TOW/HAUL & GRADE-BRAKING
I have a 2005 2500HD GMC with an unmodified Duramax/allison. I use this truck to tow an 11,000-pound fifth-wheel travel trailer, and have been concerned a few times when using the Allison’s Tow/haul mode to descend some of the steeper grades we’ve traveled. Tow/haul doesn’t appear to work as well as it should, and the truck will continue to build speed unless I also use the truck’s brakes. However, using the brakes to control speed seems to cause the transmission to downshift to fourth and then to third gear. On the steeper grades, the engine speed will reach the red zone on the tachometer, and I’m afraid to use the truck’s brakes because it might force another downshift into second gear. I’m concerned about damaging the engine.
I talked to a tech at a well-known aftermarket diesel performance company, who said the Allison Tow/haul mode wasn’t designed to control downhill speed while towing. I told him I believed that controlling downhill speed is exactly what it was designed for. So, my questions to you are, who’s right about the Tow/haul mode function? Is there something wrong with my truck? What can I do to control downhill speeds? Are the newer trucks better at controlling downhill speeds? Wayne Vessoff Via the Internet Great set of questions. Let’s begin by describing how the system should operate and what Tow/haul mode was actually designed to do.
First, you are correct, in that T/H should help control vehicle speeds when descending grades with a heavy trailer in tow. However, engine grade-braking alone might not be enough, depending on the GCVW of your truck/trailer combination and the steepness of the grade. The grade-braking provided by Tow/haul was originally designed to “assist” the truck/trailer service brakes while descending grades, not eliminate the need for the truck/trailer brakes to help control downhill speed.
T/H will/should operate as you indicated, in that it will produce a downshift (to make the T/H gradebraking more effective) if the brake pedal is pressed and the vehicle doesn’t slow within a preset time interval or if the speed continues to increase. Lightly riding the brakes can inadvertently produce a downshift because the computer is sensing a brake pedal activation, but the vehicle’s speed isn’t slowing. So don’t ride the brakes; use them to slow the truck/trailer, and then get off the pedal.
The tachometer in the 2001-2005 trucks actually has two redlines. While under power, your tach’s redline begins at 3,200 (the beginning of the dashed redline). While in grade-braking, the red-line begins at 4,800 (the solid red portion of the redline).
It’s interesting to note that GM did away with the tachometer redlines entirely on the 2006 and newer trucks. I suspect they did this to avoid owner confusion and because a redline on the tachometer really isn’t necessary anyway. With an automatic transmission, the factory powertrain programming won’t allow the engine to exceed the engine speed limits discussed here—unless you manually select a lower gear.
As an interesting sidebar, I met up with the drivers of GM Powertrain’s “Power Tour” truck/trailer in July of 2000 at the top of Pipestone Pass on I-90 near Butte, Montana. This combination of prototype truck and trailer was loaded to the maximum GCVW rating, and traveled all across America and Canada to demonstrate to the public what GM’S new truck and powertrain could do. During a descent at max GCVW on this 6% grade, the engine speed rose slightly above 4,500 rpm, but the truck’s brakes were not needed to control speed.
Current trucks employ an intake air control valve and a more efficient Variable Nozzle Turbocharger, which combine to increase grade-braking efficiency. In addition, a number of Diesel World advertisers offer exhaust brakes that can provide an equivalent of up to about 200 horsepower in engine braking performance. Check out those offered by Banks and BD.