Ok all, here is my 2 cents worth. First off note one thing, the over drive clutch in any auto trans is the smallest clutch pack in it. I would second the "hunting" comment, but on top of that, the reason most overdrives fail in a trans is heat, when in overdrive, the pump can not and does not move as much fluid thru the cooler, and adding the extra weight does create more heat, so with less fluid volume it will just build heat as it goes on, the temp gauge is a valuable asset for any auto trans. I used to tow with a Yukon, and i also felt if I kept it from hunting it would not fail...........BAD thought there, cost me dearly twice, and the final answer I got was the lower RPMs and fluid movement. Also note, the torque converters on most gas rigs only locks up late in the pattern, and most will unlock anytime there is to much of a request for throttle, and if you want to see a trans heat up, drive it with the converter unlocked....very bad, and that is what will happen towing in OD most often. picking the foot off the throttle, or brake appication will unlock it. I now tow the fifth wheel camper, and my 911CDX behind that with a Duramax with the Allison trans, 6 speed, when pulling just the boat, I can tow in 6th, which is double overdrive, temp never goes over 150, when trippled up, I use 5th gear, and never goes above 170, unless dam hot out side, then 180, also note on the lock up converter, the Alison locks up in sedond or third, and does not unlock until back in that lower gear. Guys, with gas rigs and large boats, do NOT use OD, the manual will likely tell you that also, a little less mileage is a whole lot better than 2 g or more for a trans repair......the tow button does jsut like noted before, it changes shift points on the up shift, to keep the engine more in the peak torque band longer, and with some, is also moves the downshifts up, to help with braking, but that is mostly with the diesel, along with the coverter staying locked up helps the braking.
Oh by the way, what do I do for a living......I am a service manager for a Detroit Diesel engine distributor, oh and an Allison trans distributor also. SO in the last 5 years there, I have learned a fair bit about auto trans problems. Sorry for the long post, but I felt the info was needed for you all to understand where the issues develop for these products.
