i had this question on my mind for a while and it is whats the difference between t/h and o/d i know tow haul changes the shift pattern but what does o/d do isn't it the same thing? thanks in advance

o/d is 5th gear.

From 03 on gm put a button in to allow taking it out of o/d manually.

Before that we had to install an aftermarket switch to do it. It was worth the effort.

You are right t/h only adjusts the shift points.

The Tow Haul and the Overdrive Lockout are two completely independent functions that happen to share a switch on the shift lever. I think things would have been clearer from a user interface standpoint if GM had used two separate switches.

The operation of each may make more sense if you can think in terms of two switches, one controling the T/H function (pushing the button and releasing immediately), and the other controling the O/D Lockout function (pushing the button and holding for a couple of seconds).

As you mentioned, the primary purpose of the T/H function is to change the shift points. It does not impact locking out 5th gear. Locking out 5th gear with the O/D Lockout does not impact the shift point setting slected with the T/H function button. (Turning tow haul on or off also impacts the grade braking feature, and when the torque converter locks up.)

The purpose of the O/D lockout button is to lock out 5th gear (which is overdrive). In reality, it is a way to do a down shift from 5th to 4th gear. The gear shift lever is modeled after those on the non Allison transmissions that only have four speeds. So, if you are in 5th gear (D) and try to down shift the Allison with the gear shift lever (3), you will go from 5th to 3rd. GM could have come up with a separate dash display and lever pattern for the Allison equiped trucks that had D4321 rather than D321, but that would mean more parts, more assembly line variation, and more expense. Instead, GM got around this problem by adding the O/D lockout button. This was GM's symbolic way of adding the "4" to the dash display. Pushing in the button locks out 5th gear, which is the same as downshifting to 4th gear. This works whether the tow haul feature is turned on or off.