Volvo Repair: transmission, worse case scenario, check fuse

Question
My 1990 780 turbo willnot go into 4th gear

Answer
John, first check fuse.  Next there is a white O/D relay behind the ashtray assembly.  Is the orange arrow lighting up?  Then you have to hear if the solonoid is clicking on the trans.  If it is then the worse case scenario is an internal fault inside the trans.  Roger