We got a couple 6500 GMC's in a fleet we work on, and they all downshift very hard, like enough to about throw ya through the windsheild when coming to a stop. I'm no tranny expert, but they wanted us to check one of em out. There are no codes showing up, good on fluid ( I know Allison's seem to be temperamental with fluid level). There is no spin on filter on this model, that was my first thought. I assume there's a filter in the pan? Anyone run into this, seems like a common problem, since 3 of em are doin it.

 

I'd try dieselplace.com. They have some Alli gurus on there. On the Alli 1000's it's common to feel a "bump" when coming to a stop and the trans shifts back into 1st gear. It's more pronounced when in tow haul. My '02 2500 Chevy was pretty smooth, but alot of people complain that it's a harsh downshift. Might be normal???

 

Thanks, I'll give them a shot.
This one jerks real bad, and overrevs between shifts, mostly back into 1st, like you said. I've also heard when they have a problem they can possibly bring on an ABS lite, which it did.