Auto transmission overheating question

[Conundrum]

'00 F350 7.3, 100* out, fully loaded with trailer full of raft stuff, steep grade, secondary trans cooler, low speed out and the transmission overheated fast in hi range. Shifted to low range and the temp came down and all was well. That diesel is pretty torquey so I was surprised. Are the internal friction plates slipping to much in high range? The motor was having no problems pulling the weight. Curious why this happened.

[TAFKALVS]

Low range means higher revs, more torque and less strain on things, resulting in the torque converter locking up quicker. Slow speeds and low revs mean the torque converter and clutches are slipping far more to prevent stalling, etc.

If i'm driving on a steep logging road even without a load I tend to just put things in low-range. More torque means less strain. Think about climbing a hill on a bike. 1st/2nd gear takes far less perceived effort than 7th. Your truck has plenty of power push through a disadvantaged gear ratio but its gotta work harder to achieve the same results.

At least that's my theory, ymmv.

[Conundrum]

That was our theory too and really only hope when we were looking up 4,000 feet with no cell service. You're probably right.

[hawkgt]

I would change out the trans fluid also. Over temp will degrade the quality of the fluid and easier for it to happen again.

[snowaddict91]

Yeah I would say torque converter was building a lot of heat and with little air flow at slow speeds that trans cooler doesn't do a ton for you honestly. Do you have a trans temp gauge? What was the temp?

[Conundrum]

Just had the fluid changed and do it annually based on the transmission being the weak link in this year of truck. I do have a gauge set to alert at 220*. It got up to about 228* while I was pulled over. Not super worried about it at that temp. After putting it in lo, it dropped to about 205* the rest of the drive out so not terrible. Ran about 180* the rest of the trip on the highway. I was just surprised how fast it got hot but that makes sense about the torque converter...pretty much what I was trying to pinpoint so thanks. I guess theres a modification you can do where you switch to lock it but this isn't a very common issue for me.

[TAFKALVS]

I guess theres a modification you can do where you switch to lock it but this isn't a very common issue for me.

You gotta be really really careful about locking up your torque converter when it doesn't want to... can cause just as many or more issues. (including destroyed transmission gears and melted clutch packs.)

There is a reason why you can still get a 5/6 speed manual in diesel trucks. While a bitch to drive in traffic, they run a bit cooler, have near 100% power transmission and are very strong.

[Conundrum]

I've heard that about the converter lock. I can see me not being in the frame of mind to remember to turn it off which is another reason I'm avoiding it. I think if it were really an issue, I'd get a wrist shaker. I'm actually looking at downsizing anyway now that I don't have a slide in camper so probably not doing anything to the truck except maintenance from here on in.

[cat in january]

There is a reason why you can still get a 5/6 speed manual in diesel trucks. While a bitch to drive in traffic, they run a bit cooler, have near 100% power transmission and are very strong.

That and you can lay rubber in 2nd and 3rd gear.

My first thought was to advise a fluid change as was mentioned, but sounds like you have that in hand.

[Snow Dog]

First, the torque converter is two disks (with ribs) that is fluid coupled. When it slips -- which is how it works -- heat is generated. Pulling up a steep hill means lots of slippage and low speed means little cooling. Second, there are clutches in the torque converter that lock the two plates together. No slippage means no heat generated. You want to be locked. The thing is, most older transmissions only lock in the top gear. On my old Dodge RAM 4-speed it only locks in 4th. If I turn off overdrive then it locks in 3rd. So pulling up a hill can be the worst of all conditions -- low speed and 2nd gear and lots of heat.

Going to a lower gear and high RPM means less slippage and less heat and less speed. The big killer of older automatic transmissions are guys who try and go up a hill at 60mph and put the pedal to the metal.

There are aftermarket kits that let you lock in lower gears and I think modern trannys will lock in more gears because the computer controls the engine and transmission.