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Discussion Starter #1
Occasionally (and been doing this for a long while) when pulling a steeper grade at 55mph or so in the passing lane, my Titan will drop into passing gear heading up.

When I crest the hill and road flattens out, tranny seems to want to stay in that low passing gear, unless I left off the gas all the way or drop it into "4" no overdrive manually and back to O/D.

Curious why this may be. Appreciate the insights. Otherwise this truck is a tank. Love it, still rides like new.
 

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You don't say here what year your truck is, but based on what I know of my 16-1/2-year-old, this is normal behavior. It's built into the grade logic. The trans stays in a lower gear for additional cooling, holds that gear until it's satisfied that it's cooling down.

I had a conversation with a nice guy from Nissan who was on the powertrain engineering team, and this is one of the many things he explained to me. Yeesh, that was wow, fifteen years ago...
 

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Discussion Starter #3
Sorry - 2012 Pro-4X. 130K miles. Seems like stepping up to the cruising gear with cool it faster. Am I thinking backwards?
 

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2012 SL, Texas Edition, Big Tow, Blizzard white, Tastefully modded (in my own expert opinion, lol)!
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Sorry - 2012 Pro-4X. 130K miles. Seems like stepping up to the cruising gear with cool it faster. Am I thinking backwards?
I had exactly the same thought. Seems like staying in a lower gear at high speed would generate more heat. I am not an engineer and might have that all wrong.
 

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2004 Titan SE 4x4 Big Tow
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Not an engineer;

I believe the trans is pumping fluid through the various coolers at a faster rate when it’s in a lower gear and spinning more rpm.

Something about converter lockup also introduces more heat in the equation, so if you can convince the truck out of lockup using Tow/Haul or manually shifting to 4th it will cool faster.

I recently bypassed the radiator’s transmission cooler with an LDP 4588 and running 55-70mph through hills in 65* ambient the gauge barely got off cold. It will cool off at 75mph even when ambient temp is 85-95* regardless of terrain.

TLDR; make sure your fluid is in good shape, bypass the internal transmission cooler, and most importantly, change your rubber lines.

Edit: dubyam’s comment below is the one you should be reading.
 

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Constable is on the right path. Only thing I can add is when the converter is locked, it generates less heat than when it is unlocked and slipping. And that's one of the major reasons the transmission controls keep the truck in lower gear longer than you might think is necessary - to keep the converter locked and prevent heat buildup during slipping when going to a higher gear.

Now, if you run in lower gear too long, it can cause heating of the trans, for two reasons. First, more RPM does pump more fluid, but it also builds more heat. And at higher RPM, the engine builds a good bit more heat, so your transmission sees this heat by convection (adjacent to the radiator) and conduction (in the wet cooling segment Constable is talking about bypassing). Generally, my trans runs in the 140-180deg range, depending on whether I'm towing or not, and what the ambient temps are. I've had it stay down around 100-110deg when the temps were single digit and I was doing 70mph. I've had it get as hot as 194deg towing a heavy boat (~5500-6000lbs) in stop and go traffic in the foothills of the Appalachians. Part of that is a bad fan clutch causing my motor to get hotter than it needed to during that same trip.

And Constable's advice about bypassing the lower radiator tank wet cooler, coupled with adding an auxiliary trans cooler, is on my to-do list. I check my trans lines every oil change, ever since Nissan had to put a new trans in my truck at 52k because the lines let go while I was driving. That was not an enjoyable experience, at all.
 

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Discussion Starter #7
Thanks, the tranny fluid has been drained and filled three times (I'm the original owner), and the tranny lines were all inspected in March before COVID outbreak. I do not tow anything, and occasionally haul a couple loads of wood at slow speeds.

Probably put a whole 30 miles on it since March and its last inspection LOL. But the gear down hang up seems only to happen on the highway at 55mph or higher on steep grades, and dropping to passing gear seems to require me to let off the pedal 100% or manually shift to 4th and back to O/D.
 
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