There’s one transmission code that has flashed up many times in our fleet with the Allison 3000 series transmissions…code #3500 which is a battery voltage interruption to the transmission ECU. What happens is the push button shift control does the fish eye thing and you lose shifting ability. The first place to go is (of course) the batteries where the negative and positive feed (#10 or #12 gauge wiring) to the Trans ECU gets direct power from.
It’s a hard wired connection that splits into smaller wires when it reaches the dash. If you don’t have a ProLink or laptop you can turn on the ignition to run position and press the two arrows on the shift key pad at the same time and the code numbers will flash on the LED screen. You will see in this case a 35 then a 00. To check for more codes you hit “mode” and if more than one code exists it will indicate the second code # and so on.
We recently had a bus have a “check trans” light on the dash and no speedometer. This engine was a Cummins ISC it also has it’s power feed run directly from the batteries. There are several fuses at the battery which is splitting up the positive feed wire to the engine ECU so they have to be checked out on a regular basis.
The thing to watch with a “no speedo” condition is it gets it’s signal from the transmission output speed sensor. which goes through the transmission ECU and then goes through the interface module (VIM) to the engine ECU. 99% of the time it’s a wiring problem starting at the batteries which makes sense being subjected to the environment.
Thankfully the problem was a broken wire at one of the engine ECU fuse holder wires at the batteries. Checking the source to any problem has saved our shop a lot of time. We have experienced getting off track and going on wild goose chases beyond where the actual problem is located. So I always say to myself “check the simple things first”