no OD

HI,Newbie here,I have a 1993 3500 Silverado that goes thru all the gears until it gets to the OD but will not go into OD at highway speeds,does not slip as it is shifting any ideas why this is happening.
Could it be that all it needs is a fluid & filter change as that is what it needs done now,thanks

HI,Newbie here,I have a 1993 3500 Silverado that goes thru all the gears until it gets to the OD but will not go into OD at highway speeds,does not slip as it is shifting any ideas why this is happening.
Could it be that all it needs is a fluid & filter change as that is what it needs done now,thanks
no 4th gear maybe the valve body is worn. I would replace both valves a/b..