Outer Limits 4x4 Board

Having problems with the automatic transmission. Doesnt seem to be shifting into overdrive. It has a button on the side of gear stick that takes it out of overdrive so when i press the button the light comes on on the dash saying its out of overdrive but when i press it back in so it should shift into overdrive but its not. Is there a valve or something onb the gearbox that shifts it into overdrive? Would that be the problem?
Any thoughts or help would be great