Towing Question

John R · May 12, 2003

Can anyone tell me if you can pull A 5th wheel

trailer in OD?

Also does the tow/haul switch lock out OD?

The truck is A 2003 2500HD with D/A combo.

Thanks

HillHound · May 12, 2003

Alot of it depends on the weight of your 5th wheel. How much does it weigh? The tow/haul program does alot to decrease excessive shifting in and out of OD and therefore overheating the transmission as well.

If the tow/haul for the Allison does the same thing as the other transmissions, it doesn't lock out OD. It just makes the engine rev higher before upshifting. On the '01 1500 I had you could still get it into OD with the tow/haul engaged but the engine revved several hundred rpm higher before it shifted into OD.

I think the Allison is the same way. Someone will know for sure!

BlackSilver · May 12, 2003

Yes, you can pull a 5th wheel in OD with the Duramax/Allison combo. (In fact, that's why I have one!)

The tow/haul mode is there specifically for that reason. It delays the shift points, and also increases presure so that the shifting occurs are higher RPMs and is crisper.

Putting the tranny into "tow/haul" does not lock out OD, although the Allison has that option (press the tow/haul switch a second time and hold it) if you are towing extreme loads, or you find the tranny "hunting" on exceptionally steep grades. So far, I've never had to do that, even in the western mountains.

hosspuller · May 12, 2003

This write-up might help you understand the Allison better.

Tow/haul

The 1000 Series features a 12-inch torque converter equipped with an electronically controlled, hydraulically actuated, converter clutch. The (TCM) Transmission Control Module selects the gear for each drive range and engages the torque converter lockup. This is important: the TCM controls BOTH gear selection and torque converter lockup. It is NOT controlled directly by the column gear selector lever or the Tow/Haul mode button. The reason this is important is because you are not controlling single variables when you make input selections with these controls. You are changing several things at the same time. So let's look at what these inputs cause the TCM to do.

The gear selection (D, 3, 2, 1) input changes the shift point and the high gear as anyone familiar with an automatic transmission should be familiar, but it is further affected by the mode. The 1000 Series employs two modes of operation for GMC and Chevy trucks: Normal Mode When the truck is being used for normal day-to-day driving, and Tow/Haul Mode. Tow/haul mode is available for when the truck is loaded (a button on the end of the gearshift lever activates it).

The two modes are differentiated by three primary functions:

1. Torque Converter Clutch lockup

2. Grade Braking

3. Shift Stabilization

Although the second and third, Grade Braking and Shift Stabilization, interact with gear selection, these three functions comprise the difference between normal and Tow/Haul mode and so should not be confused with the gear selection alone. Let's look at the three functions:

TCC

In normal mode, the TCC (torque converter clutch) is locked up in 4th and 5th gear. This provides for smoother up and downshifting when unloaded or with light loads, provides better fuel economy, and still provides for effective lockup under the higher loads of the upper gears (4 and 5). This is the best mode when the truck is at less than 75% of the GCWR.

In the Tow/Haul mode, the TCC is locked up in 2nd, 3rd, 4th, and 5th gears. Allison did this to avoid heat build up generated by a slipping TCC under heavy loads. This can occur when the vehicle and trailer are over 75% of the GCWR or under certain conditions such as frequent stop and go or steep hills.

Engine Grade Braking

Engine Grade-braking helps the driver slow the vehicle and avoid excessive use of the brakes. For example, when drivers carrying heavy loads encounter a steep hill, they can manually shift into a lower gear for added engine braking to slow the truck. The 1000 Series collects operating data, including how long the driver's foot has been on the brake, vehicle speed, gear currently engaged, etc., and uses it to calculate shift parameters. At a certain threshold, the TCM of the 1000 Series will cause the transmission to downshift without the driver having to move the gear lever. Manual range selection is still available to the driver. Again, in the Normal mode, it will shift down from 5th to 4th. In the Tow / Haul mode, it will perform multiple downshifts from 5th to 4th, then from 4th to 3rd, using steadily lower gears to slow the truck.

Shift Stabilization

Shift Stabilization uses the same type of advanced logic with input from the throttle position sensor, engine speed, and other factors. This feature detects and overcomes "shift busyness" that might be found in mountainous or hilly terrain. Algorithms housed in the TCM analyze the conditions, and when the parameters are satisfied it maintains the transmission in the lower gear for optimal performance. In Tow/Haul mode. the stabilizing effect this has makes driving a loaded vehicle considerably more comfortable.

As you can see, with the Tow/Haul mode, you are doing much more than telling the TCM to select a specific gear. The mode affects TCC lockup, the Engine Grade Braking program, and Shift Stabilization.

John R · May 12, 2003

Thanks for the info, I can go ahead and pull my 5th

wheel in normal drive with the tow/haul turned on?

BTW the weight of my trailer is at 10,500 dry.

Colorado Kid · May 13, 2003

Thanks for the info, I can go ahead and pull my 5th
wheel in normal drive with the tow/haul turned on?

BTW the weight of my trailer is at 10,500 dry.

Yup!

By the way, if you've ever towed that 5th wheel with any other truck you're really going to be impressed with how the D-max 2500HD handles it. :thumb: