I have a 2000 series trans in a 2004 BB that has about 150k on it. Over the years I have had a little trouble with the connector by the shift lever (going into trans) coming off and, of course, causing it not to shift. I haven't had this problem lately because I have used a zip tie to hold it on. Last week the driver complained that it stayed in low, so I went out to take him another bus and before I got there he said it was shifting again. So when I got to him I gave him another bus and crawled under his pulled the connector and it was wet. Blew it out as best I could and reconnected it. Drove it all the way back to the shop (15 miles) and it shifted fine. Pulled the connector and blew out with shop air. Put some dielectric grease around rubber seal and put it back together. I think he ran about 150 miles and it started to do it again. This time he didn't make it out of the lot. When I got here he had taken a spare and left it for me. When I put it in drive it went in pretty hard and it does this in reverse too. The connectors are in good and not loose.
No takers on this one? Well, I called my BB dealer and the Allison dealer. Between the two I figured out that there is trans fluid getting into the round connector on the right side and when I cleaned it out it started shifting. I decided to go ahead and take it up to the Allison people so they can replace the harness. Just as I was making a turn about a mile from the dealer it took off from a stop sign like it was stuck in a high gear. It took off real slow and worked its way up to about 50 mph and never shifted.
That internal harness is relatively easy to change, I keep one on the shelf. I had one once that wouldn't shift out of third the driver insisted the fuel filter was bad because he could only go 35. On the newer one in a vision the external harness loops down before going in, I pulled the loom back and it was full of wet mud cleaned it out and separated the wires and the problem went away!
My Allison dealer says the harness is bad and they also found a solenoid bad. Harness in stock solenoid not. I'll keep this in mind though for future reference. My problem is I can't get in to read codes so if I blindly put on a harness and then there is another problem in there I am still taking it to the dealer and they are draining (and discarding) that high priced oil. lol Like My grandma used to say, I guess it will all come out in the wash.