Hi Yolanda Guerero-Rebollo,
Thanks for the file.
Sorry for the late reply, but I got the 503 error today, constantly.
You asked this question in the Tutorial forum, below a tutorial that handles modeling nodes. Please don’t use the forum this way. People with questions about this tutorial might think that an answer to this tutorial is in your question. While searching for an answer, that might not be fun at all.
Your scene does not show any modeling nodes at all. So I have to move it to the Q&A forum.
Please go to the Attribute Manager> Project Settings> Dynamics> Visualization.
Here, check the enabled. Then, run your scene (play button) and do nothing, really nothing (!) while your Control> Gear is set to stop.
Observe the colors that are shown. The new lines in your scene will go to blue at one point, which indicates that the dynamics have been stopped. In this state, the engines will not start.
If that state is reached, the Motors will not restart. Therefore, the Dynamic Tag>DYnamics>Deactivation needs to be set to zero, one or the other of the two.
Now you can set a Keyframe for the Control> Gear.
Cheers