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Here's my take. On the 03 if you hit the tow haul button for 1 second or quickly you will engage towhaul. If you hit the button for 5 seconds you will lock out overdrive. You can use towhaul with or with out the overdrive lockout funtion. You can use OD lockout with or wiht out T-haul.


Gm reccomends tow haul anytime you are towing 75% of the total load rating or more. Tow haul does change the shift points and locks up the converter. It also lowers the point at which grade breaking begins. Meaning that normal downshifting would almost be unoticable, however with towhaul engaged it really becomes apparent that your truck is down shifting and staying in lower gears when using the breaks.


As far as the Overdrive lock out is concerned. This is my take. Under normal conditions your vehicle from a stand still will upshift through all 5 gears. 1,2,3,4,OD. Down shifting your transmission may or may not go OD,4,3,2,1,. If you pull the gear select lever down from D to 3 you miss 4rth. So you never get to use 4rth. gear. That is why it is so obtrusive to shift from D to 3 at highway speeds. You are actually downshifting 2 gears.


Real life tranlation. If you are towing with the gear selector in D and your engine is searching for gears your best option is to hold the T-haul button for 5 seconds and lock out overdrive. You will then be using 4rth, gear as your highest gear.


It is also a good idea to use this same logic coming down steep grades. Just lock out your OD and you will avoid a runaway condition. If you use OD lock out in combination with towhaul and your 4 wheel anti lock, huge 13" disc brakes, I think you find this as the next best thing to an exaust brake.Edited by: Bronco
 
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