After reading Brian's post and going over pages of Prevost maintenance manuals it became clear that the failure to shift was intentional on the part of the Allison control unit. There are a few conditions that cause it to avoid 5th and 6th gears, and one of those is temperature exceeding 260 degrees F.

Not wanting to stay in Flagstaff until Monday, I elected instead to drive to Las Vegas where there is factory Allison service available for the B500R. The trip to Vegas was quite pleasant and shifting was normal for the entire route, including crossing the Hoover Dam. Had the cruise control set on 65 most of the time. The amount of speed this beast can pickup on a downhill stretch is astonishing and it's now clear that I need a retarder that's available all the time, that can hold the coach to 65 or less on a 6% grade. Maybe I'm dreaming...

I had to use the retarder on several occasions and each time I kept a watchful eye on the trans temp gauge, keeping it below the 230 mark at all times. After reading in the manual that there was a specific 260 degree threshold, I found it somewhat curious that the trans temp gauge had graduations at 200, 230, and 320 degrees - clearly a non-linear presentation. This means that if the needle ever passes 230 that it's entering into "logarithmic range" (just kidding), or in other words, very hot. I didn't let the needle get much past the midpoint between 200 and 230, even though there were times I really wanted more retardation. I stabbed the service brakes instead, which is recommended over riding them. The retarder really digs in when you hit the brakes, in three increments according to brake pressure, as I recall. This makes for a very rough downshift at lower speeds and when driving around town at in-town speeds, it seems better not to retard so that downshifts are smoother.

So, tomorrow I'll be pulling into W.W. Williams to find out what they can do for me. At a minimum I think I'll have them do a full diagnostic, change the oil and filters, and perhaps upgrade to synthetic oil. Depending on the results of these tests, I'll either hit the road and hopefully make it to Glacier National Park by this weekend, or, bust.

Our spirits are still high and we're growing accustomed to our new living quarters aboard the bus. It's very nice, overall. We're in the Oasis Las Vegas RV Park and it's got everything we need. Oh, and the rear CruiseAir konked out with HI PS again, two times. These things are simply not reliable when the temp is over 100.