UPDATED Coast on the uphills accelerate on the downhills

[opnspaces]

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.

[badsix]

ppine wrote:
I can't agree with your technique at all. There is an rpm range that all engines pull best in. That is where you want to be. Gearing down and going slower will produce less heat.

Yep ^^^ also trying to keep you converter locked is going to burn it up not designed to pull a 7000 or more T/T up a steep grade. only for hi gear and not pulling, luging the trans will heat it more. that's why transmissions have gears the more the better. let it shift down and keep the engine in the rpm range that makes the most power. big rig drivers don't lug there engines they run them at a certain rpm range for the best power range and adjust with there gear boxes.
Jay D.

[BenK]

Your latest info says to me, that air flow is the main issue. Highway will have much more air flow than stop and go...where the fan is the key to keeping air flow up there

Even if you have an El Cheapo radiator, running with just the driver, it should not behave like that...heating up in stop n go, as you still have the external aux ATF cooler

Since a 2500, EVERYTHING is sized for HD usage. Thermal abilities are one of the main attributes making your truck a HD towing vehicle. The margins (head room) are there to absorb that kind of short term usage needing more HP in spurts (acceleration from a stop)

Hope that is all there is, but you never know until the situation is completely fixed.

[opnspaces]

Also I pulled the trailer back to storage and temperatures were better than last week. First thing I did was I pulled in D not 3rd to try to stay in lockup as much as possible. Of course when I hit the hill the truck forced a downshift to third. I held the RPM's at 4,000 except at points where I had to lift my foot to avoid rear ending the slow soccer mom in front of me and dropped to about 3800 RPM. The temperature gauge was much better and didn't even hit 200 on the hill though it did slowly climb there as I drove. So it was better, but I'm not out of the woods yet. below are a few things that probably contributed the differences.

First, I will admit I was distracted with general life issues and forgot to turn on tow haul. I'm seeing this as a negative and will definitely make sure it's on for the next pull.

Second, I was pulling the trailer empty as I was headed to storage.

Third, the pull was only about 20 miles. If I had to go further I'm sure the heat would have climbed.

[opnspaces]

Thanks Ben. I've been messing with this truck for a bit and I found something that to me is interesting. This is when I drive unloaded as in no trailer and only me in the truck. If I drive the freeways the temperature stays low around 160. But if I drive around town with all stop and go traffic lights, the temp climbs to almost 200 and stays there. So to me this means either my fan clutch is going, or my radiator has no real cooler built in. I think my first and cheapest move it to pull the lower coolant hose and see if I can find a way to look in the tank at the cooler.

[BenK]

SweetLou is correct, but mainly for a forced fed ICE.

Meaning the pedal (diesel's do NOT have a throttle plate) can force more than the listed displacement specification. A 6.7L can become a 8L/10L/etc...until it produces so many BTU's via more fuel injected, that the surface area of the thermal rejection system can NOT move those extra BTU's away fast enough to whatever rejection system component it has...that the metal will raise to glow, then melt

On a N/A ICE, it can ingest a bit over the displacement specification via inertia ramming...AKA scavenging effect

There are many other attributes that contribute to the BTU development of an ICE, but the above are the basic's

If you wish to delve deeper into this area, here are the 2012 GM 6.0L L29 torque/HP curves just found for this discussion. They are from the GMPowerTrain site. Mechanically, these L96's are the same. The main difference is in the state of tune. There would be differences in the thermal rejection components and sub-systems



This is for half ton pickups with L96 6.0L



This for HD pickups with L96 6.0L



This for 2500 Suburban with L96 6.0L.



This is for Express/Savanna 2500/3500 full sized vans with L96 6.0L


As a general rule of thumb, the best MPG while towing heavy is to choose an RPM just below to at peak torque. Going over that RPM will gain HP (torque x RPM / 5252), but cost MPG's and produce more BTU's

Figure what HP you desire to tow heavy at and then choose the gear that keeps your 6.0L in that range

[SweetLou]

While most are commenting to you about the equipment possibilities or things to check, I wanted to let you know that they way you drive while towing is also very important. I am on my third Dodge (now Ram) a 2013 diesel 6.7 with the 68FE trans.
From previous set up, I had exhaust temp guage which goes hand in hand with radiator temp. In order to keep the truck temp cooler I had to balance the exhaust temp and boost from the turbo. I rant the temp at around 1000 to 1100 which and of course using the correct gear to maintain RPM. It's a balancing act and it kept the radiator cooler, thus the trans cooler as well. If I ever put my foot into it, the temps shot up, so I learned to cut back. I am pulling, not racing and it chugs up all the way. Try the 16 mile Baker grade near las Vegas on 116 degree day pulling anything with never a hiccup. So, my advice also to find your sweet spot learn that pull truck and you will be fine.

[drsteve]

My 06 with 4L80E runs cooler when I leave it in D and tow haul. Selecting 3rd always makes it run a bit warmer, even on flat roads. Temp is usually around 185 or 190 towing, very seldom goes over 200. Unloaded it stays around 160-170 ish.

I have never noticed the TC locking in 3rd.

[opnspaces]

I have a question about transmission temp for any of you running a similar age range truck or Suburban.

When I am running unloaded, just soccer dading around the city my trans gauge is just shy of 200. I thought that was normal since the radiator is 195. But after reading into many of the posts I'm wondering if I'm already too hot? What are you guys seeing for normal unloaded transmission temps?

[opnspaces]

I found a GM publication called the 2001 Chevrolet Suburban Restoration Kit. Unfortunately the way I read it there is only lockup in 4th. So I guess I would have to look into an aftermarket lockup if I wanted to lock up in 3.


And the link to the manual
Link

[BenK]

GeoBoy wrote:
BenK maybe on to something with that engine radiator. I installed this in my 2007 3500 with the 4L80E transmission, https://oregonperformancetransmission.com/product/LNG-47391.html. The factory transmission coolers on the 4L80E were marginal at best.

GeoBoy, that is one of the best types of low PSI, stacked plate radiators out there. Best if the connections between each plate is brazed together and think this vendor does so



Those which have gaskets, tend to leak over time. More so in dynamic applications with vibration. Solved by brazing the plates together with past filled with brass & heated to fuse them together...they even had upset areas on the plates that touched the other plate to lessen ballooning of flat surfaces in higher PSI applications

OEM stacked plate types are good, but not as good as this type of stacked plate (flattened round tubing)

I'm of the opinion to have as much external aux ATF cooler as you can stuff in. Main caveat is that the ATF must also be plumbed into the engine radiator and in extreme cold, a by-pass to remove the external aux cooler

'Was' going to install a larger external, aux ATF when still owned the 8.5K lb boat + trailer and my 7.4L continued to get hotter and hotter while towing. Until it also heated up just commuting to work (32 mile one way, Silicon Valley can get really hot during the summer and fall), which meant something else was wrong

Installed a severe duty fan clutch thinking it was going bad. Instantly cooled it down for a while, but then it started to get too hot again.

Turned out to be plugged radiator from DeathCool (Dexcool and coined it because almost lost the 7.4L). Had the OEM radiator rebuilt (new plastic tanks and rod'ed out the core). Still have the modified 180*F thermostat in there...think it's #3

Then never needed the bigger external aux ATF cooler. Will do so when I rebuild the 1996 Suburban. As the 7.4L will be 'built' and other goodies that will increase the BTU's to be rejected

[GeoBoy]

BenK maybe on to something with that engine radiator. I installed this in my 2007 3500 with the 4L80E transmission, https://oregonperformancetransmission.com/product/LNG-47391.html. The factory transmission coolers on the 4L80E were marginal at best.

[marcsbigfoot20b]

I had a 99 Tahoe 4L60E with both OEM computer and then upgraded to 0411 LS style computer.....so I could tune it and the trans plus add tow haul which the old GMT 400 PCM didnt have.

I found 2nd gear does NOT have lock up at all...(the reason is you can manually put shifter into 2nd and it will start there from a dead stop, and if you were to do that stopped or come to a stop, you would stall the engine). My problem was my highly modified 99 would only pull 45 mph max going up the rim past Payson AZ, nice big 10 mile 7-8% mountain. I was stuck wide open at 4000 rpm in second with my ~7400 lb trailer and the trans temps would start cooking.
Brought my laptop with and tuned the trans with EFI Live to lock up in 2nd gear after 3000 rpms/at certain throttle position/speed. But it only works if you have it in D or 3. I could also change if I wanted it to stay locked up during shifts or not....I cant remember how the OEM tune was anymore. 3rd will lock up as well as 4th and using T/H it will stay locked longer/load.

My guess is your converter clutch may be going, or the pressure to it is too low.

[Grit_dog]

^Thats the same outcome I was getting with our 01 4L60.
It worth looking into as well as aftermarket lockup switches if you determine the same.
I'd think you basically never use D (overdrive) pulling a significant trailer with that rig unless downhill or flat with a tailwind.

Trans cooler may not be worth investigating. The cheap easy button is an external cooler of your choice. Simple and effective.

Also research aftermarket lockup switches. I think they exist for a good reason, although I'm not very knowledgeable about LS 4L80 combos. I do know tha the same combos a few years newer 08 and up, pull good size trailers through the mtns at work without overheating. So something is up with yours.

[opnspaces]

Thanks Patrick, yes I'm definitely going to try towing in D and see what happens. Oh and just to be clear, in no way was I picking on Ben about the 3 or D for towing. I'm just trying to figure it all out.

I did drive the freeway for a while and my findings seem to mirror Grit with only getting lockup in 4th. But it's not always easy to watch the tach and the traffic and the speedo etc. So I might try it again with one of my kids as passenger watching the RPM's on my ELM327 scan tool while I drive.

One method I used to try to verify or force lockup in 3 was

• I drove in 3 and put the tach as close to 3,000 rpm as I could.
• I held my foot still and drove for about 10 seconds to make sure the trans was in whatever state it wanted to be.
• Without moving my foot I shifted into D.
• I watched the tach drop as it hit 4 then drop again into lockup.
• Still not moving my foot I shifted back to 3.
• The tach jumped immediately to 3,000 and seemed to stay there for at least 10 seconds.

I'm assuming that the transmission would come out of lockup for the shift. If I'm wrong then my test method was a bust.

[PatJ]

Ben says to tow in 3 instead of D for the same reason you towed in 3 with your old rig, because the GMT400 manual tells you to. For the GMT800 it says tow in D with tow mode on.

Even though it is the same transmission, the software has changed significantly between the GMT400 Vortec and GMT800 LS era. The LS powertrain computer is something like 100,000 times more powerful than the GMT400 computer was. Both owner's manuals are absolutely correct the trucks are just slightly different. Just do what the manual says and you should be OK. Hopefully you had a breakthrough here, but unfortunately I think the 80 still locks up in third when in 3 (direct.) I hope I'm wrong and your right.