Volvo: 1990 Volvo 740GL Auto w/ Overdrive, glovebox, solenoid

Question
While driving down the road the car will go out of overdrive on it's own. It will not go back in by pushing the button on the shifter very often. Could it be a bad wire or switch?

Answer
You could have a bad relay, a bad solenoid, or a bad overdrive unit, going in order from easiest to hardest to repair.  Most of the time it is the relay, behind the glovebox under the dash.  I would have that checked first, then test for voltage at the solenoid under the car on the transmission.  Good luck!