I agree, but I have never gotten a Jeep transmission hot enough off-road that a cooler would be needed. But, if you do, then you should use a cooler.

FWIW, the JK seems to do better than TJ's did, but I don't have any hard test data in any side-by-side setup.

Jeff

 

That's true, but transmission fluid is also designed to be most effective in a certain operating temperature range. Too cold and it loses fluidity and isn't efficient. One source indicated that the optimum temperature range is between 175-225, but I imagine that is different depending on the specific fluid -- e.g. synthetic has a wider range and can operate better at lower temperatures.

 

Unless you are way up north in the winter, it's not going to run too cold. Running an automatic tranny at 180 plus in many stock situations doesn't make much since . . . look at the rating of some of the tranny fluids. On one of my Broncos I have the radiator by passed on on one I don't and there isn't much difference on temp fluctuation. If I were in Minnesota etc. I wouldn't by pass the radiator and then there wouldn't be a risk of the fluid running too cold. My Bronco trannies only hit 180 under a subtantial load in Jul/August. I've gone to 210 only a couple of times and that was towing in the SW desert in Jul towing 6000 pounds at 70 and even then the temp cooled back down.

The point I'm trying to make is that if an auto is going to be even moderatley punnished whether towing or offroad etc., extra cooling will give it a longer life. Arguing that an automatic tranny isn't going to benefit from extra cooling doesn't make much sense to me.

 

Too much cooling is just as bad as not enough. Never exceeding 180 at cooler line temps is not good. I live in MI and you really need a bypass system if you see consistent temps below freezing.

I did OEM trans tests in a dyno lab for several years. The test required that we did not take shift data until trans temps were 190 degrees. If the temps fell to 180 f during the tests we had to start over. Our cooler circuits did not do into full cool until 220 degrees. These temps were all pan temps, cooler line temps (before cooling) are often higher by some number of degrees.

There was talk of the OEMs using PWM solenoids on cooler circuits to limit flow to aid warm up and/or eliminate rad coolers. Never actually saw that materialize, but I'm not doing that work anymore. Stupid economy .

 

Anything below 160 (approximately) is too cold for normal operating temperatures. Using synthetic will lower that number some. But, if your fluid runs under that temperature you do not have the fluidity required and you will not get the lubrication you need. It will not cause fluid breakdown like high temperatures, but I'd rather the fluid breakdown than the transmission not get the lubrication it needs. And just to clarify the fluid doesn't just breakdown immediately when you hit 200+ -- it starts to breakdown. A chart on the Amsoil site seemed to indicate that it was 450 hours for conventional and twice that for theirs. Personally, I change my fluid more often than the recommended severe duty level, so I should never come close to any of that.

 

conventional tranny fluid starts breaking down above 220 degrees and at 300 F it is measured in hundreds of miles instead of thousands. Like I said in an earlier, with an additional cooler the radiator or equivalent shouldn't be bypassed in cold climates.

Edit: I have 3 Broncos and and F350 in the yard and the all came from the factory with an auxillary tranny cooler. I just added and additional one to one of the Broncos and the F350 along with temp gauge with the sending unit located on an auxillary tranny filter located before the fluid enters the radiator. I'm not too far off base with what they did at the factory.

Edit Edit: here is an interesting article made by Michigans Forest Fire Experiment Station: http://www.roscommonequipmentcenter....notes/nn15.pdf

 

Nothing turned up on the 1st 3 or 4 pages saying running a tranny at 120 to 150F was going to trash it. Normal "factory" operating temperatures are at 175. Trucks & 4x4s have been coming out of factories for years with auxillary tranny coolers. They aren't going to cool the tranny below ambient temparture. In fact they won't even get it close to ambient. If the vehicle is in 0 F or below 0 F, that's another issue all together even without a cooler. There are units out there to regulate the cooler flow in extreme low temps.

 

Coming with a tranny cooler from the factory is a totally different story. They obviously determined it was necessary in those cases to keep the fluid within an optimum temperature range.

 

Often the cooling is minimal from the factory depending on model and make.
However, if the vehicle is going to see sever duty such as towing or offroad use, cooling needs to be upgrade or the tranny life will be reduced dramatically.

We all know that jeeps trucks etc. come from the factory exactly the way we need them or want them . . . the have the right size lift, the right diameter tires . . . when people mod their rigs they often forget about their auto tranny whether its engine, gearing or tire size.

 

I've had my transmission slip on my a couple of times already (16k miles). Twice while pulling a trailer uphill, and once while offroard in 4Hi going up a hill that wasn't all that steep. That's why I looked for this thread...I'm thinking I need a tranny cooler--> or the dealer needs to fix my transmission.