Overdrive shift problem

Have a Mercury Tracer Trio 4-speed automatic trans. It has trouble shifting into 4th. If I drive in D, I don't have a problem, since the car then nevers shifts into 4th gear. It does shift into overdrive, but it's rough, and shutters. Is there a sensor, or switch that controls the shifting that could be bad? This car is just an around town vehicle and rarely is driven over 45 mph. Can I just drive in D and not in overdrive? I don't want to put much money into such an old car.

I have the same car and it does the same thing. The car will try to shift into overdrive and very quickly decides to shift back to drive. It does this when I am going more than 40 mph and letting off the gas peddel and causes the car to jerk back and forth violently! I wish someone could tell me whats wrong with this car. I took it to a transmission shop and they charged me $45 to connect it to there diagnostic computer and said there was nothing wrong with it. I think they are scamming me.
anyone got any ideas? email me at walterramjet@yahoo.com


Have a Mercury Tracer Trio 4-speed automatic trans. It has trouble shifting into 4th. If I drive in D, I don't have a problem, since the car then nevers shifts into 4th gear. It does shift into overdrive, but it's rough, and shutters. Is there a sensor, or switch that controls the shifting that could be bad? This car is just an around town vehicle and rarely is driven over 45 mph. Can I just drive in D and not in overdrive? I don't want to put much money into such an old car.

I did end up taking the car in to the shop. I was told it is the trans. They did a complete flush of the trans fluid which helped a little, but did not eliminate the problem. They told me to just drive the car in D (3rd gear). That burns more fuel and is a bit harder on the engine at speeds above 40, but will work. The other gears are still fine.