Auxiliary Transmission Coolers

[Serbonze]

There is an interesting discussion starting in another thread, but I really don't want to clutter up the intent of the original post.

The discussion is around the plumbing of auxiliary transmission coolers. When I had my 32rh rebuilt, they also installed an auxiliary cooler and bypassed the radiator cooler completely. It's been awhile, but I believe they said that the auxiliary cooler would keep the trans fluid cooler than the cooler at the bottom of the radiator and the radiator cooler would introduce additional heat. Note that I live in the southeast so I have no need to warm the transmission fluid, I don't ever see cold mornings where that is an issue.

Anyway, a few interesting posts popped up in the other thread:

The ATF cooling tube in the bottom of the radiator is a timed tested method that is as or more effective than a stacked plate or tube and fin external cooler (compare fluid to fluid heat exchange rates to fluid to air heat exchange rates.) It is still the primary method for cooling automobile ATF.

Unless the ATF cooling tube in the radiator is damaged or defective there is no reason to bypass it. In fact, bypassing the radiator and and re-routing ATF to an external cooler alone often results in less cooling, not more.

For a TJ an external cooler should be an add-on, not a substitute.

The fallacy with that argument is that the ATF cooling tube runs through the bottom of the radiator adjacent to where the already cooled coolant is returned to the engine. The temperature of the coolant in contact with the ATF cooling tube in a warm or even a hot engine is well within the normal operating range of ATF, never at 200 degrees.

Yeah I was just using that number as an example. But let's say it's 150 degrees, that still means that it is unnecessarily heating your transmission fluid up if the transmission was operating at 120 degrees. Like I said, some people will disagree with me and that's just fine, we all have our opinions and experiences and I have zero experience with jeep transmissions, just other experiences.

These two gentlemen bring up some good points.

It seems that there are a few running debates on how to properly plumb in an auxiliary cooler:

Transmission > OE Radiator Cooler > Auxiliary Cooler > Transmission
Transmission > Auxiliary Cooler > OE Radiator Cooler > Transmission
Transmission > Auxiliary Cooler > Transmission

I know that our TJs want to run at 210 degrees, but what is the temperature of the fluid in the radiator when the Jeep is up to operating temperature? Is it 210 degrees?

Is there a chance of heat soak from the coolant in the radiator to the transmission fluid in the OE cooler in the bottom of the radiator?

 

[glwood]

Thanks for asking these questions. The other thread was interesting to follow, but was getting confusing. I like how you simplified it. I'd like to know this also.

 

[Serbonze]

I’m not usually afraid to ask a question, I just want what’s best for the longevity of my Jeep.

 

[glwood]

As do I. I know these coolers are usually recommended for Jeep auto transmissions, but I would, like you, want to know the most effective plumbing, prior to deciding which type to get.

 

[balfred]

Lol, this is such a religious debate usually, I'm sure you will get some very passionate responses from either side. I have talked about this and my experience on a couple other forums and typically the haters saying to keep the radiator plumbing haven't ever tried bypassing it to see the difference so their stance is blindly biased. I'm really not trying to start a war, I just know what I know from experience in many autos but like I said, never done it on a jeep.

 

[balfred]

I will add this, regardless of if you keep the radiator plumbing or not, what you really need is a plate style cooler as they are more effective and since jeeps experience harsher conditions like crawling at low speed you probably want a fan to be safe. The Derale unit seems to be a popular one for wranglers and is a plate style with a fan so that's good. I have always used B&M Super Coolers but it's the same concept really, the super coolers just come in so many different sizes so it's easier to find one to fit in the grill if that's the spot you are aiming at. One truck I did it on only had room for a littler one so I had to get two littler ones and hook them together in order to have the right cooling, just something to keep in mind

 

[glwood]

I don't want to muddy up this thread, but it would be nice if someone started a thread with a list of what's available for TJs as far as transmisson coolers...

 

[Serbonze]

Lol, this is such a religious debate usually, I'm sure you will get some very passionate responses from either side. I have talked about this and my experience on a couple other forums and typically the haters saying to keep the radiator plumbing haven't ever tried bypassing it to see the difference so their stance is blindly biased. I'm really not trying to start a war, I just know what I know from experience in many autos but like I said, never done it on a jeep.

I would think, to really know, one would have to install a temp gauge and try the different combinations.

Someone may have already done this, I just don't have the data.

 

[Chris]

Wouldn’t the one way to really put this to the test be to get a transmission temp gauge and monitor the temps with the factory cooler, then monitor the temps with just the aftermarket auxiliary cooler, and then monitor the temps with both?

 

[balfred]

Wouldn’t the one way to really put this to the test be to get a transmission temp gauge and monitor the temps with the factory cooler, then monitor the temps with just the aftermarket auxiliary cooler, and then monitor the temps with both?

I would think, to really know, one would have to install a temp gauge and try the different combinations.

Someone may have already done this, I just don't have the data.

I actually did this very thing when testing the vehicles that I did. Trans temp gauge was installed to monitor the Temps of the stock cooling then I would install the bypass and the super coolers and see the difference.

 

[mrblaine]

I would think, to really know, one would have to install a temp gauge and try the different combinations.

Someone may have already done this, I just don't have the data.

Finally. I see many posts from folks about whether they do or don't need a cooler. Put a trans temp gauge in. Monitor the trans temp, not the fluid at its hottest point or at its coldest point, monitor the only thing that matters and that is the transmission temp. When you monitor engine temp, you do NOT put a temp sensor that reads the flame front on the combustion mixture as it explodes so why would you want to know the temp of the fluid exiting the converter where it is the hottest it will ever get and starts cooling almost immediately on the way through the cooling system?

Here is what experience has shown us. Fan mounted coolers under the tub work at low speed, they do not work well at highway speed. Grill mounted coolers work at highway speed, they do not work well at low speed.

I prefer coolers in addition to the radiator liquid to liquid heat exchanger because that buffers the temps and keeps them more moderate. (see part about grill versus fan driven)

 

[Michael Alzen]

Finally. I see many posts from folks about whether they do or don't need a cooler. Put a trans temp gauge in. Monitor the trans temp, not the fluid at its hottest point or at its coldest point, monitor the only thing that matters and that is the transmission temp. When you monitor engine temp, you do NOT put a temp sensor that reads the flame front on the combustion mixture as it explodes so why would you want to know the temp of the fluid exiting the converter where it is the hottest it will ever get and starts cooling almost immediately on the way through the cooling system?

Here is what experience has shown us. Fan mounted coolers under the tub work at low speed, they do not work well at highway speed. Grill mounted coolers work at highway speed, they do not work well at low speed.

I prefer coolers in addition to the radiator liquid to liquid heat exchanger because that buffers the temps and keeps them more moderate. (see part about grill versus fan driven)

When you say monitor the trans temp, would that be with a bung into the pan? I've been debating mounting a sensor in the pan vs on the return line after the cooling system, or maybe both with a toggle switch.

As for where to mount the cooler, my understanding is that the best place is after the radiator going back into the trans. If there's heat soak then the aux cooler will help take care of that, and if it's winter then you want the radiator there to warm up the fluid to operating temp. The aux cooler has a bypass for when the fluid is already cool so in winter it shouldn't over-cool during warm up.

 

[balfred]

A bung in the pan is your best bet usually

A lot of transmissions have a test port already threaded so you can just use that instead going through the pan hassle

 

[Jerry Bransford]

There's always some who push to eliminate the OE cooler when installing an aux cooler or run the aftermarket cooler before the OE cooler. Neither made sense to me so before installing my first aux cooler, I called and spoke to both B&M and Derale who are the two leading providers of aux transmission coolers. Both agreed... keep the OE cooler in the circuit which also helps warm the ATF on cold days to get the transmission working better sooner, and to plumb in their aux coolers after the OE cooler. The OE cooler serves to keep the ATF temperature within a certain range but when the ATF Is too hot from something like towing a trailer up a mountain or rock crawling, that's when the aux cooler helps provide that extra amount of cooling. That was my thinking before calling them and it was good to have it confirmed.

 

[Mr. Bills]

. . . typically the haters saying to keep the radiator plumbing haven't ever tried bypassing it to see the difference so their stance is blindly biased. . . . .

I have bypassed the radiator in favor of an external cooler in another vehicle (non-Jeep) at the recommendation of some well-meaning "internet experts" who cited a history of the particular radiator involved suffering from manufacturing defects that caused coolant to mix with ATF. However, the result was hotter ATF temperatures and an overheated automatic transmission in the middle of Death Valley that put my rig into limp mode and made for an extremely long day. The proper solution was to replace the factory radiator with an aftermarket unit without the manufacturing defect, maintain the function of the ATF cooling tube in the radiator, and use the external ATF cooler as it was originally intended - as an auxiliary or supplemental cooler.

I am no "hater" and am basing my recommendation not to bypass the ATF cooling tube in the Jeep TJ radiator on fact rather than "blind bias."

Fact: Before the coolant in an engine warms up enough to open the thermostat there is essentially no flow through the radiator, thus the ATF cooling tube in the bottom of the radiator does nothing to warm or cool the ATF.

Fact: After the coolant in the engine is warm enough to open the thermostat, the hot coolant enters the radiator at the top and cools as it passes through the tubes to the bottom, thus coolant is at its lowest temperature at the bottom of the radiator where it is in proximity to the ATF cooling tube.

Fact: Automatic transmissions are typically designed for the circulating ATF in a warmed up vehicle to range from 165-195 degrees with occasional temperature spikes when under heavy load. Most ATF can withstand normal operating temperatures of around 200 degrees for tens of thousands of miles, but begins to break down if the temperature of the ATF rises above 220 degrees.

Fact: A Jeep TJ's coolant temperature typically reaches its maximum at 210 degrees. It will always be less than 210 degrees at the bottom of the radiator in proximity to the ATF cooling tube because it has already been cooled by the radiator. Since radiators typically cool the circulating coolant 10-15 degrees as the coolant passes through the radiator, the cooled coolant will be within the acceptable temperature range for ATF

Fact: Liquid-to-liquid heat transfer is more efficient than liquid-to-air heat transfer. The ATF that passes through the ATF cooling tube will be quickly cooled to the same temperature as the coolant, which as mentioned above will itself be within the proper temperature range for ATF.

Fact: External ATF coolers installed by auto manufacturers as original equipment are typically designed to augment the cooling by the ATF cooling tube in the radiator, not substitute for it. Stacked plate external coolers are slightly more efficient than tube and fin coolers; either design will provide up to approximately 10 degrees of additional cooling. A fan for the external cooler may be necessary for slow speed work in 4WD low range to keep air flow through the cooler, but a grill mounted cooler without fan may be enough (it has been for me so far, and if needed I can add a 500 cfm 12v Derale fan).

You can by-pass the ATF cooling tube in your radiator in favor of an external liquid-to-air cooler if you want, but if won't be as efficient as the OEM system. The better practice (unless the radiator is defective or malfunctioning) is to maintain the OEM cooling system as originally designed and add a supplemental external cooler if your transmission temps frequently spike over 200 degrees or you want the peace of mind of additional cooling for trailer towing or slow speed four wheeling.

The proper routing, evidenced by most OEM auxiliary coolers, is: Auto Trans to Radiator ATF cooling tube > other end of Radiator ATF cooling tube to top of External Cooler > bottom of External Cooler back to Auto Trans. This allows the radiator to cool the ATF before it gets to the external cooler which cools it further. If you route the hot ATF to the external cooler first it cools the ATF to approximately the same temperature as the already cooled coolant which means that the ATF cooling tube in the radiator will do little or nothing to bring the ATF temperature down further.

And that's all I have to say about that.

Well, almost all:

On my "other" 4WD vehicle I ended up changing out the radiator for one without the alleged manufacturing defect so I could maintain use of an ATF cooling tube in the radiator, kept the OEM external ATF cooler (included with the trailer tow package), and added an inexpensive Derale fan operated by a thermo switch that turns on the fan when the ATF entering the external cooler reaches 185 degrees and turns it off when it drops to 165. No overheating of engine or transmission since, even on long hot days in 4WD low range and the A/C on.

 

[balfred]

Like I said, we all have different experiences and mine do not include jeep autos. I have found the bypass to be beneficial on other vehicles but that's me and my experience and yours might be different and that's fine. If you want to be right then that's also fine, I admit defeat. I just like forums where different opinions based on experiences are voiced and you get to see the difference in them like is happening here, this isn't a bad thing

 

[Hayduke]

I have bypassed the radiator in favor of an external cooler in another vehicle (non-Jeep) at the recommendation of some well-meaning "internet experts" who cited a history of the particular radiator involved suffering from manufacturing defects that caused coolant to mix with ATF. However, the result was hotter ATF temperatures and an overheated automatic transmission in the middle of Death Valley that put my rig into limp mode and made for an extremely long day. The proper solution was to replace the factory radiator with an aftermarket unit without the manufacturing defect, maintain the function of the ATF cooling tube in the radiator, and use the external ATF cooler as it was originally intended - as an auxiliary or supplemental cooler.

I am no "hater" and am basing my recommendation not to bypass the ATF cooling tube in the Jeep TJ radiator on fact rather than "blind bias."

Fact: Before the coolant in an engine warms up enough to open the thermostat there is essentially no flow through the radiator, thus the ATF cooling tube in the bottom of the radiator does nothing to warm or cool the ATF.

Fact: After the coolant in the engine is warm enough to open the thermostat, the hot coolant enters the radiator at the top and cools as it passes through the tubes to the bottom, thus coolant is at its lowest temperature at the bottom of the radiator where it is in proximity to the ATF cooling tube.

Fact: Automatic transmissions are typically designed for the circulating ATF in a warmed up vehicle to range from 165-195 degrees with occasional temperature spikes when under heavy load. Most ATF can withstand normal operating temperatures of around 200 degrees for tens of thousands of miles, but begins to break down if the temperature of the ATF rises above 220 degrees.

Fact: A Jeep TJ's coolant temperature typically reaches its maximum at 210 degrees. It will always be less than 210 degrees at the bottom of the radiator in proximity to the ATF cooling tube because it has already been cooled by the radiator. Since radiators typically cool the circulating coolant 10-15 degrees as the coolant passes through the radiator, the cooled coolant will be within the acceptable temperature range for ATF

Fact: Liquid-to-liquid heat transfer is more efficient than liquid-to-air heat transfer. The ATF that passes through the ATF cooling tube will be quickly cooled to the same temperature as the coolant, which as mentioned above will itself be within the proper temperature range for ATF.

Fact: External ATF coolers installed by auto manufacturers as original equipment are typically designed to augment the cooling by the ATF cooling tube in the radiator, not substitute for it. Stacked plate external coolers are slightly more efficient than tube and fin coolers; either design will provide up to approximately 10 degrees of additional cooling. A fan for the external cooler may be necessary for slow speed work in 4WD low range to keep air flow through the cooler, but a grill mounted cooler without fan may be enough (it has been for me so far, and if needed I can add a 500 cfm 12v Derale fan).

You can by-pass the ATF cooling tube in your radiator in favor of an external liquid-to-air cooler if you want, but if won't be as efficient as the OEM system. The better practice (unless the radiator is defective or malfunctioning) is to maintain the OEM cooling system as originally designed and add a supplemental external cooler if your transmission temps frequently spike over 200 degrees or you want the peace of mind of additional cooling for trailer towing or slow speed four wheeling.

The proper routing, evidenced by most OEM auxiliary coolers, is: Auto Trans to Radiator ATF cooling tube > other end of Radiator ATF cooling tube to top of External Cooler > bottom of External Cooler back to Auto Trans. This allows the radiator to cool the ATF before it gets to the external cooler which cools it further. If you route the hot ATF to the external cooler first it cools the ATF to approximately the same temperature as the already cooled coolant which means that the ATF cooling tube in the radiator will do little or nothing to bring the ATF temperature down further.

And that's all I have to say about that.

Well, almost all:

On my "other" 4WD vehicle I ended up changing out the radiator for one without the alleged manufacturing defect so I could maintain use of an ATF cooling tube in the radiator, kept the OEM external ATF cooler (included with the trailer tow package), and added an inexpensive Derale fan operated by a thermo switch that turns on the fan when the ATF entering the external cooler reaches 185 degrees and turns it off when it drops to 165. No overheating of engine or transmission since, even on long hot days in 4WD low range and the A/C on.

Thanks for all the info. What aftermarket radiator do you recommend?

 

[Mr. Bills]

You mean cooler?

I use a Flexalite 4116TJ in front of the radiator. It is a popular choice.

Jerry Bransford likes his Derale 20561 with fan mounted underneath his rig. Chris, our Administrator, just purchased one.

There is a B&M cooler favored by some TJ owners.

___________

Edit: I see that you edited your post to read "radiator" rather than "auto".

The answer to that question is OEM Mopar. With the possible exception of adding an auxiliary transmission cooler, keep your cooling system stock.

 

[Jerry Bransford]

Thanks for all the info. What aftermarket radiator do you recommend?

There is no aftermarket radiator that is regularly recommended by anyone with any experience. Personally I'll stick with the tried and proven Mopar which outcools anything we're aware of that sells for a reasonable cost. The OE Mopar radiator has sufficient excess cooling capacity to keep the engine cool in the hottest desert conditions, something few aftermarket radiators can match.

 

[mrblaine]

When you say monitor the trans temp, would that be with a bung into the pan? I've been debating mounting a sensor in the pan vs on the return line after the cooling system, or maybe both with a toggle switch.

As for where to mount the cooler, my understanding is that the best place is after the radiator going back into the trans. If there's heat soak then the aux cooler will help take care of that, and if it's winter then you want the radiator there to warm up the fluid to operating temp. The aux cooler has a bypass for when the fluid is already cool so in winter it shouldn't over-cool during warm up.

You want to know how hot the transmission is. Not how hot the cooled or heated fluid is, how hot the trans is.