Volvo Repair: 1993 volvo 940, overdrive units, 1993 volvo 940

Question
the 4th gear will not engage all of a sudden. Arrow indicator in the instrument panel according to the manual is the 4th gear is disengaged.  How can I engage it since I have tried to click on the button as indicated and the arrow is still there. Thanks

Answer
  Marie, you need an xperienced thechnician to look at the overdrive control system. I would first check the fuses and then the wiring at the base of the shifter for integrity. Then the switch itself. Then the fourth gear switch on the transmission and then the overdrive solenoid. If all ok, then the overdrive relay. The overdrive units themselves are bullet proof. I only replaced one in 30 years of doing 2-7-9 series Volvos. HTH

Mark