I'll post one more item here. Just back from a trip to WV, where I met some bigger hills than I ever have. ST did fine, but I'm real glad I had the additional tranny cooler.
Some observations:
Conditions
'03ST, 4:10 rear, 4x4, 255/R70-16 Michelin LTX M/S, towing 3800lb trailer with a frontal dimensions of 8'x7', 300lbs of gear in ST, all weights measured on a scale, additional 8400BTU tranny cooler, Air temp for all measurment about 85F, full synthetic tranny fluid, all temps measured at 55-60 mph
Flatland Non-towing OD OFF: Tranny temps in the 140-145 range. Water temp 195.
Flatland Non-towing OD ON: Tranny temps in the 140-145 range. Water temp 190.
Flatland Towing OD OFF Tranny temp 160. Water temp 200.
Flatland Towing OD ON Tranny temp 165. Water temp 195-200.
Mountain Towing: OD ON Tranny was hunting as I was going over moderate ups and downs. Left OD on until tranny temp got to 195, and then switched it off to let things cool down. Water temps peaked at 215. Not possible to leave on in steep uphill because it would automatically downshift out of OD and stay out...
Mountain Towing: OD OFF Going up 4-6% grades for about 30 miles. Let tranny downshift as needed to maintain 55-60 mph. Tranny temp peaked at 183. Water temp at 216. Had no problem keeping up the speed, but I was at ~ 4200 rpm. I let the ST choose what gear to be in. I did not manually downshift.
Observations: The more the tranny had to shift, the faster the tranny temp went up, especially if is was under a heavy load when it shifted. 10 degree rises in 30 seconds was not uncommon. Water temp also went up with rpm and amount of work the engine was having to do(measured by my feel of the gas pedal as the cruise was working to keep it at speed). Tranny temps took a while to cool down after getting heated up(5-15 minutes). Water temp came down fast after the load was off(2-5 minutes).
After 12 hours of driving in mountains, rolling hills and dead flat NW Ohio, a couple of things became clear. If the tranny is not downshifting out of OD on highway overpasses on flat land(especially if fighting a headwind, it will do that..), then leave it in OD. As it starts to downshift on slight hills, use your judgement, but even downshifting once every 10 minutes is enough to keep the tranny temps elevated. In the hills, remember that every time it shifts, it makes heat. OD is just another shift, and more heat. Best to leave it off. In the mountains, no doubt, leave it off and let the temps recover whenever possible on the easier stretches.
And... If you are going to tow any distances or in the mountains, an additional tranny cooler WILL be a big asset. Mine dropped tranny temps by 20 degrees in nontowing and towing, and allowed faster cooling when things did heat up than just the stock one alone...
This was my 1st time towing in mountains with the ST, and although I may have been ok with just the stock cooler, the Scan Guage showed what was really going on.
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