I posted a month or so ago that the Allison would not shift into gear (neither forward nor reverse). It would just show the selected gear (6) and a dash in the accompanying window. It would also sound a constant beep. The pad diagnostic indicated no faults.
I was having trouble with the ignition switch at the same time and thought they might be related. I replaced the ignition and start solenoid and all seemed to be well... until we took off for a trip. Five or six shifts and then the same problem returned. I couldn't figure it out so I ended up taking it to an Allison shop.
They hooked up their fancy dancy computer to the diagnostic port and it indicated shift inhibits caused by too high RPMs. The tech told me that the throttle linkage on the 8.3 was sticking and causing the engine to idle just a bit too high for the Allison's liking. I adjusted the spring (actually two springs) for more tension and sprayed some super duper penetrating oil on the linkage and it now shifts every time. The 8.3 is supposed to idle at 750 and that's about what it's doing. I never would have figured this out. The 3060 must be more sensitive to revs as I know the 4060s' idle can be quite high and still allow shifting.
jor
The throttle position sensor controls the transmission going into gear and shifts. It must know that the throttle is at idle for the transmission to shift into gear. The first place to look, even for bad shifts. A side note that all of the sensors are common on the mechanical engines, only the cable length varies. I bought mine on eBay for $100.
Where is that located on the 8.3? In my case, the tech simply pressed the linkage to lower the idle and that allowed it to shift. After I adjusted the spring tension it has shifted every time. Thanks.
jor
It is attached to the side of the engine box/frame below the air throttle. It has a cable attached to the throttle linkage.
Your's may have the King electronic throttle. It's a servo motor in a box that has a cable running to the engine. The throttle position sensor is in the pedal. A bad signal to the transmission computer can cause harsh shifts.