The reason that it wont go into overdrive is because a short in the shift solenoid or possibly if car is equipped with power comfort option the shift trigger in the switch could be dirty or faulty or simply low manifold pressure.
Check your fluids. But you may need a new transmission.
Hard to say, start with the basics-is the fluid full?
If speedometer itself is working then gears inside the assembly are damaged.
The small sun gear freewheels, and the larger sun gear is held by the overdrive band.
Turning off the overdrive is basically preventing the truck from shifting gears past 3rd. It's useful when you're going up or down steep hills. answer Dodge Ram trucks come stock with an overdrive, or O/D, function that enables them to switch between three and four gears. When the overdrive is off, the truck operates on three gears, allowing it to pull more weight and giving it more power. When the overdrive is on, the truck operates on four gears, using less power and less gas.
Overdrive just lets the car use the lowest gear. Without overdrive you are probably only using 3 gears, and with it you are probably using 4.
It could have 3 forward gears or 4. Some automatic transmissions have overdrive. Overdrive is another gear that allows for better fuel economy on the open road.
6
If you have an automatic gearbox, 4. Plus two extra overdrive gears, in 3rd. and 4th. gears. The total will make 6 different gears. European Volvo 960 with 2.0 liter turbo engines have manual transmissions with 4 gears plus manual overdrive. The later S90 was also available with 5 gear manual (reverse was synchro!)
In Nissan Sunny (and some other cars too), overdrive is ON when the button is DEPRESSED and there's NO LIGHT in the dashboard. The automatic transmission will utilise all the 4 forward gears (the 4th gear being the overdrive that helps in fuel economy -- similar to 5th gear in a manual gearbox). This is the recommended setting for normal driving. When the button is NOT DEPRESSED, you turn OFF overdrive, and the indicator LIGHTS UP in the dashboard. In this mode, the automatic transmission will NOT shift into 4th gear.
Select the " D " with the circle around it for overdrive Select the " D " if you only want to use the first ( 3 ) gears and no overdrive
Gears is what transmission is for. No gears. No transmission!