View Full Version : OT Transmission Cooler

06-29-2008, 09:20 AM
Hi Guys,
The shop is packed with mill, lathe, rotary table and index head all crated for the move to Alaska. Though my household goods and shop are to be loaded on an ocean-going container and shipped to Alaska, I will be driving cross country (from Wisconsin) and connecting with the ALCAN Highway. My vehicle is a 1994 Toyota 4-Runner (6 cyl with automatic transmission) and I will be pulling a 5' X 10' UHAUL trailer. For the most part the trailer will be empty as I will use this as a means of transporting my UltraClassic Harley during bad weather; otherwise I ride the HOG and the Frau will drive the car.

When I came from Alaska to the Lower 48 I pulled a trailer and had problems with my transmission overheating when going up long "slow grades" or in mountainous areas. When overheating occurred, or just prior to the transmission light coming on, there was a noticeable loss in pulling power. I had to pull onto the shoulder and drive slower in a lower gear or I had to completely stop and let the transmission cool before proceeding. I had no *known problems* on "straight aways".

Just recently I was advised to get a transmission cooler. My question are:

1. Was loss of power due to lack of horsepower or was it the direct result of my transmission over heating?

2. Will a transmission cooler "fix" this problem or would I be waisting my money having the cooler installed?

Harold :)

06-29-2008, 10:04 AM
I wouldn't think the transmission overheating related to the loss of power unless;

1.The transmission was slipping - this is unlikely or you probably wouldn't have made it back.
2. The engine computer senses the overheating and reduces the power to try and compensate. Without a separate transmission cooler the transmission fluid is cooled by pumping it through the radiator (in a separate chamber from the engine coolant) so an overheating transmission can also add to increased engine coolant temperatures.

A transmission cooler is recommended for towing. They are relatively inexpensive and a wise investment in my opinion. Every "factory" towing package for automatics I've seen includes a transmission cooler.

Did the engine overheat too? Do you remember your water temperature?

I wouldn't think an unloaded or lightly loaded 5x8 trailer would be outside the towing capability of the 4-runner, especially if it is the typical U-haul low profile trailer. Slowing down helps a lot too - most of the extra load is from wind resistance.

If the transmission has on overdrive lockout feature make sure that is activated, or set the gear selector so that overdrive isn't engaged.

If it were me I would put the transmission cooler on.

A.K. Boomer
06-29-2008, 10:10 AM
Harold, First off I hope to god you immediately dumped that fluid ASAP -- if it got that hot then it IS to some degree (no pun intended) thermally broken down,

Im hoping that your power loss was due to the fact that you were gaining altitude (normal for any N.A. engine), otherwise you really got problems, if it was due to added friction inside the trans then there is no disputing that something is hashed to some degree, the torque converter can slip with overheated fluid also - so if the engine was reving higher yet you werent going faster this can be a bad situation and the heat can snowball real fast.

A cooler will definitely help and so will good driving habits, one of the worst heat factors with an auto in the mountains is "shift cycling" --- do not allow the trans to choose its own speeds going up steep grades, every time it searches between gears is added heat in the internal clutch packs - find a gear suitable and manually stick it there, As far as on the highway most vehicles are incorporating a torque converter lockout system, its the best thing that has happened to the automatic transmission in a long time and in fact it basically converts it into a planetary manual gearbox as far as efficiencies, in fact its why some auto's are achieving better highway MPG numbers than there manual trans rivals - the auto's one-up some manuals because they get to run in thin fluid. Torque converter lockout systems only get used when the ECT senses all systems go, this means its under mild steady loads - then an electrical signal is sent to a servo that opens a hydraulic port that controls the dork converter.

06-29-2008, 10:10 AM
Synthetic ATF might help, too. AMSOIL sells it, probably others as well.

A.K. Boomer
06-29-2008, 10:17 AM
Very good suggestion SGW, in fact you would be hard pressed to find a component (including engine) that would benefit more (heat wise) with syn. than the auto trans, many companies have syn, atf, including Castrol and valvoline.

06-29-2008, 10:27 AM
Hi Guys,
Thanks for all the replies.

There was a question asking if the engine over heated. The answer is "no". Not one time did my engine over heat.

Having said that, I do remember the transmission slipping on more than several occasions while under load BUT this was only during the time that the transmission light was on. When I pulled to the side and stopped, allowing the transmission to cool, the transmission no longer slipped until I got to a steep grade.

I will immediately take your advise of having the transmission fluid replaced.

Having no need to pull a trailer while I have lived in Wisconsin, I have *never* noticed my transmission slipping. I have made several trips to Kentucky without incident.

With this added information, hopefully you guys will have a better understanding of the problem I had (past tense) and one I wish to prevent. If it takes changing the fluid and using synthetic I will do it; if it takes adding a cooler then I will do it. I simply want to make the trip uneventfully.

Thanks A Bunch!

A.K. Boomer
06-29-2008, 10:38 AM
I will immediately take your advise of having the transmission fluid replaced.

You will at the very least give it the best odd's of survival, BG also makes an ATF fortifier and my bro who has used it in heavy duty service vehicles swears by it, it also gets along quite nicely with synthetics, You may or may not have done any damage to the Hardened steel mechanical components of your trans but I can assure you that you have put your friction discs through a few changes, Pull your dipstick out and smell the fluid, it really should have a nice smell (and color), if there is any kind of burnt smell then thats proof once again, even if there's not, get rid of it... Good luck.

06-29-2008, 04:03 PM
One suggestion, when you get the fluid changed. Use a shop that does a complete trans flush. They have a machine that pumps out all of old fluid and replaces it with new. If you just drop the pan it only changes about 30% of the fluid.

With the combination of a big cooler and synthetic fluid i got 150,000 miles from a o/d trans in a 91 chevy pu. I also used an oil pan with cooling tubes in the bottom which increased the capacity by 2 qts. As mentioned in an earlier post, always tow with overdrive locked out.


ps. Do not tell the shop you have had problems with the trans, many shops will not flush a trans with previous problems.

06-29-2008, 05:51 PM
One suggestion, when you get the fluid changed. Use a shop that does a complete trans flush. They have a machine that pumps out all of old fluid and replaces it with new.

ps. Do not tell the shop you have had problems with the trans, many shops will not flush a trans with previous problems.

Nice suggestion Lee. Thanks!!

I also talked with a friend and he suggested that in addition to the Tran cooler that I consider installing an engine oil cooler. I've never heard of such. Of course, I had never heard of a Tran cooler either.

Have any of you guys had experience with an engine oil cooler and would that be worth the additional money?


06-29-2008, 07:11 PM
Have any of you guys had experience with an engine oil cooler and would that be worth the additional money?


Yes, but since you're engine wasn't overheating I don't think it would be necessary.

In addition to the trans cooler I also concur with suggestions for synthetic ATF and a complete flush and replace, not just draining from the pan.

06-29-2008, 07:25 PM
Engine oil coolers are a good bit less straightforward than transmission coolers, and although it would probably be a good thing, it might not be as directly cost-effective. Usually installing one of these requires moving the oil filter base, and adding an adapter. However, since your vehicle is a Toyota, and these things are modified, raced, etc. with some regularity, you might investigate what is out there.

As far as a transmission cooler is concerned, I would absolutely go for it. This is a cheap and easy modification because the transmission is already equipped with cooler lines that run to a rudimentary cooler/heat exchanger built into the radiator. An auxiliary cooler simply plumbs into these lines and hangs on the front of the radiator.

Depending on the model of transmission, you might want to consult some tranny experts on the synthetic fluid question. I have some familiarity with the Jeep Cherokee Aisin-Warner transmission which is shared by some Toyotas, and many people are of the opinion that these will slip more with synthetic fluid, although they are very robust and dependable with regular fluid. It's probably more important to make sure you have clean, unburned fluid than to worry about what kind it is.

Mark Hockett
06-29-2008, 07:52 PM
If your vehicle has a V6 it might already have an engine oil cooler. It mounts under the oil filter and has two hoses running off of it. Here is what the adapter looks like,

Also if your 4Runner had a factory tow package it should have a trans cooler, but if it doesn't you should add one for the trip. There are two styles of trans coolers, One is much more efficient than the other. The style you want uses the cooling fins as the tubes (33% more efficient than the tube style)
Here is a link showing one,


The style you don't want has round tubes in an S shape. Here is a link showing the one you don't want,


It might be possible to get a factory trans cooler from the dealer if you don't like putting aftermarket stuff on your vehicles.