I recently purchased a used -- 40,000 miles -- '04 2500HD CrewCab LT Duramax in May and just completed my first real trip towing our 6,500 lb. boat and trailer from Denver to Lake Powell in Utah.

It was a fantastic trip. Used to pull with a 6.5L turbodiesel. The Duramax is a whole new world of comfort and power.

I did have one question about the tow-haul mode operation. After assending the first major pass west of Denver through Eisenhower Tunnel on I-70 with ease, on the downside, when I would tap the brake to control speed, the engine would downshift and the rpms would jump well into the red zone. This would require my getting on brakes fairly hard to back the rpms down to an acceptable rpm of 3200 or less.

I thought the tow-haul mode was suppose to reduce brake use. What I experienced was an increase in brake use. I stopped using it altogether on downhill slopes.

Am I doing something wrong?

Welcome...
In answer your question about tow haul mode and the Grade braking feature of the Allison transmission, The tow haul mode will move the shifting points to the rpm range that is best suited for that purpose. In other words, it electronicly moves the shifting to the rpm range that closely matches the torque and horsepower required to pull the load. The grade braking feature will force a downshift to slow the vehicle down without using the brakes. Sometimes this will cuase rather high rpms, but it is ok, the 6.6 is balanced and rated for 5200 rpm. I have never dealt with the high rpm, but others say its ok. I know others out there can explain better, but I hope this answers your question.

Bill

The dashed red area of the tach is where grade braking wil operate at. the solid red area is at 4800 rpms. The truck will up shift if it gets to that point.

The grade braking feature work in T/H or regular mode. It is active all the time.

Let the truck do it's job and you'll be fine...

Too hard a tap on the brake. It went clear down to third gear to put it in the red zone. Most interstate hills on I-70 don't require a down shift just a little braking will do it. This is something you need to practice. Later! Frank

I know I when I drive through the mountains and I start gaining speed on the downhill side, I will tap my brakes slightly until the transmission downshifts and then just coast down. Sometimes if the hill is really steep and I am still gaining speed, I will brake a little more until it downshifts again. I have had mine up near the solid red line several times but never into it. It does upshift when it gets there. You basically have 2 redlines. The first is when you are accelerating, your redline is 3200 rpm, which is the start of the dashed red line. The second is the redline for grade braking, the solid red line. So as Dave stated earlier, let the truck do its job. Don't try to brake more to get the rpm's down under 3200 because you may not be able to. The more you brake the more the transmission downshifts. :)

Jeff

The tow/haul is not designed to do all the braking. Later! Frank

What makes the grade braking so scary, is if the fan has or is kicking in.

Without the fan, the dmax just PUUURRRRRSSS........

My .02

In the tow/haul mode, you have no choice but to let it do the braking. As soon as you touch the brakes it downshifts, if you continue to hold the brakes it downshifts again, until it can't downshift any more because of either redline or if in 1st gear. So, instead of holding my brakes all the way down the hill, I hold them until the tranny shifts to a gear to maintain the speed I need.

Jeff

One other point. The computer will not allow the engine to generate power in the dashed red zone, i.e., you won't have fuel to the injectors. Of course if you jimmied the rpm limiter this is not true. So when your in the dashed red zone, all you can do is coast. That is the way it was designed.

Jim

You do have a choice Jeff by how hard you get on your brakes. If your trailer brakes are adjusted properly you can go lightly on the peddle and slow the truck without going down too soon to the next gear. I have 18K miles towing my 10K Wildcat all over the Rockies. I have found very very few interstate hill that even need to go to third gear. Now driving down steep winding secondary roads is another story. I have seen red on several occasions and was happy to. Later! Frank

Right now I have the brakes on my trailer set so they come on just a little bit stronger then the trucks as I am applying them. I can just feel the trailer start to slow me down a second sooner then the truck. There are 2 hills between Sacramento and Reno that I have no choice but to let the tranny downshift to 3rd. Most of the other hils it will downshift to 4th, if I am not already in that gear, and it does fine. My statement about is referring to the longer/steeper hills where you have no choice. If they are that steep, I am braking a good part of the way down and the tranny is in 3rd or 2nd if I am in the 40mph range. I have only had the tranny downshift to 2nd in the mountains up in Canada a few times, nothing around here.

Jeff