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Questions about automatic transmission cooling and operating temperatures continue to be asked. I've been making measurements on both the Automatic Transmission Fluid, ATF, at various points and the engine coolant. I've also tried to follow threads about issues with auto trans cooling. I'm going to first put the main points as brief descriptions with minimal explanation and will provide more information later.

For brevity I'll refer to the ATF heat exchanger in the radiator as the rad-cooler and the ATF heat exchanger in front of the AC condenser as the air-cooler.

The coolant temperature at the bottom of the radiator where the ATF rad-cooler is located is always cooler than the measured engine coolant temperature. The coolant is transferring heat to the air as it flows through the radiator. See Section 1.

The engine cooling system should be inspected regularly as part of general maintenance. Keeping the coolant level within the required limits will ensure maximum performance for both engine and trans cooling and passenger heating. See Section 2.

The ATF rad-cooler does not heat the ATF. The ATF is always hotter than the coolant in the bottom of the radiator. This includes during warm-up and during high load use. ATF will heat up to operating temperature during normal driving without any heat input. See Section 3.

The ATF temperature is more dependent on how the vehicle is being driven than the ambient temperature. High load will heat the ATF even in cool weather. If the converter is not locked up and the engine speed is below about 2500 rpm significant heat can be generated in the converter. See Section 4.

Driving at a relatively constant speed in 4th or 5th gear with the torque converter locked up is about the easier driving condition. There is good airflow for cooling and minimum heat generated in the converter and trans. Measuring ATF temperatures for these driving conditions is not indicative of the cooling capacity needed for either city or off road driving. See Section 4.

While there are reliability problems with the rad-cooler in early years and good reasons to consider bypassing the rad-cooler, bypassing removes a significant component of the trans cooling system. While there may be sufficient cooling capacity for light load driving with just the air-cooler, moderately heavy load driving may overheat the system. See Section 5.

Auto transmissions don't usually fail catastrophically from moderately high temperatures for short periods. The degradation occurs slowly but continuously in both the ATF and the seals. Controlling trans temperatures with sufficient cooling and intelligent driving, and regular ATF changes will maximize transmission life.

Adding an ATF transmission temperature gauge will ensure vital information to be able to manage transmission temperatures, knowledge is power. When temperatures start climbing, the driver can reduce throttle input to control temperatures. In addition, cooling capacity can be increased if high temperatures are regularly observed.

Section 1 - Engine Cooling:

The stock Xterra engine cooling system is a well designed high capacity system. I've never seen coolant temperatures above 205F while driving off road in 100+F in 4LO and the AC running. If all components are functioning, overheating doesn't seem to be a problem for the Xterra.

The engine cooling system consists of the liquid coolant, passages in the block and heads for coolant, radiator, pump, thermostat, reservoir, heater core (actually a small radiator), and hoses to connect the components. The coolant is the medium to transfer heat generated in the engine for removal in the radiator and to heat the interior through the heater core.

The pump circulates water through out the system. The thermostat regulates the engine temperature by regulating coolant flow into the pump from both the radiator and the bypass to maintain a minimum coolant temperature flowing into the pump. This is not the coolant temperature exiting the engine but the input to the pump. When the coolant temperature is below the minimum for the thermostat, all of the coolant bypasses the radiator. When the coolant temperature is above the maximum for the thermostat all of the coolant flows through the radiator. In between the two temperature limits, the thermostat allows for a mix of coolant from the radiator and the bypass to keep the engine in the most efficient operating temperature.

In general, the cooling system functions by the coolant absorbing heat from the engine and then losing heat to air as it passes through the radiator. Of course this is after the coolant has reached the minimum temperature for the thermostat to has opened. As heat is lost from the coolant as it flows through the radiator, the temperature drops. The temperature of the coolant at the bottom of the radiator will always be lower than the measured engine temperature.

Section 2 - Engine Cooling System Maintenance:

To ensure maximum performance the coolant level must be maintained between the require fill limits. If the coolant level is low, the first indication is reduced heater output at low engine speeds. If the level is low enough the engine will run hot and possibly overheat. The only way to accurately check the level in the reservoir is when the engine is cold and running. If the engine isn't running the level may appear artificially high and be between the max and min limits even though it is low. More detailed information is in this thread, Heater Blows Cold At Idle, How to Correct and More.

The cooling system is well designed and will purge itself of air if there is adequate coolant. The reservoir tank is part of the pressurized system and is not an overflow catch tank. It is both an expansion and an air separator tank. The high point of the system is the top of the radiator where there is a bypass hose that goes to the reservoir.

If there is air in the from a system flush or because the coolant level was low, after filling the system with adequate coolant, any air trapped in the system will gradually be flushed into the reservoir and separated. Coolant minus air flows back to the system from the bottom of the reservoir. This is the reason that there must be coolant in the reservoir while the engine is running. If there isn't coolant in the reservoir air will be circulated back into the system.

There are no special processes or incantations required to purge air from the system after normal servicing or topping off. If the level is well above the min line with engine cold and running and there isn't heat at an idle, there are other problems with the system than low coolant level.

Section 3 - ATF Temperature During Warm-up:

The ATF rad-cooler is located in the bottom of the radiator in the coolest coolant. After a cold start, all of the coolant flow bypasses the radiator. Hot coolant from the block only starts to flow in the radiator after the thermostat opens. The thermostat opens gradually and modulates the flow of coolant into the pump from both the bypass and the radiator. Before the thermostat opens, the coolant temperature in the radiator is close to the ambient temperature.

The question of whether or not the ATF rad-cooler heats the ATF continually gets asked. The definitive answer is there is no heating of the ATF as it flows through the rad-cooler. This should be obvious from the way the engine and auto trans cooling systems are configured and the flow of coolant before the thermostat opens. The temperature of the ATF in the pan provides additional data to confirm that the auto heats up internally as the vehicle is driven. The ATF in the pan reaches 100F quicker than the engine coolant temperature reaches 165F which is below the temperature that the thermostat starts to open.

From temperature measurements of the ATF flowing into and out of the rad cooler, there's never been a time where the temperature out was greater than in. This is from measurements taken during warm-up in ambient temperatures down to 20F. This is additional data that confirms heat is not transfer to the ATF as it passes through the rad-cooler.

Section 4 - Managing Heat in the Trans:

The auto trans generates heat in both the gear clusters and the torque converter. The amount of heat generated is dependent on how the vehicle is driven. High load under 2500 rpm will generate considerable heat in the torque converter because the converter is not above the nominal stall speed, where the converter is hydraulically locked. Under high load below that rpm, the converter is multiplying torque from the difference in the input rpm and output rpm, commonly called slipping.

Under most driving conditions, the high load input doesn't last very long. Long uphill grades and many off road trails can have load conditions where the converter is slipping and generating large quantities of heat, especially if the trans selector is in "D".

The VQ40 engine in the 2nd Gen Xterras is a strong engine that can provide plenty of torque at lower rpm. This performance coupled with the programming of the auto trans valve body or computer makes for a very drivable vehicle under many conditions.. Unfortunately this drivability can create considerable heat under some conditions. I've personally experienced trans overheating while driving off road.

While driving the Alpine Loop near Silverton, CO, transmission ATF temperatures were being monitored with a data recorder for review after driving more than for real time monitoring. The trans was in 4LO and Drive because the engine seemed comfortable and the trans wasn't hunting for a gear. Near Engineers Pass in the morning the trans went into a limp mode and wouldn't move until manually shifting to 2nd. On the return leg driving up towards Cinnamon Pass the trans went into limp mode again and had to be manually shifted into 2nd to move forward. The ambient temperature when the trans overheating occurred was 55-60F. I was not driving aggressively. The ATF cooling was stock, both the rad-cooler and air-cooler were in use and there was no fan on the air-cooler.

Reviewing the data in the evening revealed how hot the ATF had gotten during the climbs. What was surprising was how fast the ATF cooled down after shifting even though still climbing. More information in this link and in general at ATF Temperatures and Cooling System Performance.

Even though the ambient temperatures were cool and I wasn't driving aggressively, the trans was overheating. From the data it was obvious that using a lower gear to keep engine rpm up to minimize the heat generated in the converter. This is an easy way to manage ATF temperatures without having to spend any time or money.

To monitor ATF temperatures real-time an ATF gauge with the sensor in the pan was installed, How-to: Trans Temp Gauge Installation. A gauge allows for convenient tracking of ATF pan temperatures and provides time to back off on the throttle to keep heat buildup under control. For off road driving I personally will ease up when the ATF temperature gets to about 190F. This is based on the type of conditions I encounter off road with the intention of being very conservative to avoid any preventable problems.

From the data collected, the air-cooler does not cool the ATF if the vehicle speed is below 20-25 mph. There's no temperature drop across the cooler. Because of this information I decided to increase the cooling capacity of the system at low speed by adding a fan, How-to: Install Air-ATF Cooler External Fan and Adjustable Controller. The addition of the fan significantly increases the cooling at low speed, but it still won't remove all of the heat that can be generated in the trans. Intelligent driving is still required.

Above 50-60 mph any additional airflow from the fan doesn't increase heat loss. Driving between 45 and 70 mph on a relatively level road with trans in 5th gear and the converter locked is the easiest load on the trans with minimum heat buildup. On the other hand, towing or driving long grades can add significantly to the heat generated in the trans. I've pushed on long grades and have observed the ATF temperatures climb to high levels, backing off before overheating. I don't have any data from towing but have no doubt that it would be easy to overheat the trans, especially in hilly country.

If I was aggressively driving in hilly country or trailer towing I'd have an ATF temperature gauge and add an additional air-cooler in addition to the OEM cooler, even with the added fan. The added fan does not increase the cooling capacity at speeds over 50 mph so an extra cooler is need to increase capacity. I'd locate the cooler where it has good airflow and not behind or in front of the factory air-cooler. The reduced airflow reduces the cooling capacity compared to free flowing air.
 

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Section 5 - Cooling System Modifications if the Rad-cooler Is Bypassed:

The reliability problem with the factory rad-cooler is well documented. The only thing known with certainty is that the rad-cooler can develop a leak and then coolant and ATF mix with disastrous results. The few units that have been dissected after failure have shown the leak to be at one of the brazed joints. There has not been evidence of corrosion from either the ATF or the coolant.

The OEM Calsonic radiator is the only manufacturer that uses aluminum for the rad-cooler, they apparently hold the patent. Brass has been the traditional material used for decades for rad-coolers with a very high reliability. If corrosion from the coolant was a problem it should also be a problem from the other aluminum parts in the radiator. If the Matic-J ATF was corroding the cooler there would have to been similar corrosion in the trans which has many aluminum parts in contact with the ATF.

I mention these observations because they are consistent with people who have experienced the rad-cooler failure. In most cases they claim that the vehicle had been serviced regularly Regular maintenance does not appear to be a potential solution for avoiding the problem. This supports the belief that the design of the piece parts using aluminum and the manufacturing process are the root cause of the failures. While not every rad-cooler fails, there have been enough fail with a very high collateral cost of the failure that something should been done.

The most obvious response is to just bypass the rad-cooler and rely on the air-cooler alone. It's a no cost modification that can be done quickly. The downside is a major component in the trans cooling system is removed, the main cooler at low speeds. Depending on both how the vehicle is driven and driving conditions, the loss of the cooler may not significantly affect the trans cooling needs.

However, just bypassing the rad-cooler and calling the job done is taking a chance on encountering a different problem. Waiting to see if trans problems develop and then trying to resolve the issues is a bit Pollyannaish. If your choice is to bypass the rad-cooler, I'd recommend monitoring the ATF temperature and adding a fan to the OEM air-cooler as the minimum modification to restore some of the lost cooling capacity. With the data for your specific driving needs you can make additional modifications if needed.

My personal choice to remove the failure prone OEM rad-cooler was to replace the radiator with a KOYO A2807 that has a brass rad-cooler. I did this at the same time I flushed on my 09 Xterra. This was the second flush when the vehicle was about 4 1/2 years old.

May you all enjoy many miles of trouble free driving.

Jeff
 

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AWESOME post. When I saw this thread pop-up, the title had me thinking "Oh great another 5 sentence question from someone on SMOD..."

Then I saw the username and knew that Jeff has had some excellent and informative posts on transmission cooling and observed temperatures.

THEN I clicked into the post and read this wealth of great information.

Small suggestions though - perhaps update the thread title and add some formatting for readability (i.e. bold or bigger txt for the sections).
 

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Jeff,

Thanks for taking the (I'm sure considerable) time to put this together ... should be required reading for anyone who wishes to dig into their transmission or powertrain. The info on the cooling system, alone, is extremely valuable.

This post also lays to rest many of the points of 'common wisdom' that no-one seems to have taken the time to develop a definitive answer for - eg: ATF is *not* heated by rad-cooler, a properly functioning (and filled) cooling system will purge itself of air given sufficient (short-ish) time, etc.

I'll add a link back here, to my sticky-ed post on salient trans threads; but IMO, this thread should be it's own sticky.

Cheers,
-Jonathan
 
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First of all, nice job Jeff.

I hope owners have paid attention to this information and realize that low speed ATF cooling is disabled by simply bypassing the rad-atf-cooler and not replacing this cooling capacity. I believe the long term effects have not yet been seen by many who have done this alone. Low speed cooling is needed in many instances besides off roading.
 

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I second Skibum's posts on making this (and maybe the other two threads into a sticky.
Between this thread and the two other ATF Temperatures and Cooling system threads, there is a ton of good info on the entire subject.
 

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Excellent post!!!

Finally, some comprehensive testing/monitoring to prove/disprove some of the commonly held beliefs regarding the ATF cooling tube in the Calsonic radiator. For example, its the radiator at the root of the SMOD problem, not the ATF, also the ATF cooling tube in the radiator only cools, it does not heat the ATF during warmup.

One of the things I find particularly interesting is that a fan added to the OEM auxiliary ATF cooler does nothing above 50 mph. Since I tow up some grades, this information has persuaded me to replace my stock radiator (bypassed ATF cooling tube) with the Koyo unit in addition to adding a pusher fan to the OEM auxiliary ATF cooler (for slow speed trail running).

Thanks again.
 

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Nice overview Jeff.

I'd like to add a little regarding why you experienced high ATF temps when the trans was in Drive. IME, if the trans is left is D, it tends to shift up to a higher gear sooner than ideal. Example, climbing a hill and letting trans shift itself often means the trans goes into 2nd or 3rd when it should really remain in a lower gear. This ends up causing TC slip and excess heat. I'm sure Nissan engineers have the trans programmed for "general" driving situations and not necessarily for more extreme use. In the quest for fuel efficiency, lower rpm is generally a good idea, but not when someone is on a steep climb with lots of slow and go.

I was thinking about this same subject over the wknd on my wheeling trip. There were a lot of 1st gear uphills and if I left the trans in D, I noticed it upshifting, even though RPM was below 2000 or so. When I slowed a little, the trans would be in 2nd or 3rd and I'd feel the TC slip, holding the higher gear instead of downshifting. I knew this wasn't ideal and would heat the trans up.

Bottom line, manually shifting the auto trans can make a big difference in TC slippage and heat...
 

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Discussion Starter #11
I forgot that when pasting the text from Word that url addresses need to be added. The links have been added to the body. Sorry for the confusion.

Jeff
 

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Discussion Starter #12
Nice overview Jeff.

I'd like to add a little regarding why you experienced high ATF temps when the trans was in Drive. IME, if the trans is left is D, it tends to shift up to a higher gear sooner than ideal. Example, climbing a hill and letting trans shift itself often means the trans goes into 2nd or 3rd when it should really remain in a lower gear. This ends up causing TC slip and excess heat. I'm sure Nissan engineers have the trans programmed for "general" driving situations and not necessarily for more extreme use. In the quest for fuel efficiency, lower rpm is generally a good idea, but not when someone is on a steep climb with lots of slow and go.

I was thinking about this same subject over the wknd on my wheeling trip. There were a lot of 1st gear uphills and if I left the trans in D, I noticed it upshifting, even though RPM was below 2000 or so. When I slowed a little, the trans would be in 2nd or 3rd and I'd feel the TC slip, holding the higher gear instead of downshifting. I knew this wasn't ideal and would heat the trans up.

Bottom line, manually shifting the auto trans can make a big difference in TC slippage and heat...
Thanks for reiterating some of the content.

(from the opening points)
The ATF temperature is more dependent on how the vehicle is being driven than the ambient temperature. High load will heat the ATF even in cool weather. If the converter is not locked up and the engine speed is below about 2500 rpm significant heat can be generated in the converter. See Section 4.

(from Section 4)
The auto trans generates heat in both the gear clusters and the torque converter. The amount of heat generated is dependent on how the vehicle is driven. High load under 2500 rpm will generate considerable heat in the torque converter because the converter is not above the nominal stall speed, where the converter is hydraulically locked. Under high load below that rpm, the converter is multiplying torque from the difference in the input rpm and output rpm, commonly called slipping.

Under most driving conditions, the high load input doesn't last very long. Long uphill grades and many off road trails can have load conditions where the converter is slipping and generating large quantities of heat, especially if the trans selector is in "D".

The VQ40 engine in the 2nd Gen Xterras is a strong engine that can provide plenty of torque at lower rpm. This performance coupled with the programming of the auto trans valve body or computer makes for a very drivable vehicle under many conditions.. Unfortunately this drivability can create considerable heat under some conditions. I've personally experienced trans overheating while driving off road.

Even though the ambient temperatures were cool and I wasn't driving aggressively, the trans was overheating. From the data it was obvious that using a lower gear to keep engine rpm up to minimize the heat generated in the converter. This is an easy way to manage ATF temperatures without having to spend any time or money.

(and more from the provided link)
I'm definitely changing the way I drive the X. The engine has so much torque at low rpm that it masks the need to downshift. The auto calibration doesn’t let a lower gear to be selected when it would be easier on the trans and converter. I suspect it is to maximize gas mileage, even if it happens at the expense of trans life. To be far to Nissan engineers, these conditions are more extreme than most owners would subject the X’s and these conditions are outside of the design requirement parameters. It's too bad the trans doesn't have a selectable performance mode for challenging trails and roads.

I think that as long as a gear is selected to keep the engine rpm above 2200, the stock cooling system with both Rad-ATF and Air-ATF coolers working is adequate. Not all of the logs have both cooler data, but those that do show the effectiveness of the Rad-ATF cooler, especially at low speeds. The Air-ATF cooler is providing cooling at lower speeds but nothing like the Rad-ATF cooler. It is worth noting that the ambient temperatures were typically in the 50 - 60F range, not high by any standards. The trail and highway grades are also high and need to be considered when reviewing the data.

Cheers,
Jeff
 

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Jeff, I see that you're in Albuquerque. Ambient temps at altitude would be lower, but air density and its ability to absorb heat from the exchangers also affects total heat transfer of the "system". What altitude did you experience these high ATF temps?
 
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Discussion Starter #15 (Edited)
Jeff, I see that you're in Albuquerque. Ambient temps at altitude would be lower, but air density and its ability to absorb heat from the exchangers also affects total heat transfer of the "system". What altitude did you experience these high ATF temps?
The altitude near Engineer's and Cinnamon Passes would be over 12,000 feet. I'm guessing the air density would be about 70% of STP density. The lower density would certainly reduce the heat transfer.

You're clearly thinking about this. If you haven't already done so, look at the temperature plots for the two incidents in this link, http://www.thenewx.org/forum/showpost.php?p=474945&postcount=35. The temperature rise is impressive. I did not feel I was driving aggressively, the trans wasn't hunting for gears even going around hairpin corners.

However, the reduced heat transfer isn't the main issue. As you alluded to in your first post and I've tried to explain, more heat can be generated in the transmission than can be removed by the cooling system under certain driving conditions. While generalizations can be made, it is difficult to predict how much heat is being generated without measuring at least the the ATF pan temperature.

Cheers,
Jeff
 

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Jeff well done this should be a sticky . This explains in detail how the coolant and trans cooling system work to those who are not mechanicaly inclined . Like you stated its very important to monitor trans temp when weeling and towing on bypassed radiators. I myself are bypassed but running larger trans cooler but i still need a fan mounted for wheeling and driving around town to keep air moving threw it.
 

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Jeff- Finally an article in detail and plain English- My X just turned 100,000 miles and I am just doing whats recommended for that service- after reading your article I don't thinkI will bypass the rad cooler- I just want to flush and refill the system.I have had no problems with the tranny so I think i will go for that. I am concerned about the aluminum radiator - what was the cost of replaceing the radiator you mentioned and is that just a precaution you are taking? All help is greatly appreciated! Paul
 

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Thank you for this thread. I've been waiting for someone to dispel the myth of cold-weather "warming" from the rad-ATF cooler.

After reading your data, I am more confident in my decision to bypass the stock rad-ATF cooler, especially since I added a cooling fan and a second air-ATF cooler.
 

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Thank you Jeff! Very good information. I would also like to add my two cents from my experiences....Heres a little background on my truck: It is bypassed, I have a secondary trans cooler sandwiched on the back of the factory one, and a fan on the front pushing into the two coolers towards the radiator. I also have a bullydog tuner to monitor temps.

In the winter, my temps stayed good for the most part, except when using 4 hi for a considerable amount of time, for example in snow and going up hills. This made my trans temps climb significantly and even with the trans cooler fan on, it didn't help too much. I was still getting temps near 200 degres, or slightly higher. I found that 4 lo is MUCH easier on the transmission, but obviously it is not ideal to drive on the street in 4 lo.

Now, when the weather got hot out, I've experienced that I have been able to keep my trans temps for the most part under control, very rarely seeing the 190* temps, however my engine has started getting excessively hot. I've seen temps as high as 220* while climbing the big hill getting up to rausch creek.

Now, I realize that my setup having the two coolers sandwiched together is most likely the issue. I realize that I am probably not getting very good airflow through the two, and I'm basically heat-soaking the radiator which is why it isnt cooling the engine as well.

My solution to this is to get rid of both the trans coolers I have in there now. I purchased a larger B&M cooler from summit racing and I will be installing that, as well as the fan. However, Chris (killswitch) recomended that I put the fan between the cooler and the radiator, and attatch it to the back side of the cooler, so it will be sucking air through the cooler and then pushing it through the radiator. He said this is how his is set up and it is very effective this way. I have not yet had the opportunity to switch my setup like this, however I will soon. I have the parts, just need the time.

I will definitely report back after I make the switch so we can see how it turns out.

Again, THANK YOU for the information. Very helpful. I hope my info helps a bit too. Thanks to chris also for sharing info with me, its a big help.
 
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