Tow/Haul issue

[chrisinkanata]

Noticed today when pulling our 5th wheel trlr that when in tow/haul, the trans won't shift into overdrive unless I disengage the tow/haul at about 60 mph. Shifts right up then and I flick it back into tow/haul. Kind of a pain. Works perfectly in 'normal'. 02 GMC D/A as per my sig. Any thoughts?

Chris

[DmaxMaverick]

Maybe you just didn't notice it before, but that's the way it works, normally. I have oversize tires (PCM corrected), so mine is actually at about 65 MPH, but was about 56 MPH with OEM tires. Just a flick of the T/H switch with moderate throttle, and it'll upshift. Heavier throttle input will change this, as well (higher road speed before the shift is allowed). Try the same thing unloaded, and you'll find it the same result.

[chrisinkanata]

My bad here, I should have been more clear....I've towed heavy trailers for years, over tens of thousands of miles with this truck. Many different terrains. I fully understand the higher shift point in TH and how that shift point is affected by grade (both up and down), wind conditions and so on. If I'm on a steep enough grade, it won't up shift at all obviously. But as I crest the grade, level off and start down the other side, it has always up shifted. In this scenario, likely in the 55-58 mph range. If I'm on a level plain, no wind, it'll up shift at 55. Down a slight grade, 52 mph range. Into a headwind, maybe 65 mph. Point is in manipulating the throttle keeping in mind the conditions and being reasonable, it always up shifted. Not now. Regardless of the conditions, speed etc., no up shift in TH. Tried at 70 mph, flat road, steady speed, no dice - stays in fourth. The only way it'll up shift is if I disengage TH. Looked in my GM shop manuals as well as Alldata, but there is no info on this situation.

Chris