tow mode & o/d lockout---both all the time

[porthole]

Someone tell me about using the cruise control while towing. Never heard that before ... newbie

I use it all the time, would not buy a vehicle without it . Even my motorcycle has cruise control.

That said, with the GM's, the Dmax-Allison combo use the cruise to its best advantage.

Grade braking is activated with the cruise, the truck will attempt to maintain the speed going up as well as going down a hill.

Although, the program tends to downshift the trans to lower gears if the speed picks up going down a hill. This tends to be unnerving to some folks as the engine speed may approach 3500 rpm - and it makes a lot of noise. But that is the program attempting to keep the vehicle speed at the cruise set point.

The ECM is designed to not hurt the drive train.

It needs to be in Tow Haul mode as well to receive maximum benefit.

In T/H not only are the shift and downshift speeds and line pressure raised but the torque converter is locked in the lower gears. This allows the massive amounts of torque available in the Dmax/Allison combo (massive torque not available in SOBs) to be used most efficiently.

Torque converter lock up also makes best use of engine braking as there is no slippage between the engine crankshaft and trans input shaft.

I have found though, that when in T/H mode and cruising at a speed under 65 that the trans will not shift into 6 th gear. If I am at that bubble, I will momentary turn off T/H to allow the trans to up shift and then switch it back on.

 

[lwmcguir]

Just to clarify this a bit, the Torque Converter locks no matter whether you are in TH or not. The computer tried to lock 5 times and if it fails you will get an Tow Haul error in the message center. It will lock when the rpm is stable. This is usually due to a bad/leaking solenoid valve that is fairly easily replaced. One nice feature of the TL transmissions is the ease of changing the transmission filter. You don't want to neglect this.

 

[porthole]

And to further clarify, the OP has a 2005 GM, my comments were for the 2006 and newer with "cruise braking" feature.

As to TC lockup, with the Allison in "normal" mode the TCM will lock the converter in higher gears and unlock with your foot off the throttle. This yields the smoothest shifting.
In T/H the converter is locked in 2-3-4-5-6 and remains locked when you take your foot off the accelerator. The converter will also stay locked until vehicle speed drops significantly. This aids in engine braking.

One of the main advantages to using T/H when towing (Allison) is that with the converter locked up the trans will run cooler, as there is no "power loss" through the "fluid coupling", essentially direct drive (e.g. manual transmission)

Here is a good Q&A page for the Allison

http://www.allisontransmission.com/service/faq/index.jsp?CategoryID=11#239