube motor, but dont attempt to remove the motor, I did, and wasted two days of my life trying to get it back in and working reliably!!