Tow haul mode button - what exactly is it doing in machine language
It limits upshifting to higher gears (OD) where the load will constantly require the computer to adjust gearing causing unnecessary stress, wear & heat. Of course it's at the expense of higher rpm and gas mileage.
It said Use this mode when you are towing heavy loads. It changes the shift points of the transmission to match the heavier loads. It delays the shift for higher RPMs and also downshifts earlier on a grade to maximize engine breaking.
Great video by Kip at PromasterOnly about Tow Haul
Doesn't do a thing for longevity or reliablity or breakage
Engage all the time smooths out the shifting
Our 2018 Promaster 2500 class B rv is always heavy, around 9,000 lbs, so I engage tow haul in all stop or slow and go traffic scenarios. There is a small sacrifice in fuel economy but nothing of concern. Definitely feel the better engine performance. Another great and informative educational video. Spivey!
Should we have it on always? Effects on transmission?
You wouldn't want to shift too soon when towing or having heavy payload then have engine lag struggle and put more stress in Trans where it's heats up more.
I use it when climbing any hills or mountains. Flat I turn it off
It can be helpful going down mountains, should help with brake heating and wear.
I never use cruise in the mountains. Too much wear and tear on the transmission. When going uphill I don't mind if van slows down. I'm mostly in very light traffic areas or 4 lane roads. I always stay in slow lane.
while driving uphill, if you feel it gear down to maintain speed and then shortly after that, gear back up again, tow/haul should be enabled. Turn cruise control off. This is not a car.
More here