Transmission overdrive

I have a 1998 grand Prix GT just over 100k 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.

The shifting in a 4T65E tranny is handled by 2 specific solenoids via the PCM computer. Solenoid A and solenoid B. A is the 1 -2 upshift. B is the 3 - 4 upshift. They are located under the tranny driver's side cover assembly with the valve body. Sounds like the B solenoid is either sticking or defective. You can try a tranny additive such as Sea Foam or Lubegard.







http://i5.photobucket.com/albums/y186/lizzywiz/gtp.jpg
Thrasher CAI, DHP v1.0 PCM, SLP Headers,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 72mm TB,
TB spacer, MSD 8.5mm wires, Autolite 103,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials
1/4 ET: 13.210 sec. @ 104.490 mph
0 - 60 ft: 1.945 sec.