UPDATED Coast on the uphills accelerate on the downhills

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Link to the update Jan 17 2021

I learned something new after 14 years of towing the same trailer. I took my first trip in the new to me 2001 Suburban pulling my 5,600 lb trailer. My older Suburban did not have a transmission temperature gauge; I always wondered if I was overheating the transmission on the hills. The new Suburban does have a transmission temperature gauge which has always read around 195 F. unloaded around town.

I hitched up the trailer and headed for the desert. At first everything seemed to go well. Then I hit the first decent sized hill which is about two miles long and I'm guessing at least a 5% grade, and the gauge started to move.


I remember thinking "Hmm this is interesting the temp goes up faster than I imagined". But I was at the top of the big grade and although still going uphill the climb is very moderate, or so I thought. Well about 10 miles further into the mountains with the gauge still creeping upward I'm met with a new indicator.


I considered pulling over but I was about to crest the hill into a bit of a downhill section so I pushed on and the light soon went out. This happened two more times before I hit the real long descent into the desert where the gauge finally dropped down to 200 and stayed there.

Had a great trip, but was concerned with the impending trip home. That long downhill that cooled things off coming to the desert was now going to be a long uphill going home.

As expected the first decent climb greeted me with the Trans temp hot light and I decided to pull over and have lunch and think about my options while things cooled. I obviously decided I needed to add a big external transmission cooler. But that wouldn't help me with the here and now, an hour of mountain driving from home. So I thought about what causes the most heat in an automatic transmission, torque converter slippage. Since slippage causes a lot of heat I decided I needed to keep the transmission in high gear with the converter in lockup as much as possible.

So I started driving like I had a Prius, slow on the uphills and faster on the downhills. I tried to always be light on the throttle and find ways to keep the transmission from downshifting which would cause slippage and heat. What I realized was that even in the mountains there are a lot of small downhills to go with the uphills. So I would accelerate every time I started down a hill. When the uphill came I would start steady at the bottom and slowly lift my foot to keep the transmission in high gear and therefore in lockup as long as possible. My one rule though was I wasn't going to go slow enough to hold up traffic.

Well it worked surprisingly well. I managed to maintain speed and not hold up traffic while slowly lifting my foot on the uphills. The transmission gauge still showed higher than normal temperature. But it never got into the hot range. Even after the few big hills where I had to basically floor it to keep up, the gauge came down fairly quickly.

So I learned a few things.

1) I most surely was overheating my old Suburban transmission driving the same roads in the past.

2) Accelerating on the downhills and going easy on the up hills is surprisingly effective at keeping the temperatures down.

3) I now need to change my transmission fluid, even though the current fluid is less than a month old.

4) And I need to start researching an aftermarket cooler.

Anyway I was surprised how altering my normal driving habits to going easy on the uphills really made a difference in the heat of the transmission.