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UPDATED Coast on the uphills accelerate on the downhills

opnspaces
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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.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup
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1320Fastback
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We Live and Die by EGT.
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afidel
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drsteve wrote:
And to think I get nervous when the trans temp exceeds 200...


I was thinking the same, over 205 and I'm letting off even if it means slowing way down, don't want to boil the fluid! It's actually the only gauge that's ever made me alter the way I drive, engine RPM has never bothered me and none of my other gauges have ever been much out of the sweet spot.
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1320Fastback
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Looks like you got it sorted out Opnspaces. I am San Diego local too and have gone though Alpine out to the desert many times. Not sure if I've taken our current setup on S2 yet. 9/10 I go up over Palomar Moutain as I am north county but if we are going to Glamis or Superstition/Plaster area I'll take I8 out.
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2005 Forest River T26 Toy Hauler

opnspaces
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Yes I'm definitely familiar with the trans cooler on one side and oil cooler on the other. Like you say, catching all the different fluids is a bit of a challenge. My 6.0 also has the second cooler in front of the radiator.

I'm glad to hear that your transmission temp basically mirrors mine. I was a bit suspicious that after the radiator replace the temp would climb to 200 and basically just sit there no matter how hard I pushed it when towing. It almost seems like there is some kind of thermostat on the cooler line that is mostly closed during normal driving. But after reaching 200 it just opens wide and lets the fluid flow.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup

BurbMan
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Glad you got it fixed! I had the same truck with the 8.1 (but same trans, 4L80E) and it would run at 175* but never got hotter than 200* no matter the hill or outside temp. Our TT was close to 9000 lbs loaded.

The transmission actually has 2 coolers, the cooling line goes from the trans into the right side of the radiator, then out of the radiator and into an external cooler mounted in front of the a/c condenser, then back to the trans. The cooling was very effective and never had the need for an aftermarket cooler.

Not sure about the 6.0L, but the other side of the rad on the 8.1 had an engine oil cooler. That was a fun job replacing that, dealing with oil, trans fluid and coolant in the same bucket 🙂

Anyway, now that you have replaced the rad, I would drop the pan and change the filter in the trans at the next fluid change. These came from the factory with Dex III fluid, the new GM fluid is Dex VI, which is full synthetic and backwards compatible with Dex III. Of course there are different opinions about using Dex VI in an older vehicle, but I found the full synthetic ran cooler than the Dex III.

hornet28
Explorer
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Radiators are not all made the same although they may say they are for your truck and will fit doesn't make them right for all purposes. I just replaced the radiator n my dually this past summer. A local shop ordered me one and when it came it was less fins per inch then the stock one I refused it and looked for another. I finally found one online that had the correct number of FPI. If I'm remembering correctly the difference was about 7,000 fins total. I experienced the less fins per inch problem back in the early 80's with a crew cab BBC I towed with. I needed a new radiator and had a shop put it in. I experienced rising temps just going up overpasses towing. Shop tried blaming the truck and tried finding something wrong and they couldn't. Finally one day the owner called and told me he'd found there were 2 different radiators available for my truck and that the other one had over 3,000 fins more for cooling. He purchased and installed the other one at no charge. Problem immediately disappeared

dodge_guy
Explorer II
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Glad you found it. Those cheap radiators are great for price, but not for proper cooling. I'm guessing the cooler was very restrictive if it loooed like an OEM cooler in the tank.
I'm sure the fluid will be fine after the 2nd change.
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opnspaces
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parker.rowe wrote:
Great thread.

I would service the transmission, after heating the fluid up like that a few times. Check that fan clutch too. It doesn't make sense since the engine stays cool, but installing and HD clutch in my old truck made a noticeable difference, even though the old one seemed good.


Fortunately the transmission pan has a drain bolt in it. So I have done a few drain and refills on it because of the high temps. I'll probably do at least one more after this last trip just to be sure the fluid is fairly fresh.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup

opnspaces
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UPDATE Sunday Jan 17, 2021

This is a follow up on this post from last year. As reported in the previous pages I was fighting high transmission temperatures when pulling my trailer through the mountains and deserts around San Diego.

After many months of deliberation on the different methods to cool things down I decided that I really didn’t trust the new radiator that the used car lot had installed when I bought the Suburban. So early December 2020 I ordered a factory GM radiator from an online dealership for $250.

I have just finished my second desert trip and the transmission temp is much better. This trip was in the high 80’s pulling some huge hills with my foot on the floor. These grades are steep, I would guess around 6 percent and like I said my foot was on the floor for about a mile uphill. I was really trying to put stress and heat up the transmission as much as I could. Below are two pictures from today.
This first is the transmission temp with the new radiator when I’m just cruising while towing.
Normal cruising


This second picture is after driving for 45 minutes uphill out of the desert. I took it near the top of a long steep hill on the S2 in San Diego. I was trying hard to heat the transmission up. And happily I could never get it much past 200 degrees.
Middle of uphill on S2


All in all I’m satisfied that I made the right choice on the radiator. In hindsight I wish I had thought to cut open the replaced radiator to see if there was even a cooler in it. But after my first tow with the new radiator I was so disgusted with the old one that I just threw it in the trash.

Moderator edit to fix picture URL's.

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

parker_rowe
Explorer
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Great thread.

Just to add more real world experience, my old 96 burb, similar to your old one, never exceeded 200F trans temp pulling hills. It had the factory external trans cooler and the in radiator one in series. Same trailer as in my signature.

It would lock up in third, and OD, although when pulling it was always in 3rd. It would not hold OD at all with the factory 3.73 gears. My camper is very similar in weight to yours...slightly heavier when we are loaded up.

When going up hills, I would try to ride the converter as much as I could, keeping it locked while in third gear...however some hills required 2nd gear with my foot to the floor. No lockup there.

Our new suburban has a 4l80, which also locks up in 3rd. You have to be over a certain speed and under a certain % of throttle position for it to do so (just like the old 4l60).

I'm not 100% on the criteria for the gmt800 trucks, but I was under the impression that in tow/haul it would try to keep the converter locked longer. Forcing it into 3rd might not be needed, but it should not prevent the convertor from locking up.

If the computer was commanding lockup, and not getting it, you would get a check engine light.

I agree with everyone, you truck, with 4.10s and a 6.0, should have no issues with that camper. Sounds like an inspection of the cooling system is in order.

How big is your factory external cooler? I would service the transmission, after heating the fluid up like that a few times. Check that fan clutch too. It doesn't make sense since the engine stays cool, but installing and HD clutch in my old truck made a noticeable difference, even though the old one seemed good.

Love these suburbans, our old half ton was a great truck. But once we started doing long 2-3k trips and hitting steeper grades, I was worried about sending it to an earlier grave.

So we sold it and got our "new to us" 2500 burb like you did (I've always wanted a big block suburban), and unfortunately have to work out all the kinks we already addressed in the first truck. But I think it will be worth it in the end.
2015 Starcraft TravelStar 239TBS 6500 GVWR
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opnspaces
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PatJ wrote:
Wondering what you learned here? Did you pull lower hose and look at in rad heat exchanger?


Thanks for checking back. No I haven't had a chance to pull the hose yet. But the drive home from the desert this morning convinced me that I need to do it sooner rather than later. The biggest problem is I haven't found a drain **** on the radiator and I don't want gallons of antifreeze hitting the driveway from just pulling the lower hose.

So in the interim I rigged up a poor mans cooler to keep the heat down while I try to save up some money for the inevitable parts swap. My son's car was headed to the junk yard so I pulled the windshield washer jug and tubing out if it before sending it on it's way. I mounted the jug under the hood of the Suburban and ran a line up to a patio mister pointed across the transmission cooler. While it's not perfect,(okay yes it is really redneck) it does keep the temps from hitting the red zone.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup

opnspaces
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Groover wrote:
Have you thought about cleaning the existing transmission cooler and radiator? Even though you can't see it there may be a surprising amount of dirt lodged in them.


Yes I hosed them out with copious amounts of high pressure water from the garden hose. The radiator is brand new so but I hosed everything out anyway. Unfortunately it made no difference.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup

PatJ
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Wondering what you learned here? Did you pull lower hose and look at in rad heat exchanger?
Patrick

Groover
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Have you thought about cleaning the existing transmission cooler and radiator? Even though you can't see it there may be a surprising amount of dirt lodged in them.