The trans-temp runs in the normal range - usually about 175 degrees unless we have a heavy load, then never higher that 225. 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. If it's not the sensor, and it's not the wiring, and it's not the fluid level, then what? It's usually towards the end of the day, but not always. One day it will do it once, and the next day it will do it 8 times. So far, it looks like it might only be after 4 hours of driving.

There doesn't seem to be any rhyme or reason to it. We checked the trans fluid and filled it, then it didn't seem to do it again. One day, we were able to make it repeatable by hard braking. It happens on the highway, or on city streets. 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. 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. The driver pulls over and turns the truck off, waits for a minute and turns the truck back on. 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 wires were slightly tight so we adjusted them to give them more slack. Still no problem - we checked the wiring all the way up to the transmission control module. The truck went 24 hours afterwards with no issue, and then we still had the same problem. The error it gives me is an output speed sensor fault. The third (and newest) 2019 T880 has developed a problem where the transmission goes out of gear very sporadically.

Allison transmission codes p0726