High transmission temp.

The lower fan speed is only the icing on the overheating cake (sorry). In 4hi, the lower rpm makes the fan less capable of cooling, precisely when the transmission is making ungodly amounts of heat. It’s making much more heat than normal because slow speed/low rpm with load forces the torque converter to stay in it’s fluid coupling mode.

Once you get an OBD app and dongle to have live temp readings (another recommendation for “Car Scanner” app) make sure to pull up live data on torque converter state (on/off) and fluid temps post converter. You’ll be shocked, under any amount of load, the post-torque converter temp reading will immediately and quickly increase whenever it’s unlocked/slipping. There’s no stopping the increase until the converter turbines lock and they stop spinning the fluid. The 4runner, nor any vehicle, does not have the cooling capability to deal with a constantly unlocking torque converter.

In effect, the transmission fluid itself is being used as a sort of “clutch” by the torque converter. If you compare the torque converter to a clutch in a manual trans car, slipping it should only be done briefly during the time it takes to match the crankshaft speed to the transmission input shaft. If the difference is too great, you don’t slip the clutch to keep the engine from stalling, because it would cook the clutch pretty immediately. Instead, you should downshift to get a better match between engine speed and transmission input shaft.

So basically, by going uphill in 4hi, you basically asked the automatic transmission to slip the clutch to keep the engine from stalling.

edit to add: in 4Lo, you may still see the torque converter in an unlocked state often, but the improved gearing makes for very little load being transferred between crankshaft and transmission. Low load means low heat.

 

Doing a drain and fill Thursday, an obd reader on order and will try 4L.

 

I just got back from a trip towing with my OBDlink. 95 deg outside, towing 3500 lb on hilly roads at 70mph in S4 it stayed around 195ish. When bigger, slower hills were encountered pan temp got to 220 and converter briefly saw 232. I was thinking about a cooler but not sure no longer than I see temps over 220. I don't see the benefit yet and may just change the fluid every 25-30k. I agree OP should have been in 4L, it's much easier on everything, also looks like you have an aftermarket front bumper and maybe a winch? If so be extra mindful of airflow. you might also check your fan clutch, at 107k miles it could easily be shot, when worn out they no longer engage drawing more air as they should when temps start climbing.

 

Yeah bumper and winch have me thinking of adding one on my truck, they seem easy enough to install and aren't overly expensive. The only downside I see is they block a bit of the radiator, so food for thought. Pic of my current front-end setup.

 

Talking with a mechanic, he mentioned the bumper and winch blocking the flow as well.

did a drain, fill and will be ordering the iconic FJ transmission cooler.

 

Went down this same trail (Merus) not too long ago in my 4Runner. Great times to be had at that place.

 

You just taught me something. TY!

 

The transmission cooler still needs the fan for airflow, so rev red!