We are about one month and 7,000 miles in to our tour of the US in our 2000 4Runner 2WD, and I've started having what I think are overheating issues with the transmission.

A couple days ago, I pulled out my monitoring phone (OBD II Bluetooth adapter + Torque Pro on an Android phone) on the highway, and saw that the transmission fluid was hitting 230 degrees F on long, gradual uphills. I discovered if I could prevent the engine from having to rev at all, meaning cruising at 2,000 rpms, the temp would stay in control (under 200 degrees F). I can cruise at around 50 with overdrive off or 65-70 with overdrive on. If I push beyond that the transmission slowly heats up to 230 degrees F and sometimes higher.

Yesterday we were driving in Big Bend NP, a slow, steep, winding road, and the transmission temperature kept rising and rising until it hit 250 degrees F. I pulled over and waited for it to cool down a bit before continuing. This seems way high to me.

The fluid level still looks right - when warm, I cycle through the gears, and the dipstick is showing smack dab in the middle of the hot lines. Fluid was changed about 20,000 miles ago. When I start the truck in the morning, transmission temp looks basically like ambient air temp, so I don't want to blame the sensor. I don't have an external transmission cooler, and the Denso radiator has about 25,000 miles on it. Engine coolant is mostly stable between 176-185. Note that about 2,000 miles back I rolled the 4Runner on Vail Pass in CO. It was turned off immediately, sat on its side for about 45 minutes, then flipped upright and towed back into town. We added some oil and coolant back in, trans fluid level looked good. I monitored temps when we drove it back over the pass and everything looked fine.

I'm not sure what to look for to see why it is overheating. Any ideas?

on hills/mountains, I turn the overdrive off. engine rpm will drop as well as trans temp

Are you getting and whining noise when in drive? Mine had somewhat of the same issue, over heating but functioning fine for the most part. turned out to be the torque converter coming apart causing extra heat due to slippage.

I'll give this a try. What concerns me is that I haven't had to do this on much tougher/longer hills at higher elevations to keep it 'in spec' even a couple weeks ago.

I have not noticed a whining noise, it sounds normal to me. I'll listen closer though. What was the fix for you, new transmission?

Ive heard of higher trans temps in higher altitude. Did it happen only at high altitude.

Nope. In fact, we were fine in the CO mountains. Now we are in Texas, 2000-4000 ft elevation or so, and it is overheating.

It's not overheating. It's getting hot.
If you change the fluid regularly it should be ok.

I find sometimes running with O/D off actually increases the temps.

If you install a cooler in series with the rad, temps will drop significantly.

Remember, before we got scan gauges or monitoring apps the temps were this high and we didn't know. Things seemed to work ok for millions of vehicles.

I rebuilt mine and am also in Texas. I don't ever see temps over 205 now with a external cooler and bypassed the radiator. I also agree that yours is running hot but not overheating.

I appreciate the perspective! I tend to obsess over small problems with the truck, so it's good to hear that. I will see about putting a trans cooler on - its weird finding a place to work on it because we are on the road full time right now.

Wouldn't Engine RPM's be higher with the O/D off?

How old is your radiator?
Are the AC condenser fins clogged with bugs and dirt? Night driving in south east TX and Louisiana (I lived there) area pulls millions of bugs into the AC condenser which could get clogged up in days.

Fan clutch functioning properly?

I had no problem while I pulled my car in the back of my runner from east TX to west TX with all my family members in during a HOT summer.

I think a transmission cooler in series (not separate) will help. The problem is that the engine puts out a lot of heat and the transmission also puts out a lot of heat. Sometimes that can just overwhelm the system. Also make sure there are no obstructions in front of the radiator such as lights and check the fins on the ac radiator to make sure there is no blockage so cooler air can pass through to the radiator. As a test the next time you see it get over 230 open the windows turn off the ac and crank the heat on full, If the temp drops pretty quickly you know you have reached your cooling capacity

on level ground, yes they would go up.
but going up some of the hills around here, and mountain passes, the rpm drops with the same road speed.
try it on a long steady uphill road

I learned this when my trans was running hotter than I wanted ( torque pro) and others on the forum told me to turn off the OD