Peterbilt Automatic Allison Transmission Information Please?

Are the engine brake controls on the lever on the right side of the steering column? If so try pushing the lever up and down. Down to downshift and up to upshift.

 

You sure the mode isn't to allow you to shift it manually?

 

It could be if the company hasn't had that option removed.

 

Its an Allison and more than likely has this kind of selector:
All the mode button does is toggle between economy and performance. From what I understand, performance mode operates just like a regular car automatic. Step hard on the throttle and it downshifts. In economy it won't downshift on a hill if you step on it unless the RPM drags down below peak torque RPM.

If you want to manually downshift a gear, should just have to push the down arrow.

 

It depends on RPM. If the engine rpm will be too high it will not downshift.
I would decide what gear I needed to desend, slow and select the gear I needed.
It also worked well to limit upshifts.
You can arrow up or down at any time to limit top gear.
Mode button can be configured for different shift programs depending on application.
Play with the different settings and figure out what works for you. One thing about allisons is they are programmed to not hurt anything and save themselves.