I am a new truck owner. My truck has an Eaton 15-speed (model RT-12515). I can shift through 1-5 just fine, but I can't get gears 6-10 to engage. The other five gears are deep reduction gears. When I bought it, the control valve was leaking air so I replaced that. When I pull the control up, I can hear the transmission engage the piston. Everything seems to work and sound as it should, but those upper gears don't seem to work. The gear positions that would normally be gears 6-10 are just staying in gears 1-5. Any suggestions? I know I am overlooking something. I have been reading through the manual and can't find the missing link. Thank you for your help.
All the numbers and years kind of run together to me but I would start here. At the top right of that trans looking from the rear is an air piston housing with two air lines connected to it. Those hoses should have swivels on them and easy to disconnect. The front one should have air in low range and the rear one in high. If you have that put a pressure gauge on the high side line. Should have around 65 PSI. If not chase the air system. If so pull the back box. Probably a bad sync.
@BoxCarKidd advice is good. I think (and could be misremembering) that the pressure through the regulator should be 57-59 psi. To simplify things, i use shop air and a rubber tipped blowgun to put 120 psi into both ports of the range piston cylinder. If it shifts, you know it's back, if not, you know it's forward. Edit. Dry o rings on the range cyl. piston will cause your symptoms even with the 57-59 psi, but will normally shift when you hit it with 120 psi
I finally got it fixed thanks to someone who helped diagnose it. It turns out that whoever did the transmission swap for the prior owner had two air lines reversed. Fortunately it was an easy fix and it drives and shifts great now.
