B500 minimum operating temperature

[Bill B /bus]

I never had a problem with starting the engine, building air, then engaging the transmission,. Showing,on the pad, 6 and 1. Indicating Trans in gear and in first gear.  Imagine my surprise when I got "mode on service".  And no ability to make trans engage.
Towed to Allison facility. I had spoken with the Service Manger in diagnosing the problem. They discovered a power glitch after driving into the shop. During the tow about45 minutes the airline parted from the bus. The driver started the engine to maintain air. Somehow the transmission fixed itself during the tow.
After rerouting the power to clear that problem and verifying the computer had no faults I paid the bill. We headed down the road on Friday after supper in the facility lot. Stayed over night at a shopping center. Next morning no go on transmission engaging. Then the light dawned. Low Temps were a factor. Trans engaged at about 110 degrees. Subsequent overnight restarts required higher and higher Trans Temps.
One is there a low temperature setpoint to prevent Trans operation? If so what is the temperature setpoint?
Two the temperature increased to about 150 degrees over several days. The leads to believe the temp  sensor is failing. I really hope not as that is not a nice job to drop the pan.

Thoughts/comments. In the Rio Grand Valley for the winter so not time critical.
Thanks for your time,
Bill
L