@Douglas,
"The gardens are free EXCEPT on days when the fountains run."
There were no fountains operating the day I was there. In fact, one of them was torn apart for re-construction. I was told by people at the site that my Paris Museum Pass would not provide admission to the gardens (except for the Trianon Castle and Estate of Marie-Antoinette). This would seem to be consistent with the information on the Museum Pass website which states "Not included: shows, musical water shows, musical GARDENS, conferences, restaurants and transport services."
Entering the Chateau was quite a different story. I just waved the Museum Pass and the guard just motioned me through without even looking at the validity dates.
Perhaps I didn't understand the details correctly, but that was my experience.
Cheers!