I explain, when you check the starter engine outside the vehicle, what the pinion should do is move forward, until it is parallel to the window in the front of the start. The pinion must be kept advanced all the time that the starting of the starter engine lasts turning to the resting position as soon as you cut the current at the start.
The starter engine has another cylindrical body with two fat screws and a small terminal, (the starting relay or also automatic called). If you have disassembled the start of the van, you will know that one of those screws comes the direct battery positive, the other screw is just below the first and is connected to the electric motor input and the small terminal comes the positive cable from the key.
To do the test, hold the starter engine on a bank screw, put the dough to the motor housing, connect the positive to the fat screw and bridge another positive to the terminal, at that time, the start must be launched while the attack pinion comes forward.
If you play with the positive clamp in the Gordo screw below, what you are testing is only the operation of the electric motor, not of the complete start, therefore the attack pinion will do what you comment, give a blow to the next and go back to the resting position even if the electric motor continues to turn.