This last weekend I was coming down a steep long hill. I was towing my 13,000 bumper pull toy hauler with my 03 Duramax Ally in tow/haul mode. When I started to break it down shifted as expected and when to 3,500 RPM. Then it shifted down again and went 4,200 RPM. I took it out of tow/haul and it upshifted and went to a comfortable 3,500 RPM. Can it spin 4,200 down a hill and not hurt anything. What RPM is trouble engine breaking ??

In the past 5,000 miles towing I never saw the engine breaking go over 3,500, but I never descended such a long steep hill in the past.

S c o t t

If i remember correctly the rev limiter for decents is 4500 and the limter upon acceleration is 3500 from factory. I have let mine rev up to 3500 on decents before with no issues, it made me nervous the first time it did it though, sounded like the engine was coming out from under the hood. If the truck gains speed at that RPM then i start using the brakes and manually down shift it for more control of the RPM's.

I have had the same thing happen to me, except I was hauling a 47' sundowner through WV The first downshift hit 3,500 , and the next pegged the tach right out. scared the S***t out of me. Truck still runs fine, but I have never heard of It happening to anyone else.

I just want to know what the RPM limit is on the LB7 and how long it will take that RPM engine breaking. I know the LBZ is different. I also want to know how long it can take the RPM. It seems with the LB7 goes into the engine break mode it doesn't have half the stopping power my 08 6.4 Ford had, but I can say the LB7 almost gets double the MPG so I am ok with it.

IIRC the LB7 is 4500 or 4800. The allison.com has that info.

The tach has two areas that tell you where it is safe. The dashed part is where it stops fueling, but it can run up to the solid part during grade braking. I'm pretty sure the solid part is at 4800, same as an LBZ.

Thanks for the info. and that makes since on the tach it does have the dash and solid line. That is scary to hear that diesel over 4000.

Another towing question. What RPM do you guys pull hills at and does the Duramax like to rev or does it have a sweet spot RPM wise pulling large hills ?? My 7.3 Fords if you hit 2500 it would pull any hill. I haven't found the right RPM in my dmax yet.

S c o t t

Mine seems to be about the same, 2500-2800 on hills, and cruisin down the highway she pulls great about 65 mph,just a hair under 2k that seems to be the sweet spot for me. If I break 70mph my mpg are reduced drastically .

Gm had many calls about this " my truck is tacking in the redline coming down hills" So in 2005 1/2 when the second gen. LLY came out, they raised the redling to 4850 RPM to keep people in control an sane when the grade breaking kicks on going down a hill.

IIRC its designed to spin 4800RPM with out fuel, and on the fuel its designed to spin 3250.

This is in your owner's manual. Under acceleration, your redline is 3100 RPM. Under grade braking, with tow/haul mode on, your truck can rev above 3100 to 4800 with no damage whatsoever.

This is in your owner's manual. Under acceleration, your redline is 3100 RPM. Under grade braking, with tow/haul mode on, your truck can rev above 3100 to 4800 with no damage whatsoever.

That is good to know. I also got nervous on my truck when it was grade breaking in the red around 3200 rpms. The question I have is, I downshift into 3rd on steep long grades, with the tow haul mode on, is this also OK, or should I leave it in drive and slow down to 55 and let the truck downshift? or does it matter?

That is good to know. I also got nervous on my truck when it was grade breaking in the red around 3200 rpms. The question I have is, I downshift into 3rd on steep long grades, with the tow haul mode on, is this also OK, or should I leave it in drive and slow down to 55 and let the truck downshift? or does it matter?
Technically, you can leave it in D and let the grade braking do its thing, but personally I'd downshift to third anyways if its a steep enough grade.

I am more comfortable downshifting it into 3rd, sometimes it takes the engine braking to long to kick it down to 3rd, or I have to do a lot of braking to get it to downshift, thanks.

I am more comfortable downshifting it into 3rd, sometimes it takes the engine braking to long to kick it down to 3rd, or I have to do a lot of braking to get it to downshift, thanks.


In tow/haul mode, if you cover the brake peddle so the brake light is on, and the truck continues to accelerate, the TCM will downshift. At least that's what mine does. If you brake and are decellerating, it won't downshift.

What I like to do is use the cruise grade braking. I set the cruise about 10 mph less than I want, and let it do its thing. If it starts to go too fast I manually grab a little trailer brake to help it slow down to a safe speed. It works like a charm. I have a long windy decent to my house and it holds me back pulling doubles at 26,000 gross.

The LMM has NO red, orange, or green, areas on the tachometer and the owner's manual says that the on-board computer will not allow the engine to rev to a danger level, even though it sounds terrible the first few times you hear it in tow/haul mode. GM obviously decided to remove the red line because it worried owners.

TPW, I will try the cruise control trick. I have been letting the TCM drop it to 3rd, then use the trailer brakes to hold it back. This way I don't see the 4,000 RPM decending hills.

read


the


owners manual


:)

ben