2019 T880 Allison Output speed sensor problem, going out of gear.

I have a T880 roll off with a 4500RDS. Two trucks work perfectly. The third (and newest) 2019 T880 has developed a problem where the transmission goes out of gear very sporadically. The error it gives me is an output speed sensor fault. That's easy enough. We replaced the speed sensor. The truck went 24 hours afterwards with no issue, and then we still had the same problem. Still no problem - we checked the wiring all the way up to the transmission control module. The wires were slightly tight so we adjusted them to give them more slack. Still have the same problem.

Here's what happens: While driving, a gear icon shows in the dash and the allison gear selector shows the selected gear on the right, and nothing on the left leaving the truck essentially in neutral (but it does not say "N"). The driver pulls over and turns the truck off, waits for a minute and turns the truck back on. Then we're back in business. The truck could work for another few hours, or even a day before the problem comes back, or, it could happen 8 times in a row within a few minutes. Sometimes the driver thinks he hits a bump and then it happens, or sometimes it's while hard braking, or sometimes it's while gently accelerating, or sometimes it's just while moving. It happens on the highway, or on city streets. One day, we were able to make it repeatable by hard braking. We checked the trans fluid and filled it, then it didn't seem to do it again. There doesn't seem to be any rhyme or reason to it. So far, it looks like it might only be after 4 hours of driving. One day it will do it once, and the next day it will do it 8 times. It's usually towards the end of the day, but not always. So I need help. If it's not the sensor, and it's not the wiring, and it's not the fluid level, then what?

Here might be another factor: the PTO has a leak (and has since been replaced) and we were having to add about 2 quarts of transmission fluid daily until it was fixed. The trans-temp runs in the normal range - usually about 175 degrees unless we have a heavy load, then never higher that 225. The truck has 35000 miles on it and the transfluid level is ok with 79% oil life left.

I spoke with the local Allision warranty shop who does not take appointments and he said to leave the truck with him for 4 days before they can look at it. I can't be without a truck for that long, so I am trying to do everything first.

If anyone has any ideas, I'm all ears. Thank you in advance for any suggestion.

 

It's actually P0720 - and thank you for replying.