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Open Roads Forum  >  Towing

 > Coast on the uphills accelerate on the downhills

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opnspaces

San Diego Ca

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Posted: 01/06/20 07:21pm Link  |  Quote  |  Print  |  Notify Moderator

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.
[image]

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.
[image]

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.


2001 Suburban 4x4. 6.0L, 4.10 3/4 ton
2005 Jayco Jay Flight 27BH
1986 Coleman Columbia Popup.

Lwiddis

Morro Bay, CA

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Posted: 01/06/20 07:40pm Link  |  Quote  |  Print  |  Notify Moderator

Very interesting report. TY for posting.


Winnebago 2101DS TT & 2020 Chevy Silverado 1500 LTZ Z71, 300 watts solar-parallel & MPPT, Trojan T-125s. TALL pole for flags. Prefer USFS, COE, BLM, NPS, TVA, USF&WS, state & county camps. Bicyclist! 14 year Army vet - 11B40 then 11A - (MOS 1542 & 1560)


CarnationSailor

Carnation,WA

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Posted: 01/06/20 07:51pm Link  |  Quote  |  Print  |  Notify Moderator

Regarding pulling over and letting things cool down, I was advised by a transmission shop some years ago if the transmission overheats, I should pull over, but keep the engine running until the transmission cooled down.


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opnspaces

San Diego Ca

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Posted: 01/06/20 08:09pm Link  |  Quote  |  Print  |  Notify Moderator

I agree with idling the engine to cool the transmission. According to my owners manual the light goes on if it gets hot. But if it gets really hot the light changes to say "Trans hot...idle engine". Since it only ever barely touched the red and never went to idle engine, and fortunately always a the top of the grades I just let it coast down the other sides and it turned back off.

valhalla360

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Posted: 01/06/20 08:54pm Link  |  Quote  |  Print  |  Notify Moderator

If it's a lot of short up and down, I let gravity do the acceleration and deceleration just keeping the speeds reasonable (try to keep it below 70mph and above 50mph). This also helps avoid a lot of downshifts. You feather the throttle on the uphills so it doesn't drop out of top gear.

On longer or very steep hills, that doesn't work. Also, you mention desert...how hot was it outside?

Newer transmissions can lock up more gears, so it's less of a problem.

I'm guessing you are significantly heavier than 5600...empty is 5000 for your jayco. GVWR is 7500 and most people find that's closer to where they are in real life. Add in a load inside the suburban and you were likely pushing the truck to it's limits, so not surprising if she was running a bit hot.


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opnspaces

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Posted: 01/06/20 09:45pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks valhalla360 that's exactly what I was doing, trying to feather the throttle to keep it locked up in high gear. Yes it was a trip to the desert, but this time of year the high is only about 72 F. It was definitely cooler than 75 degrees F outside.

As far as the 5,600 weight. Since this Suburban is new to me I was trying to see how well it worked vs my old 1996 Suburban. So I had actually weighed the truck and trailer on my way out of town (my first time ever weighing a loaded setup by BTW) and the trailer was 5680. Wait a minute, no I think you are correct, because I was forgetting the tongue weight which was 1,000 lbs.

I always assumed I was at 6,200 loaded based on guesses and other peoples reported weights on this forum. But looking at my link below I think the trailer actually weighed 6,680 lbs. [emoticon]
Here's a link to my weights in another post in this forum. Link

As far as pushing the truck to it's limits, I think that probably only comes into play in it's 167,000 miles. The Suburban is a 3/4 ton with a 4.10 gear ratio and rated to tow 10,000 lbs. Oh I also looked today at the Chevy hitch and it is rated for 1,500 lbs weight distribution.

ACZL

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Posted: 01/07/20 02:17am Link  |  Quote  |  Print  |  Notify Moderator

Could always engage Tow/Haul or drop 1 gear and keep it there. RPM's may run higher, but you are drawing more air thru the rads and if eng fan kicks in, all the better for more air being sucked into eng compartment. Ironically this keeps tranny temps down and avoids shifting. Make sure you are running full synthetic for all oils as well.


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2003silverado

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Posted: 01/07/20 05:42am Link  |  Quote  |  Print  |  Notify Moderator

Have you thoroughly cleaned the existing tranny cooler? Lots of bugs and road grime accumulate over time and reduce cooling efficiency.

Dave H M

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Posted: 01/07/20 06:03am Link  |  Quote  |  Print  |  Notify Moderator

So I went to the OP to check up on the age of the TV. Cus I was surprised no one mentioned the TC lock up issue.

I may be wrong but think the TC on that vehicle stayed locked up when down shifting or locked up again after the down shift was completed.

I am a Ford motor head so what do you all think about the chev?

Hannibal

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Posted: 01/07/20 06:30am Link  |  Quote  |  Print  |  Notify Moderator

That’s odd. When I drive our Prius, I set cruise control and let it rip. Same with the truck and trailer.


'10 F250 XLT CC SB 5.4L 5spdTS 3.73 106k miles
ex '95 Cummins,'98 12v Cummins,'01.5 Cummins,'03 Cummins; '05 Hemi
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