This will get responses all over the place. I'll get my 2 cents in so everyone can disagree early.

Nope. Don't do it. You'll either have too much cooling or too little, and that will change with the weather and your driving. The reason it goes through the radiator is A) water is a MUCH better conductor of heat than air, and B) you want it to be the correct temperature, not just random bulk cooling.

With a radiator exchanger, you will nearly always be able to shed enough heat while also maintaining the proper viscosity/heat.

Cooler isn't better, correct temp is better.

 

Curtis is right on. I did some research years ago and found out the exit temp of a radiator should be about 150 degrees (bottom of radiator). The trans cooler in the bottom of the radiator sees a constant temp which in turn provides a constant temp at the transmission. Note the valve body in the trans is aluminum and other parts in that valve body are steel. Critical that the temp in the trans be where designed. One other factor is a torque converter that is designed to lock up. Years ago I was stuck in traffic in my 33 Willys which had a 4L60E trans with a temp gauge. Trans temp went up to 210-220 area then traffic cleared and once up to speed and locked up the temp came down quickly.

 

The in-radiator coolers have advantages, but there are also external coolers with a better design. The most obvious design improvement is a stacked plate cooler, which works much better than a simple serpentine tube with fins. The cheap coolers in the parts stores are usually just a serpentine tube.

Long, Mfg. Also makes some high capacity stacked plate external colors with a built-in temperature bypass. When the trans fluid is cool it does not circulate through the finned part of the cooler, so it warms up faster. Many of their coolers also include metal mounting brackets, so they don’t rely on plastic ties through the radiator.

 

I believe Ford, at one time, had a bypass for the cooler (OEM). Not a bad idea.

 

For the most part the TH350 isn’t hard to keep in range unlike the 700R4 and its progeny. If you’re towing and live where the weather is hot and the engine is given to overheating then there is something to be said for an external cooler.

There are temp bypasses available that put in line can bypass the external cooler when needed.

Coolers can be plumbed to the external first to chop off the high heat then to tge radiator to warm or cool the fluid based on engine coolant temp.


So there are some ways to slice this problem.

Bogie

 

I have done that in the past, and it never made me sleep all warm and fuzzy, although you're right. There are solutions. Too complicated, and without a trans temp gauge, I was always afraid of one of those bypasses failing and toasting things.

I also can't imagine that buying and plumbing all that would be cheaper than just getting a radiator with the extra circuit and doing it "right."

You'll get a lot of other opinions on putting an external cooler before or after the radiator. I always say before. For the same reasons I mentioned before, if you put it after, you will almost certainly overcool the fluid. Putting it before means you either undercool (too small) but you're still helping the radiator shed heat, or overcool (too big) but the radiator can return it to the proper temp. Putting it after means you'll have correct temp fluid leaving the radiator 95% of the time, and then overcooling it to send it back to the transmission.

 

I have the original TH350 trans in a 69 C10 pickup. It's had a small stacked plate cooler for around 25 years, which I think is plumbed ahead of the radiator cooler. I never have measured the trans fluid temp, but perhaps I should some day. I've always thought the cooler does some good, especially here in Texas heat. The trans was rebuilt over 30 years ago, and still has crisp shifts, so maybe I should leave well enough alone.

 

For whatever it's worth. I work for an aftermarket transmission parts company. Some of our stuff has been in winning drag week cars etc. etc.
I asked our high performance product line manager about coolers some time ago and he recommended the following:

TRUCOOL LPD47391 13 plates rated for 45,000 BTU -- sufficient as a stand alone cooler. There may be others out this is what he recommended.
Tru-Cool Max. Twice the Cooling for Your Transmission. Maximum Protection for Your Vehicle. | Tru-Cool

 

I just remembered that have a buddy who is using a Derale cooler like this, or similar, as the only trans cooler in his car. It's controlled by a temperature switch in the side of the 700R4 that closes at 170 deg, as I recall. It actually keeps the temp in the range of 170 even though it's mounted horizontally under the car.

DERALE Electra-Cool Remote Cooler, 16 Pass, -6 AN Inlet 13700

DERALE Electra-Cool Remote Cooler, 16 Pass, -6 AN Inlet 13700 With its own air supply available when needed, the Electra-Cool is one of our most popular remote coolers and can literally be mounted anywhere space permits, making it a popular addition for performance compacts, hot rods, muscle...

motionraceworks.com

 

The Trucool trans coolers I referenced in post 9 used to be called the Long, Mfg. Low Pressure Drop (LPD) coolers. Looks like they are now owned by Dana. Excellent build quality, and very effective coolers.

At one point in time the B&M trans coolers appeared to be a rebadged Trucool, but I don’t know if that’s still the case.

 

I was told that many of the aftermarket coolers are not really intended to be run stand-alone. The one I referenced above by TruCool is capable of handling the load of a transmission however.

 

I am in the SoCal desert. I currently have six vehicles that run - all automatics. One might be using the heat exchanger in the radiator. The way it is parked I cannot get under the hood to look. All the others are stand alone.

The 63 F100 came with a standard trans, so it did not have a heat exchanger. For several decades it ran a 425/T400 switch pitch with a heat exchanger in front of the radiator. It is now in the middle of a frame off complete redesign. It is getting an exchanger with fan on the frame.

The 12K# E350 7.3/4R100 Power-stroke - was bus now RV - uses what was the the rear AC condenser - on the frame, about four feet long, with three fans.

The 9K# E350 7.3 IDI/E4OD has a heat exchanger with fan on the frame.

The 4K# 460/C6 Wagon has an exchanger with fan in the front off to the side of the radiator.

The 65 Mustang 427/AODE and 73 Mustang 351/C6 both have exchangers in front of the radiator.

Most now have trans temp gauges.

I will continue to use stand alone, because now when I do a system everything gets a triple pass radiator, and they don't have trans heat exchangers.

 

Using this as a stand alone trans cooler under rear of bed with a 195 electrical switch and a 8" fan/homemade shroud. Zero problems cooling TH350 with custom converter that will stall to 4100. And best thing is it does not contribute to overheating the engine.
https://www.amazon.com/gp/product/B004XONRX6/

 

This works just fine to keep my 700R4 cool no matter how hot the mercury gets. The fitting in front of the cooler is for a trans temp gauge.

 

Is that all you're running for a cooler? I read that the manufacturer wrote, not for use as a stand alone cooler.

 

I was always wondering, if a person uses an external cooler with the radiator, how is it plumbed? On a th350, does the top line or the bottom line go to the external first? Which is the output, I guess I'm asking?

 

My preferred is for the ATF to route to the external first. The idea is to take some heat load off the engine’s cooling system but to also use that system to add back some temp to the ATF when the weather is winter cold. I try to keep the ATF around 160-180 degrees F.

If you live where winters are really cold you might want to include a bypass thermo valve to isolate the external cooler.

Bogie

 

I was having engine overheat issues when I put that in. I figured that anything that removed heat load from the radiator could only be a good thing. So...separate trans cooler hanging between the front frame horns behind the front bumper where it'll get Plenty of air flow but still be protected from road debris.