If your looking for a short "fix it" answer, it's not that simple...maybe...
With the check trans light on, it's telling you something is up. It might be communication, might be the TCM is in crisis, or it could simply be no power to the TCM.
I am not familiar how a Gillig is wired up but typically in school busses the TCM fuse is usually located right there in the battery box. I did have one situation on an Amtran FE where the fuse was actually hidden from sight. It was found under the bus sandwiched between the brake air valve and the bus body, conveniently ty-wrapped there.
First thing you need to do is find that fuse and make sure power is going to the TCM.
Now to touch on communication which is secondary at this point until power is verified.
What I say next only applies to the "pushbutton" shift selector.
If your push pad is dark when you turn the key that means no power to the TCM. In a school bus, if it is dark the engine will typically not start. this is a safety feature.
You said yours starts once but then you have to cycle the key switch?? This process does not call any special attention as to restart the bus, you need to cycle the key anyways but tranny still does not engage.
When it starts, does the push pad display a gear or does it display what is called "cat's eyes"?
If it display a gear then communication is fine. If it displays cats eyes then there is either a communication issue or a safety interlock issue causing it to go to cats eyes. Interlocks could be physical or could be software within the TCM.
Can't really tell what to look for next unless we know exactly what you see when you turn the key.
If you have a "bump" or "handle" gear selector and TCM does light (check trans light) up than I would look at the NSBU switch to see if it is in conflict. Might be dirty, worn or connections failing.
Please turn the key, tell us what you see and also let us know pushpad or bump....
Going back to communication, if the communication circuit resistors went bad, that was cause a check trans light to come on but it would still go into "limp mode" gear. usually second or third only. If it was a broken wire somewhere, which happens more often than people realize, a pushpad would stay dark and not light up, hence the bus not going into gear.
If it was the TCM itself gone bad, many times the TCM will just go into limp mode.
Your mechanic stated there was no communication from the data port. I question that because the bus starts. That means there is communication between the Engine ECU and it's sensors so I would not focus efforts in that direction just yet. that would be something to look at after you ruled out TCM power.
Was your mechanic looking at engine data or transmission data? this would be helpful to know. If he saw no tranny data then again, TCM power needs to be verified!
I also forgot to mention, sometimes...the tranny will not go into gear if the engine is idling too high. How high...IDK??? what does your engine idle at? my best guess would be above 800 rpm...
also, Allison TCM's will go into neutral if voltage is less than 11 volts. So with the engine running checking voltage to the TCM would be helpful.
What your mechanic said about the brake light switch, that too is a possibility but easily checked. If your TCM is powered up with no cats eyes, then simply unhook the brake switch connector. If yours is a simple two wire then try jumpering it after testing it "disconnected". see any changes? power going to brake switch connector? I do not think though this would cause the check trans light to come on but...you never know.
Which Gillig bus body style do you have?
