The only time you should turn off overdrive is either when towing or descending a long hill ( to allow for some engine braking which saves wear on the vehicles brakes and helps them from overheating ).
The car should automatically be in over drive. What that fancy button does, is kicks your transmission out of overdrive and downshifts so to speak. It helps with hills and hauling a trailer. It does not harm the car
I have an automatic 1999 Toyota 4Runner and I usually turn on the OD uphills or on the freeway. Can this harm my car or the engine?
You use OD when you are doing regular driving. Turn it off for big hills (mountain passes) and for towing a trailer that is causing the engine to struggle. Putting it on for hills is exactly what you are not supposed to do.
I leave mine on except for one 5 mile long hill, or towing a car on a trailer.
If it harmed your engine they wouldnt fit it to your car.......use your noggin ffs.
