Volvo: volvo 240 dl , overdrive stuck, volvo 240 dl, volvo 240

Question
i was using my 1989 volvo dl wagon , and i was riding in a place with many hills, in which i pushed the button to change for overdrive every time i was riding on a hill . then the overdrive kept active as the arrow indicating overdrive kept lighting. i changed the overdrive button , but the problem remains, and the transmission is working hard because the overdrive is active. i just want to deactivate it, and know which is the broken part

Answer
The two likely culprits are a bad overdrive relay, located behind the glove box, or a frozen overdrive solenoid, attached directly to the overdrive unit on the transmission.  My bet is on the relay, they tend to fail the most often and can be replaced for $40 or so.  Good luck!