The following has been copied from a forum, where the discussion kind of ran the gauntlet of topics about towing. It is a good explanation of the way that the Tow/Haul feature of a transmission works. In his case, Rodeworthy specified that his description applied to the Duramax engine and the Allison transmission.
I present this here for the enlightenment for those who may not understand Tow/Haul and its benefits. With some drivers, they still will pull a lot without using the Tow/Haul feature, but they usually monitor the transmission temperature. I would guess that as that temperature goes up, they will engage their Tow/Haul mode.
All within quotation marks are the comments of those other than me.
“In the days when I dealt with such things torque converter lockup meant a sliding splined gear slipped over the torque converter output shaft to physically "lock" the output shaft to the input shaft thus removing any possibility of torque converter slippage affecting the load -- it, (the torque converter) is effectively by-passed or "locked out".
Tow-Haul mode with a Duramax/Allison does not lock up the torque converter as described above but the torque converter clutch (TCC) provides similar function and behaves differently in the two operating modes -- Normal or Tow-Haul.
In Tow-Haul Mode:
During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking.
During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.
Tow-Haul shift mode significantly changes the transmission's shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads.
Upshift points are raised at light to mid throttle position to use more of the engine's available power for accelerating. Downshift points are raised to enhance engine braking to help slow the vehicle.
Like others I routinely tow with Tow-Haul mode off. I will deploy it when I want extra control on hills and mountains or in traffic with frequent speed changes. There are times the transmission will not shift into sixth gear in Tow-Haul mode on but it always will in Normal mode. At my cruising speed (59 MPH) in sixth gear my engine is at its peak torque curve at 1600 RPM. I believe that is my optimum fuel efficiency point too.
I also use Tow-Haul mode under the same conditions when I am not towing to achieve the same control level on hills and in traffic.
If the transmission is used to retard speeds on a long grade it is most important to monitor the transmission temperature.
Then, there was some comments from Wingnut60 in response to some comments and some questions I had made, specifically this:
I have to chime in and also thank you for such a good explanation. BUT, I also have a question. Would your explanation also be similar with the Ford transmissions with the Tow/Haul feature?
The reason I ask is because I had the impression that ANYTIME one was towing their RV that they should be in Tow/Haul. But, from your explanation, it sounds that it might even be more economical on fuel if one is NOT using Tow/Haul in flatter country. Could I be assuming right, or am I way off base?
Now, Wingnut60’s comment:
Since I know you have a 450, as I have, I can pass on from experience that you can use T/H or not AT ANY TIME. These trucks/transmissions are very 'smart' and for the most part, will do what needs to be done at the right time.
The 6.4 will tow my 17000+ MS in OD just as well as it will with the T/H engaged, but I don't use it much unless in mountains that require some extra help in braking. Use of the T/H will often delay the final upshift to OD and you will find, as RW and Huck said, running above 2000 rpm at 60 mph (4th/D) when the truck will easily pull in OD to save fuel.
Often, if you are pulling into headwinds, or a grade that hasn't caused the truck to downshift, you will hear the fan clutch kick in to cool the engine that wouldn't have occurred if in T/H--that is one situation that T/H will help over not using it.
Also, be aware that you CAN'T force the TorqueShift into D (4th) with the shift lever--it will always skip 4th and go to 3rd--the only way to access 4th is to let the computer find it for you when needed--so this is another area where T/H may help--it will keep 4th engaged when needed and you can't get to it any other way. This is the only irritating design I have had a problem with in my 450--they should have given us a way to hold 4th manually if needed. As mentioned above, if I encounter a long grade in OD, the truck will gradually lose speed--use of T/H will sense this and downshift; if you try to use the shift lever, it will downshift to 3rd and cause a higher jump in RPMs than necessary.
Another TorqueShift peculiarity--if you put the shift lever in 2 or 3, it will START in 2 or 3 and stay there; there is no way to start in 1 unless you have it in 1--not usually a circumstance that causes a problem, but something to be aware of.
It's a great truck and the TorqueShift works hard and long, just need to be aware how it shifts if you are using the options available.
By the way, went to AZ in Feb--all the way into 30+ headwinds, got an outstanding 6.0 mpg average for the westward trip. That was painful, and without the extra tank, would have been troublesome looking for fuel.