How do I engage/repair overdrive in a 1975 mercedes?

I have a 1975 mercedes 280c (automatic tran) that shifts and drives fine, but won't kick into overdrive after it reaches the 3rd gear. I am moving and need to drive it via interstate. So does anyone know if it would have on/off switches like newer cars? If it is faulty, what could it be? Someone once suggested it could be the solenoids.. any confirmation on that and what types of places can fix it?