Still searching but... Just took our new to us 1999 U320 out for a maiden trip. Everything going great except about the last 10 minutes of our trip we had a weird issue with the transmission. About 260 miles into a 280-mile trip everything was going great. Mostly all highway driving. When we got into town, at two traffic lights, when we went to start out it sounded like it was in Neutral and would not go. In fact, it started coasting backward at one. I quickly put in Neutral and then back in Drive and it went fine. Since I was trying to get going and out of traffic I can't tell you exactly what the shifter read. No codes showed up on Silverleaf that I can remember. Any gut reactions as to what happened.
The first thing to do would be check trans oil level using the shift pad
Read the DIAGNOSTIC CODES from the Allison shift pad.
Ignition on, engine off:
Press up and down arrows at the same time.
Press them again.
That will bring up d-1 and any stored codes.
If there is a d-1, press "mode" to bring up d-2.
Repeat until you get a d- "some number" and - -.
Look up the codes in your Allison owners manual or on Allison's website.
Let us know what you find.
D1 35 00
D2 - -
Allison transmission codes:
35 00 POWER INTERRUPTION
Check all electrical connectors to ECU and on transmission
Could bad coach batteries or ones that are in their death throes cause the issue?
In my opinion, low voltage could cause issues.
If the voltage on the coach batteries is ok (and relatively stable) with engine batteries being charged by the alternator while the engine is running, then less likely so.
At idle at a stop light the alternator is putting out lower voltage/amps. Check the voltage at start batterie with engine off, and again at engine idle.
I would check voltage when this occurs AT THE BREAKER panel in front of the passenger's seat. Identify the two feeds to the Allison ECU and check both for proper (i.e. over 12 VDC) voltage.
Will do. I know the batteries are going. I have to use Boost to start and Silverleaf was showing dropping below 12 on the trip. It would fluctuate between say 11.7 and 12.2 on the trip. Before the trip, when Roger and I were running through the Silverleaf screen, I believe it got down to 10.6 with engine off, ignition on.
Low and/or erratic voltage can damage electronics.
Fully charge the batteries and have them load tested.
BUT, unless totally shot, the alternator should have been able to maintain them at 13+VDC while driving. So, you may have more than one problem. But start by verifying that you have good batteries and that the connections are clean and tight AND that the cables themselves are still in good condition (ends clean shiny copper, not green copper oxide).
I had a similar transmission event. I thought it was weak batteries due to the power interuption code. When I removed the batteries I found light corrosion. I cleaned all the battery connecitons and found the ground to the engine block wasn't tight. Starting problems dissapeared and and no additional transmission events. Glad I checked everything before buying 3 new start batteries.
Coastal, In case you don't know where the Transmission ECU is located, mine is behind a carpet covered panel at the drivers left foot, The panel is held in place with screws buried in the carpet. There are 2 large wire bundle connectors on the ECU. Our coaches are different years, but suspect it is similar on your coach. Sometimes unplugging and replugging helps.