no overdrive

I have a 1998 3.8 v-6 just over 110k on it. Out of the blue it has quit shifting into overdrive. There had been no indication of any problem before this. Is there any electrical sensors or solonoids I might check. This vehicle has been serviced on a regular basis and shifts fine in other gears. When the cruise is set and you are going 55 the rpm in 3rd gear is 2500. On a long hill or downgrade the tach will drop to around 1100 but once you put any gas to it it will shift back into 3rd. Throwing no codes.

Common issue here the "stovepipe" shaft to the o/d clutch unit can strip and no connection to the planetarys. A scan with a trans capable scanner will allow a trans tech to see slippage errors and/or command the od, if none out it comes. This exact problem occurred with my 3800 LTZ 98 Lumina.

I am havein the same problem with my 2001 MC. no Overdrive, I have no Idea what it is, the tranny just isnt shiftin to fourth like you said. I need to know how to fix it. If anyone has any ideas please let me know.

Thanks
Henry