Transmission won't go into overdrive

I have a '88 Grand Marquis with less than 80K miles on it. Recently, when I get to 40 mph and the tranny shifts into 4th gear overdrive, the engine just whines and the car coasts. When the speed drops down to 25-30 range, the transmission engages again and I can drive as long as I stay below 40mph. I have to put the transmission into D2 to get it to run 'normally' but without going into overdrive. Anybody have a clue as to what's happening here?

There is a shaft that runs between the direct(3rd and 4th) clutch to the lockup clutch inside the torque converter. The splines have ripped out of the lock-up clutch or the direct clutch has burnt up. Thats about it. Good luck.

Thanks for the reply. Maybe you can give me an idea as to what it should cost to repair something like that?

I see too many Leo Durocher style ads that are designed to sucker you into mounting costs that always lead to a 'complete overhaul'.

I had a bad experience with a national chain. The problem was solved simply with an oil and filter change but the guy tried to convince me that he had to disassemble the tranny to determine what was wrong. He refused to change the oil and filter as I requested, claiming it wouldn't fix the problem. I took it to a gas station nearby and they did what I asked. I drove away happy.

This is the way I do it. First, pull transmission then you check the splines in the converter and on the intermediate shaft. If one of those are defective then replace it. If those two parts are OK, then the transmission has to come all apart=rebuild

That sounds like a reasonable approach. I took the car to a nearby shop and allowed him to take the tranny down. I may go over and discuss his methods with him, keeping in mind how you would approach the problem.

I may not save any money but perhaps this little education will help me avoid unnecessary costs.

Thanks for your input.

No problem :)