Just bought an LMM truck last week and very pleased with the purchase. Took it out and towed our 25' travel trailer (7,000 lbs) over the Cascade mountains and it did great...no surprise. My question is how to best descend steep grades at 50+ mph? I tend to keep it in Tow/Haul mode and use the integrated brake, even though trying to guage the right gain. Any insight on how to drive in these conditions would be greatly appreciatied. - jwc

As a truck driver I was always taught descend a steep grade at the speed the truck will hold you back, mind you rigs have a jake brake. IMO I would slow the truck down below 50 at the crest or just before and then let the truck hold you back as best it can and let god and gravity pick you back up to speed, cresting a steep hill at 50+ mph is looking for trouble if there is ever a problem halfway down the hill. It is a 1 in a 1000 chance but it is always there. It you weight it would probably be a bit better at braking than say 15000 lbs but do what feels comfortable to you.

Put it in T/H and let the Allison do it's thing.

At the top put it in M and drop it down manually to slow yourself, and don't worry the Ally won't let you drop it into to low of gear. If its too much of the truck to hold back use the brakes a little to get ya ~5 MPH under your desired speed and get off them until you get over that speed again.

Thanks for this feedback and I will put it into practice. I think I crested the hill at too high a speed and when coming down the engine revved to 3,000+ rpm while in T/H mode. Is using the Manual gears a better option over T/H mode....guess I'm still a little confused when to use these features.

Always use T/H "M" may help a little more I don't know for sure. Use what ever your comfortable with and don't worry about 3000+ RPMs going down hill.

I've got about 14k lbs. I slow below 50 before the crest and put it in Second. On the steep hills it will raise on the tach but, still in the dotted area of the tach.

I agree with the first post. . . slow the the speed the truck can hold back.

Your brakes should be available for emergency NOT for riding the whole way down the hill.

Going downhill fast may lead to bad times. My buddy has his brakes catch on fire. His brakes went out and just around the next corner there was a truck run-off. He got very lucky. His truck didn't stop until the last 15 feet of the emergency area. His 38x15.50 tires didn't dig into the soft dirt/gravel like semi tires would have. After that he went much slower down hills.

At the top put it in M and drop it down manually to slow yourself, and don't worry the Ally won't let you drop it into to low of gear. If its too much of the truck to hold back use the brakes a little to get ya ~5 MPH under your desired speed and get off them until you get over that speed again.
exactly! works for me

There is a small handbook for your allison that came with the truck. RPM's while under engine braking can go all the way up to the solid redline (higher than while accelerating) on your tachometer safely.

There is a small handbook for your allison that came with the truck. RPM's while under engine braking can go all the way up to the solid redline (higher than while accelerating) on your tachometer safely.
Maybe this is a dumb question but does this also apply to MANUAL trans. trucks?? If i'm descending a steep hill in 5 th gear should shift up to 6 th in order to stay out of the red zone on the tach??????
Dana

I have a dumb question also. My 2007 doesn't have a red zone marked on the Tach. What is the max RPM I should watch for? (or are they smart enough to not allow you to exceed the max RPM now)

From Allison

A. The TCM is programmed to command downshifts only when the resulting increase in engine speed will not exceed factory-set limits. Specifically:
For the LB7 and LLY diesel engines: Maximum engine speed immediately following a preselect or grade braking downshift can be as high as 4,000 RPM. After a grade braking downshift, if vehicle speed continues to increase, an upshift will occur at 4,800 RPM engine speed.

From here http://www.allisontransmission.com/service/faq/index.jsp?CategoryID=11#104. Good stuff:D .

exactly, dont go down the hill faster then your truck would go up it, and use your brakes "stabbing" 5mph elow speed let off till about 5 over speed, hten brake until 5 under speed.

You get to meet some of the nicest people when you go too fast towing down a hill. The wrecker driver, state trooper, driver of the vehicle in front because you can't stop in time. And they all want your name and address so they can stay in touch.

You mentioned the cascades. When I've been coming down (cabbage pass)in my 05 pulling doubles, I'll start out at 50 mph with T/H on in 4th and about half way down, I'm at 42 mph and still loosing speed, so I will give it a little gas and it will upshift and I let it go until about 53 mph then apply a little brake so that it down shifts again and holds.

I drive for a living too.. Take it easy people, get movin down those hills too fast and you'll end up in one of those bozo traps.
Smoked brakes, a tow bill and a speeding ticket sure arent a very good way to have fun.. Kinda funny to watch sometimes.

I leave it in tow haul and set the cruise. 75 up, 75 down and it downshifts to slow down on its own.

Then again I'm only towing 6000lbs worth of open car trailer and car.