Just now I heard a call to the radio show - a person’s Volvo 240 seemed to stick in low gear - ours had this problem and it turned out to be a malfunctioning electronic overdrive turn-off switch - our mechanic disabled the switch (which means we cannot turn OFF overdrive by pushing the little button on the shifter - no prob!) - and voila!
Could also be the speed sensor which tells the trans. when to shift. If your speedometer doesn’t work, this is most likely the problem.
And if it’s not expensive, do the repair!
I had a 240 and I miss it every day (despite the turbocharged awesomeness of my V70).