What makes a transmission heat up?

I have a stock 2004 S10. I know that shifting in and out of overdrive too much is a bad thing, so when going uphill, or when travelling in hilly country you need to shift out of overdrive.
What is it exactly that is hard on the transmission? - selecting overdrive but not going in - the actual shifting from drive to overdrive - driving with the torque converter unlocked - none of the above
I've read numerous posts, but I haven't been able to put a finger on what exactly causes the overheating problems in a transmission or premature wear on the parts.
I guess my torque converter locks in both Overdrive and '3', is this correct? and if so is it relevant to driving in a way so as to protect the tranny?
Any help or a link to a source would be appreciated. Thanks.
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JRK,
Heat in a transmission is caused primarily by two things:
1) Torque convertor slippage 2) Shifting gears.
For #1, the TC produces heat as a by-product when it is in replication mode (NOT locked), the more it's replicating torque, the more heat it creates. So, an AT will produce more heat in 4th gear, unlocked, than it will in 3rd gear, unlocked, becuase the engine RPMS's are higher in 3rd gear hence to engine is producing more torque that the TC doesn't have to "make."
Shifting gears causes hear as the frictions apply the selected gear. The longer the shift, the more heat is produced.
Doc

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So when I was driving at 60 mph with a trailer behind my Blazer, in 3rd, and thought I was overrevving the transmission, it was in fact a better situation than being in 4th?

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wrote:
Just my personal feelings, but when I'm towing, I lock out OD... More of a peace of mind thing, really.. I figure that you can run forever without OD and not hurt anything, but might add heat or strain on the tranny if in OD at the wrong time..
With our setup, (99 ram/360/3:55 6,000# TT), even running on flat ground it seems to pull better and use less fuel if OD is locked out.. In OD, it takes more pedal to do the same speed, so I'm guessing that it's more efficient for us to just not use OD.... YMMV
Like Doc said, shifting produces heat, especially downshifting on a grade... I know my truck is going to downshift going up the hill, but it seems like starting with OD locked out is easier on the tranny..

Mac
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yp that's what I've been hearing from Gary and others for quite some time on this group, so I made sure to lock out OD.
Still-- seemed like it was revving awful high. Probably just my not being used to the tranny not using it's OD.

hilly
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Think back a few years back before OD trannys werre the norm. I had a '64 Chevy pickup with 3.73 gears and no OD. It would run about 3000RPM at 65. It would run like that all day long if you wanted it to. I had a '73 GMC that was the same. Enter OD transmissions and now it is a sin to run much over 2000 RPM at 65 MPH. You have to be crazy to run that engine at 3000 RPM!! (or so many believe!) I run my '93 Caprice, pulling my boat, all day long at 70 in drive. It is still turning less RPM that the old pickups were. Greg
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Thanks for the info, Greg. I have no experience with iron that old.. ;)
Makes sense, though

being
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wrote: <snip>

Greg... I use a similar guide when I'm towing.... I think of a 5 speed stick... you can run full out all day in 4th without hurting it... When we took a trip from Ca. to Wa. and back last month, we were pulling grades at 40 or 45 mph and turning about 3,300 to 3,500 rpm... Once we got used to hearing the truck stay in "passing gear" all the way up the grade, everything was fine.. flat land, we held about 60 or so at about 2,500 rpm.. (not towing, we turn about 2,200 at 65 mph in OD)
Mac
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wrote:
probably what you're hearing is the engine being in the "sweet spot" of the power band... not usually heard unless you're towing or have a BIG foot.. lol
In my limited experience, engines seem to do better at a little higher rpm when towing... unless, of course, you can afford a turbo diesel.. (someday, someday)

Mac
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Ditto He needs to try and drive a old 'Benz with 4 on the tree. The mech. at the station I worked at in the '70's got a laugh when it took me 10 min. to figure out how to get it in reverse to bring it into the bay. I figured it out. Damn furrin cars.........LOL
--
John
"anything you say can & will be misquoted & used against you"
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<<< Heat in a transmission is caused primarily by two things
I have a "tow/haul" mode on my 01 Suburban 1/2 ton. I don't tow much, but does that help with transmission heat or tear?
-Brown

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JRK wrote:

I don't think you NEED to shift out of OD, but I and I'm sure many others do.
I personally do it because all of the shifting irritates me.

I've found that my tranny temp rises when I'm driving around in 3rd under the rated stall speed for my converter. IE: in traffic never going much above 35-40mph. For my full sized truck running the 4L65 tranny, it's around 1800rpm brake stall.
I can basically drive around all day at 2,000rpm in 3rd, and not generate much more heat than if I was locked up in OD.

My computer is set to apply lockup in third as early as 43mph... lockup in fourth as early as 36mph. The thing is, if I manually select 3rd gear, I never once feel the lockup take place. "Torque converter clutch lock during shift" is not turned on, so I know that's not happening.
One thing I hear that takes place, is a "partial apply" of the lockup clutch in fourth when you go up a moderate grade or feed on moderate throttle. I haven't logged any data about this yet, or felt it myself, but the TransGo kit for the 4L60's are supposed to delete this "feature".
TCC lockup will be more of a kickdown of a gear, rather than a smooth (partial) release. That might be where the "don't tow in OD" thing comes from. It's probably pretty easy to cook a TCC if you drive around in this mode for too long.
With that said though, I have no problems pulling 5,000lb loads at 65-75 on the highway, in OD. Around town, I use 3rd.
-marc
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