Scroll down to the bottom for a TL;DR

I read a comment on another thread that brought up a good point. They said that for about 2% of the 4runner owners this automatic transmission was not designed for. I'm not saying this is true, but that's how it feels from my experience. Here me out, this might be a long post.

I've installed a B&M 70264 transmission cooler in series with my radiator with a pusher fan on the aux tranny cooler. In the last 15k miles I have replaced the following; radiator, thermostat for a cooler one because of the supercharger, new water pump, new fan clutch and new coolant. I read all of these threads and I can relate to their numbers. The problem is when you use your 4runner outside of what 98% of people use their 4runner for. On flat ground, stop and go traffic my tranny temps usually stick around 165-190 depending on temperature outside. Even when I'm going up short inclines there are times where it usually sticks around 180-195 degrees, but once you crest it levels out and starts going down. This changes as the duration of the incline goes longer.

My problem is when Im towing(around 3000lb) really steep inclines for long periods of time or when off roading on really steep inclines going really slow for long periods of time. Yes, O/D is turned off. The button is not clicked in. Flat ground towing I stay round 180 all day long. Even short inclines it won't move a lot.

my last trip with minimal camping gear and no trailer, air temperatures are in the 90's, all day long on relatively flat ground with the occasion of hill that you need to crest, transmission temperatures barely move from 165-180. As soon as I hit a longer climb i start to see 200*+ but level out fairly quickly once I crest. Now when I get into the mountains and I'm off roading, very steep incline, going very slow(2-3mph) and it's for longer periods of time(15-20+ minutes), the temperature went to 250* capping out my gauge. At this point I pulled off on the next flat spot and waited 5-10 minutes and let it come down 30* before going again. This would give me another 5-7 minutes before I would hit 250 again, by the next time I was near the top. My pusher fan doesn't do squat for cooling under these situations and it feels like the aux cooler only really slows the heat process, but with enough time the temp will keep rising if you continue with the same conditions. Short to medium steep inclines on rolling hills, where you level out for a short period of time, temperatures usually hang around 180-195 while going fairly slow(10-15mph).

It feels like this transmission generates so much heat during the long periods of time either slow crawling or under heavy load while towing. I'm running into a few people with similar stories. Am I part of the 2% where this tranny cooling fails to deliver? Around town, stop and go traffic, short to medium duration inclines even in 100* ambient temperature it does just fine. It's like the cooling system doesn't effective keep the transmission temperature down when it's under load for a long period of time. Maybe thats normal and they didn't design to for those conditions.

I'm contemplating whether I should put another aux cooler in series with everything else. How will this affect the cooling system for the engine if I have two large transmission coolers in front of the condenser? Will this finally solve my issue? Anyone running two aux coolers up front?

TL;DR
Under unique situations I've hit 250* transmission temperatures with an in series external transmission cooler with pusher fan, should I consider adding a second one and will that affect engine cooling?


Edit: I should add that I'm running 255/85/16 KM2 tires, aftermarket rear bumper, winch plate with winch up front, sliders/skid plate and geared with 4.88s. So I do have some added weight that attributes to the extra heat.

Well there are Limits to Everything. It would appear to me you've bumped up against one.

Just to be sure, your dash OD off light is On, right? So you usually hit what kinds of RPMS on your long, steep inclilnes in 3rd or 2nd gear?

Yeah, I'd go bigger on external cooler or a 3 row radiator (w/internal tranny cooler). But overall you're bumping up against physical limits I think.

Luck and enjoy the Runner.

Yeah I know it's off for sure. When towing it very easy to tell if its on or off. Tranny temps go up fast if OD is on, dash light is showing. RPM's are high, 3000-3500 and sometimes past 4000 when I'm struggling on the really steep inclines. This is even with my supercharger. I hate how much I have to push the engine to maintain any respectable speed on the roads when towing. I've started to look at 4th gen 4.7 V8, but I can't bring myself to buying another vehicle for the soul purpose of towing. I can't bare to get rid of my 3rd gen since I've dumped so much money into it. Other than the slow crawling high tranny temps and crappy towing, I love this runner.

The amount of heat the torque converter puts in the trans fluid is huge when you are not driving around with the lock up clutch engaged in the TC.

Sadly this engines lack of bottom end torque and Toyota’s programming causes the TCC to unlock at any hint of a load.

I thought about making a latching push button circuit the manually lock up the TCC and it would unlock by me hitting the button again or by me hitting the brakes. I even drew the circuit and may have posted it here.

My 04 V8 Grand Cherokee would lock the TCC at 30mph if overdrive was disabled. And it had the ass to do it too.


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Bypass the radiator completely, I bet that brings them down some.

I'm not sure how bypassing the radiator would bring it down, I wish this was the case. There has been a few threads regarding this and the consensus is to run in-series for better cooling especially for applications like off-roading and towing. Running the aux cooler alone will make it worse, unless you know something that I haven't heard. With my current setup it's only problematic in unique situations like slow speed crawling or towing under long steep grades.

I'm definitely seeing the limitations of the 4runer unfortunately. It's like trying to turn a Ford into a Ferrari, doesn't matter what you do, it's always going to look like a Ford.

There's limitations to these rigs for sure but there's a definite fix for you when you're off-roading and climbing sustained grades. I'm guessing you are wheeling in 4hi. If you climb sustained grades in 4hi, your auto trans temps will go through the roof. If you put it in 4lo, your temps will drop dramatically.

Now, when you're climbing steep sustained grades on the road, 4lo can't help you unless you have manual hubs you can unlock or you have some other type of wire mod to disengage the front 4wd actuator and be able to use 4lo without binding the front axles and differential.

Anytime I'm off-roading and climbing sustained grades, I put it in 4lo and just drive slower. I'd rather keep it in 4lo and travel at 20mph or less than have to stop multiple times to cool my trans down. Since I have manual hubs, I've used 4lo on steep pavement climbs as well.

In the recent past, I was in Death Vally and the ambient temp was around 100 degrees. On a very long sustained road climb, I pulled over to let my engine and trans cool off. I ran the rpms up to 1500 to pull more air through the cooling fins and quicken the cooling process. This road was a fairly major road and I couldn't have gotten away with crawling up it at 20 mph so I did the next best thing and that's pulling over and giving your rig a rest when it's prudent to do so. This is why it's so important to monitor your engine and trans temps. It could make the difference between a short break on the side of the road and a break down where you're calling a tow truck because you fried your trans or blew up your motor.

Check out this video:

Pretty simple actually. Says you're armored. Probably have the same issue I had, where the radiator cooler was actually blocked by the bumper and essentially ineffective at doing anything except keeping the fluid in there at a constant 190 plus to match the engine, then up in the 230's if I was driving in town. It'd do the same thing, come back down when I got some forced air moving around it, but everything was too blocked to let the fan actually help any at all. Bypass with a fullsized cooler, now it sits at 140ish pretty consistent, occasionally up to 180 if I'm pulling hills and always drops back down immediately.

Just a thought, something to try. Obviously what you're doing now isn't working.

Last week I was driving my front-end-rebuilt-2000 limited up a long sustained steep dirt road to get to a trailhead. I have completely bypassed the trans cooler and realise that my B&M by itself doesn't have much capacity for a lot of cooling, so I just keep an eye on the fluid temps to be safe. Temps rose and got over 200. I'd stop for a few minutes to let it cool down and as soon as I started moving again it hit a max of 225. Today I searched and discovered that this isn't too bad of a temperature as long as it isn't constant so I was OK. I had forgotten about Timmy's 4Lo trick.

Another option to consider is to get a manual transmission 4runner. I have an auto and a manual (plus my mom has an auto that needs work to get back on the road).

Tomorrow I'm taking a shuttle to get back into Squamish so I can ride my bike back up that road to retrieve it. But I'll be driving back down this time so won't be able to test Timmy's 4Lo solution but I'll remember it for next time.

I appreciate the feedback, i was under the impression that these coolers in the radiator are liquid cooled. I wouldnt think air flow matters directly, unless you were seeing high engine temps. How big of a external cooler did you put in? I think it would have to be pretty big to effectively cool it when your rock crawling or towing up a steep incline for a long period of time. I read many accounts of people only running an external cooler only which is fine for normal use, but they get similar spikes going up hills and such. To be clear my temperatures are fine most of the time. It's in certain situations that I see it rise.

I cant recall if how much it was in 2wd and 4hi. It wasn't a technical climb but it was really steep and long. I usually save 4lo for the really technical stuff. I can't recall what temperature were in the past when I was going slow, but I feel like I have been in 4lo with similar temperatures, but I cant say for sure. I will definitely try 4lo next time and see if my temperatures are more manageable. What are you running as far as cooling, stock/external cooler? I'm thinking of adding another aux cooler.

Would like to know your results. Keep us posted. Your probably right, stressing over the small stuff. I did a drain and fill today. I've ran up to 230-250 multiple times within the last couple months and my tranny fluid looked bright red, barely any discoloration from when I did my last drain and fill about 3k or 4k miles ago. Honestly looked new. I'm probably overreacting in all of this. When I hit 250*, its never for very long. I mean really it's only really harming the fluid when you see those temps? I have no issues drain and filling more often to keep good fluid in there.

I'm running a Hayden 679. I've got it mounted as high as you can go to get above the winch plate. I'm fairly certain that it is larger than the stock cooler is. Yeah the transmission will get warm, but it just gets warm and stays there rather than continuing to climb. I did put a fan clutch in while I was at it and sitting at an idle with the AC on after driving across town it'll drop from 170 to 150 in about 5 minutes. Again, I'm not saying that'll cure the problem, I'm just saying it may be worth a try.

Your statement that you reserve 4Lo for only really technical stuff tells me you probably were in 4Hi or 2wd for a lot of the situations your trans got very hot. I guarantee you will see a huge difference once you start using 4Lo for all consistent grades off-road. It doesn't have to be steep for your temps to spike. If just has to be consistent and long and your temps will spike.

I'm running a stock radiator and a B&M 70264 external trans cooler ran in-series with the radiator cooler. I have a CBI front bumper and skid plates so the air flow has diminished some through the cooling fins of the radiator and the external trans cooler. I do see higher engine and trans temps in this rig as compared to my 2000 which is mostly stock. I estimate I'm carrying around an extra 700 lbs all the time with the bumpers, skids and roof rack on my 98.

Where is your gauge sender? The OEM sender (available via OBD-II on '99+ trucks) is on the send line to the OEM cooler, the HOTTEST location for trans fluid. That's why turning off OD lowers temps ~20*F at that location almost immediately. The average fluid temp is ~20*F lower than the OEM gauge shows in most situations (and lower than that if you have an extra cooler inline).

Speaking of OD off - it only helps lock the torque converter above 45mph, it won't lock below that and won't help trans temps below that speed. OD on allows the torque converter to lock above ~50mph (stock tires/gears), but will unlock much quicker under load. If you are towing with high loads or below 45mph, OD off won't help trans temps much because the torque converter still can unlock.

Using 4LO on steep/long/slow climbs isn't a trick - its what you are SUPPOSED to do. It puts less than half the torque load on the transmission with that extra 2.28:1 gear reduction.

I'd keep a close eye on engine coolant temps too - it is possible your engine fan clutch is weak and not pulling enough air while you are at higher RPMs.

-Charlie