Allison transmission

I drove a manual transmission for many years and got used to shifting to neutral as I coasted up to a stop light, etc. Out of habit, I started doing the same thing with automatic transmission cars and Suburbans I’ve had, and I didn’t have any problem. However, I now have a 2004 GMC Sierra 2500HD with the 6.6L turbodiesel and 5 speed automatic Allison transmission. One time, while only moving less then 5 mph, I shifted into neutral as I was coming up to a stop light. When I did so, I heard a big “clunk.” My question is, is there something different about regular automobile or light-duty truck transmissions that would allow this on-the-move shift that the heavy-duty Allison does not? Or, do I have a problem with my truck’s transmission?

This is just a stab in the dark, but modern truck transmissions are very good about downshifting early to use engine braking in order to assist your brakes. If you were rolling along with your foot off of the gas and the transmission in a low gear (and the engine providing maximum braking), putting it in neutral would unload the drivetrain (transmission, driveshaft, differential) and that could well account for your clunk.

To add to this: knock off the shift in to neutral business. In an automatic transmission, it’s doing nothing but causing wear.

Agreed. It’s a bad habit on an auto transmission and will do nothing but cause you problems.

That clunking was most likely in the drivetrain. Leave the gearshift lever alone until you are ready to park the vehicle. Let the Allison do its thing, they are not cheap to rebuild ($1,400 for the overhaul kit alone) My average ticket on an Allison is over $4,000 depending on what they tear up.


I don’t recommend doing that. Don’t start jerking your pump around. The clutches will wear prematurely. You could put the shifter in reverse and I know that wouldn’t help.