I've got a new 2012 Silverado 2500HD (Duramax/Allison) and was wandering if there is a speed limitation as to when you can change the shift position from D to M?

The reason is I want to take advantage of this in the mountains out west, and don't want to have to stop first if I don't have to, but unsure the characteristics of how it handles being changed while in motion.

Thanks,
Roger

 

Welcome to the forum. No, you can switch back and forth at any time. The TCM is not going to allow you to do something illogical. They took the powertrain control away from you years ago. All you've got left is some influence over gear ranges as long as the ECM and TCM concur.

 

Thanks... It looks like a great feature for descending the rockies! After 3 years of tranny issues in my F250, just wanted to know what to expect. The computer in my F250 would kick out the transmission under heavy load when descending sometimes and that was a very unnerving experience to suddenly find yourself freewheeling (in effect coasting) downhill at nearly 17,000 lbs GCVW.

Roger

 

Try the Tow/Haul mode plus Grade Braking as well to see if it works better than the M mode. See the owner manual.

 

It is starting to make more sense... I got out the highlighter and started marking up the points of interest between the different options, and which ones work together vs the ones that don't work together.

I can see the advantage of tow/haul mode, but on the steep descents of the rocky mtns it seems like M_anual mode could be the better option to prevent downhill momentum from allowing an unwanted upshift and acceleration.

I'm really looking forward to the first climb/descent to see how it all works!! It has to be way better than what I'm used to in the mtns!

Roger

 

Mine is an 08 LMM so I do not have the turbo vane assist that you have, but even without, in tow/hall mode when coming down steep grades just give the brake a firm push and the truck will take over keeping the speed down to include shifting down a gear. I use the M for keeping in fourth going up steep grades. Give it a try, you will be supprised how controlable your load is with the allison.

 

Shhhh getz you a 18 spee Fuller, jam dem gearz

but dat allison will control ya on dem hills if you know how to engine brake