I would like to try and help with your current problems.
I need to know the engine serial number, as well as the Transmission model and serial number. Is it an Allison?
The B500 has diagnostics built into the key pad.
Turn the Key on, at the key pad for the Allison, push the up and down arrows at the same time, this will put you in diagnostic mode.
Display should show D-1 then a four digit code. Write these down.
Then hit the mode button, this should show D-2, keep doing this and record all codes listed.
Once you ran a code check on the transmission, let me know what you find.
Allison will force the lower gears and even lock you into range if there is a problem.
I'm sorry, I dont show a 15-16 or 15-11 code for the Allison B series.
I will keep an eye out for your next posting.
If it will help you, I add the Allison Code Manual to my answer. If you get codes that match, I can follow up with the information to support that codes test and repair.
Your request timed out, but below is a direct download of the Allison Code Manual if needed.
"Click HERE for the Allison MD PDF Code Manual"
I will get the book out and copy the code testing and repair pages for these codes.
Give me a little bit. Will get it loaded shortly. If all this helps, please accept my answer and let me know if there is anything more you need.
I'm sorry, I was wrong, the book I have is the parts book. It will be tomorrow morning before I can get you the Testing information related to your code.
Well the book only has you check for an open between the snsor and the computer.
Most of the time, the problem is found at the main connector just a few inches from the transmission. I have also seen the tone wheel come loose in the Allison causeing problems. Hope this helps,