Yes, there is linkage between the controls and vale that you can check. Below is a scan of the page from the D14 operator's manual. Beyond this things are internal and you will want the service manual. If you don't have copies of the Operator's and Service manuals you should get them. You can look at the parts book to see the linkages as well. If you don't have a parts book you can go to https://www.agcopartsbooks.com
and sign in as a guest user. On the sign in page there is a link that will take you to AGCO Publications, where you can get manuals. This site's store has manuals as well. I would get the AGCO or quality reprints of their manuals first before getting I&T or other third party manuals.