I have a John Deere 12. Its about 3 years old. I believe the hydrostat is starting to go bad. Tractor no longer will climb steep grades. Also when I first start going forward, you hear a load mowning sound. Tracto alsop seems slowe than when new

I know the K46 is a sealed unit that you are never to service.

Is there a way to change the fluid? If so do you think this would help?

Hi,
Yes it is said to not be serviceable. You can remove it and remove the plug where the factory filled it and invert it to drain. It uses 10W30 oil and should measure 30mm below the top of the hole when full. To see this, go to this link, type in "L120", hit the plus sign to expand, pick Sectional Index, Power Train, and Transmission. Item #33 is the plug mentioned, and #32 is the magnet that is supposed to trap the metal particles and keep them from recirculating.
John Deere - Parts Catalog (http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePa geServlet_Alt)
Peace,
Clarke