Ive Been having a problem with me 48re for a couple of years now ever since i put in a sun coast valve body. What is happening is intermittently is when I start moving it wont shift from 1st to 2nd till 2800 to 3200 rpm same with 2nd to 3rd.... the funny thing is that if i run the throttle pedal through its range of motion (stab it too the floor quickly) it shifts normally.......Now, I just had the transmission rebuilt by Garmon diesel , new electronics , the whole works....but still having the same issue......im not getting any codes....i was running a smarty but its on stock till i get this resolved....sounds like symptoms for a bad throttle valve motor (ttva) on the trans but again no codes, its the only part that is not new on the trans and was on the old one. Im taking it back to the shop so we can get it figured out...just trying to see if anyone can offer any help on the diagnosis or potential problems......

 

Update....It was the suncoast valve body.....replaced it....
works great now!!!!

 

WOW!!! That was quick! Did you put in a new Suncoast?