Yes the torque converter is the only thing that puts heat into the trans fluid.


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The more I read other threads on transmission temperature the more I feel like I'm stressing over a non-issue. On various tundra threads, it's not abnormal to see tranny temps of 230-240 even higher when pulling up an incline. As long as it's not staying in those ranges for long periods of time, its sounds like it's fine. Worst case is to change out fluid more often which I already do as a preventive maintenance thing.

The hydraulic pump that pumps the fluid for the trans also makes heat, as does other sources of friction in the trans. The torque converter is the largest (and most variable) source of heat through.

-Charlie

This is the camp I have been in for a long time. I see people here freaking out over 200*F trans fluid... not sure why. It is easy to do a quick drain and refill of the trans every 15k miles, or even 30k if you use high quality fluid. Regular fluid changes will keep everything happy. Occasional spikes to higher temps are fine, it won't blow up the trans. Try to keep it under 200*F for most operating conditions and 160-180*F is ideal.

All those rules for trans temp were for gauges in the pan with much older technology fluids. Our temp sender (the OEM '99+ OBD-II sensor) is in the SEND line for the trans, just before the fluid gets cooled and sent back to the pan. The average fluid temp is lower than what you see on your gauge...

-Charlie

Installed a Long 4490 cooler in series with the radiator cooler this past weekend. It was the absolute largest cooler I could fit.

I'm running heavy 33's with (heavier) Tundra steelies on anemic 3.91 gears. (15" wheel truck when new).

My trans regularly stayed in the low 200's-220's, just cruising on flat land. It was lugging and needs a re-gear terribly..

And now, after the cooler, I can not get the temp above 135! It will occasionally hit 140 on grades.

Isn't that too cold? There is an operating temp for trans fluid, just like engine oil/coolant, yes?

Run it in series with the radiator instead of bypassing it. If you are worried about it going in too cold to the transmission, you can run it transmission sender line --> radiator --> cooler --> transmission return line. Most instructions tell you to keep the stock cooler in the system anyway.

Heat is the killer of automatic transmissions. While it's not abnormal to see temps in the 230-240 range if you're going up a steep enough sustained grade, that doesn't mean it's not doing any harm to your transmission. What's considered long? I start thinking about pulling over and letting my trans cool down once I reach 230 degrees. If I'm on a long enough grade, 230 becomes 240, than 250, you get the point. I believe a rig driven by someone who frequents the mountains and 4wheels is going to have a trans that lasts fewer miles before needing major repairs or replacement compared to the soccer mom who uses her rig to take the kids to school and go grocery shopping. The difference between the 2 drivers is the heat that was generated in the trans while they were traveling.

The idiot light on the dash is there for people who don't monitor their trans temp. When it comes on, it's telling the person, "Hey idiot, you're frying your trans. Better stop before you destroy it." The idiot light comes on at 300 degrees.

If I can avoid high temps, I do. An external trans cooler, knowing that turning off OD when you're in that speed range of around 40-60mph to lock up the torque converter and knowing to use 4Lo when wheeling up sustained grades are all things that can help you keep your trans temps down.

It's just like temp spikes in an engine. Can your engine handle it. Yeah, more than likely. Could it also accelerate the demise of the engine, sure it could. I monitor my coolant and trans temps so I know when it's prudent to give my rig a rest. I'd rather give my rig a little rest and delay my arrival to wherever I'm going instead of burning up my engine or trans.

Update: just got back from another camping trip. About 900 miles driven in total, maybe half of that on dirt roads. 4 Low definitely helps with keeping temperatures down. Thank you for this advice. @mtbtim Had extended periods of climbing going about 5-15 mph on steep grades and never saw temperatures over 200*. I also tried L in 2wd, I assume this just keeps it in 1st, which seemed to help as well. Can anyone confirm this? Even felt like 2-2wd on the shifter did better than D in certain situations. I saw max temp of 230* in D-2wd before switching to 4 low. It would have surely been over 250* if I hadn't been in 4 low for some of the climbs.

Overall very happy with the results and was easily able to keep my temperatures under 210 the whole time. Experienced almost all types of terrain. Only myself and maybe 300 pounds of extra gear on this trip. Now if I could do something about towing, I'd be set. I appreciate all the advice that was given.

you are correct. L keeps it in 1st. 2 allows it to use 1st and 2nd. D with OD off allows it to use 1,2,3. D with OD on allows all 4 gears to be used. I'm pretty sure the torque converter gets locked out in different speeds depending on the gear. IE... if in D with OD off once you hit 40 or 45 it locks out the torque converter. that would apply to each "gear" IE L,2,D. Although it make work different when in 4wd LO.

Personally I love 4LO when wheeling any excuse I'm in it. More torque, slower crawl, less strain on transmission. wins all around. although it will not go into 4th while in 4wdLO. (at least in my 99 with the J shift.)

Yeah speed is limited with 4 low I started using L and 2 for this reason and the temperature was managable and was able to have a little more speed. You are correct, 4 low only uses 3 gears and i can maintain 15-20 mph comfortably.

Sorry to restart and hi Jack the thread a bit. I'm in a similar situation. Armored and winch up front. 4.88 gears onn 33s. Running a 70624 B&M in line with stock radiator. I can do a 15-20 minute drive from my house up a steep hill ( last 5 to 8 minutes of the drive) and hit 210. Once I top the hill it does start to come down. I haven't tried going up steep mountain grades yet as I'm still breaking in my new gears... I guess my question is knowing the stock cooler is at the send line ( hottest location) couldn't you run an independent temp sensor at the return line from the aux cooler to know the cooled temp? Meet in the middle and use that as a guide of real fluid temp (i.e. OEM send reads 220, aux return reads 180...real temp likely 200. ) . Or should we all just stop worrying about these low to mid 200 temps to get up grades if things cool down once over the hump so to speak?

I run lucas transmission fix and have been for over 100,000 miles and my transmission temps never go above 160. Most of the time it steadily runs around 120 to 130. This transmission has 325,000 miles and shifts smooth and consistently. Since I don't tow with this 25 year old SUV I never manually shift or take it out of OD. When the hills get big I use ECT all the time. The transmission has been rock solid on this 96 4 runner limited for over 25 years. I always have liked Lucas Transmission fix on older cars but on this 4 runner it has been very good.

Where are you sensing trans temp?


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The shop installed it in the line leaving the trans and going into the cooler. The cooler is not in line with the cooler in the rad. Air flow more than any single factor keeps the temps low.