My Allison 3000 tranny had been dropping out of Drive while driving. I checked the diagnostics and I got a code of 3500 which means the electrical connection to the transmission computer is loose. After having the local Allison service shop run their extensive diagnostics on the tranny to find nothing wrong, the tech told me he hears a relay or circuit breaker trip repeatedly when the ignition key is turned to on.
We checked it out and it was the circuit breaker on the bulkhead under the bed that controls the dash AC condensor fan. Sure enough the fan motor is locked up, smells like it is burnt and it's supply wires are frayed.
I have removed the fan - 4 rusted screws - and now will call FOT to see what to replace it with.
The tech told me that the transmission likes a good connection + and ground and it can drop of drive with a relatively small voltage drop. It appears when the breaker popped, that was more than enough.
Bottom line, its always fun when the problem symptoms have really nothing to do with the cause.
Sounds like you found a good (conscientious and aware) Allison tech, too! Feel free to share the shop and tech info with the forum :)
I remember that happening to me many years ago. I got to the Allison shop and they diagnosed it for 7 hours and traced it to the main ground by the engine. They brought me back to show it to me wiggling the loose ground wire and I had lights blinking and all kinds of fun things going on. No doubt that was it. $700. to find it :(
Seemed like all the techs there were very knowledgeable, in fact, over the phone when I first called them, they said not worth trying to find the loose wire, just run another one directly from the tranny to the battery (I suggested using the lugs near the battery) --- but when they found the breaker popping, they said don't bother running the wires until find out what is causing it to pop.
The shop is Trans Diesel of Central Florida. I went to the Lakeland location. Trans Diesel of Central Florida - Detroit Diesel Engines and Diesel Engine (http://www.transdiesel.net/locations.php)
That is a known issue on our coaches. I turned on my dash ac and it stalled the engine. They put the ecm and condenser fan on the same breaker.
Hopefully its now known by a few more so they can save some $s.