My 2001 duramx/allison 1 ton is doing some strang stuff.
When decelerating at the end of a stop, like going 1-2 mph, just about ready to stop. You feel this bump. It feels like the guy behind you just hit you. I think its the allison shifting into 1st. But its not a down shift.
It feels more like when the truck is in park and you put it in drive, of corse you are still rolling. It did it 3 times this weekend. The truck was empty.
This thing is stock. Is this normal?

Typical problem with MY01's, it's a delayed 2-1 downshift. There's a service calibration for the transmission control module that fixes it.

Is this something I have to go to the dealer and get or can I update the computer myself? A friend has the snap-on scanner.

You will need the dealership to do it with the Tech2.

I always thought it was the fuel sloshing. I've only noticed mine doing it when it's almost full. Of course, mine won't shift into reverse now, maybe it is the tranny.:mad:

The MY05 have had that fix for a while so it's probably not that.

There's an old trick with the Allison that will help you find if you have an electrical issue or hydraulic. Unplug the transmission control module (it's under a shroud just right of the engine fan- make sure the engine's off), start the vehicle up and shift to drive and reverse. With no TCM the trans should have 3rd and reverse only. If it doesn't move in one direction or another it likely has a fried clutch, a stuck valve, or an internal leak somewhere.

2001 Crew cab dually with an allison and having the same problem. Seems to happen when I am making a slow stop. Sudden stop and does not do it. When it happens the anti-lock seems to pulsating the brakes. The transmission seems to come completly out of gear for a second and then engages back in gear suddenly. If you try to take off too soon it jumps really hard.

Same Prob As Charlie - Dealer Replaced Tcm - All Is Good