Ford used to use Mercon in their older transmissions; this fluid was usually sold as Dexron/Mercon in parts stores which would seem to indicate the two were essentially the same. Some years ago Ford dropped the Mercon fluid and told the dealers to use Mercon V in its place. I have never seen a problem caused by using Mercon V in a Mercon/Dexron transmission. In fact, the 722.3 I rebuilt in my own car two years ago got filled with Mercon V and operated just fine.

Your lack of reverse is not fluid related unless it's just plain low on fluid. It's probably the B3 clutch discs coming apart which is a common 722.3 problem. Also could be a cracked B3 piston (part of the front pump assembly). See my reply in your other thread.

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Erich Loepke
2010 Ford Focus
Currently Benz-less