While driving on the highway today, lost the top gear. Truck shifts at very high rpm on other gears now as well. transmission fluid and oil both look alright. is it time to take it to a tranny professional and get it checked out?

 

Need more info on truck.

Engine, tranny etc.



But i'm gonna take a wild guess here, you could have the 3 speed C6, and the higher RPM shifts are probably due to a bad vacuum modulator valve.

It is on the passenger side of the transmission, facing the rear, it's a lil round thing held in by an arm, with a 1/2" bolt.

There will be a vacuum line going to it, just remove the line from the modulator, if oil comes out of the line, or the modulator it self, it's bad. Replace it, and avoid driving the truck until you do replace it.




However, if your truck is an F150 (or Light duty 250) and have the AOD tranny, it might be just the TV cable out of adjustment.