Transmission Cooler Flow Direction (Types + Diagram)  (2024)

Updated on March 9, 2023

Not many people might know this, but the automatic transmission fluid gets much hotter than engine coolant, even during normal driving. And if you use your vehicle for towing or racing, the transmission fluid temperatures can become a real problem. That’s because as the transmission fluid starts to overheat, it loses its lubricating properties, which can destroy your transmission in no time.

However, the same thing happens if the transmission fluid is too cool, which is why we will explain the different types of transmission cooling systems together with the flow direction. But before we go into more detail and cover all the popular transmissions, here is a quick answer to the main question.

In most cases, the cooler feed line (flowing from the transmission into the cooler) goes to the top of the cooler, while the bottom is the cooler return (flowing from the cooler into the transmission). And that’s the transmission cooler flow direction. However, on the transmission side, there are no rules as to which line is the feed or return, so you have to check for your specific transmission.

Transmission Cooler Flow Direction

As already mentioned, the flow direction of the transmission cooler is such that the top hose attachment is the hotline, and the bottom one is the cold line. The hotline is the fluid returning from the transmission, and the cold line is the fluid returning into the transmission.

However, that’s just a rule of thumb, so it could be the other way around in some cases. Moreover, some coolers have both inlet and outlet on the top of the cooler, which throws our initial assumption out the window.

So, to avoid making a mistake, you can check if there are any markings next to the hose attachments. For example, you could see in and out or inlet and outlet next to the hose attachments indicating the flow.

In other cases, one hose or the hose fittings could be marked with blue and red colors, in which case the blue flows to the transmission, and the red flows into the cooler. Also, if you have just bought an aftermarket cooler, you can check the manual you got with it to see the flow direction.

Also, if the cooler is already installed on the vehicle and it’s functional, you can check the hose temperatures. You can do that by touching the hoses and noticing which one is warmer, but do this only when the engine is cold because these hoses can cause significant burns.

Alternatively, you can get a laser thermometer or borrow one and use it instead of your hands. And if you choose this method, the colder line of the two returns the fluid to the transmission, while the hotter line feeds the cooler.

And as a last method to check the cooler flow direction, you can find what the cooling hose attachments on your transmission are and follow them back to the cooler. You can find that by checking online, getting a workshop manual for your specific model, or you can check the list below and see if your transmission is included.

Transmission Cooler Flow Direction on Different Transmissions

In the following chart, you will see a return and feed line for each transmission. The feed line refers to the line flowing from the transmission into the cooler (hotline), and the return line refers to the line flowing from the cooler to the transmission (cold line). Also, this information is coming from hummerforums.com.

Types of Transmission Coolers

Radiator Cooler

In most cars, the transmission cooler is integrated into the coolant radiator. On the cold side of the radiator is a pipe with hollow walls, and through those walls flows the transmission fluid. Then, since the pipe sits inside the radiator, the coolant flows through and over the pipe, thus taking the heat away from the transmission fluid. This system also warms up the transmission fluid quicker because, even though the coolant is cooler than the transmission fluid at operating temperatures, it’s still hot.

Furthermore, this type of cooler is cheap to produce and isn’t problematic. However, if the walls of the pipe crack, transmission fluid will mix with the coolant leading to a lot of problems with the engine and especially the transmission.

Moreover, the cooling properties of this system aren’t as good as a standalone transmission cooler. Also, the temperature of the transmission fluid transfers onto the engine coolant, which slightly increases the engine’s operating temperatures, and that’s never a good thing. Still, for most OEM applications, the system is efficient enough.

Stacked Plate Cooler

A stacked plate cooler is a standalone cooler, so to speak. Meaning the transmission fluid flows through a designated radiator and has nothing to do with the engine coolant. This system is the most efficient in cooling the transmission fluid, if not too efficient.

The only problem with this system is that it can lead to overcooling and can delay the transmission fluid warming up to operating temperatures. However, this can be mitigated by installing a thermostat. Also, the transmission fluid radiator is often placed in front of the engine radiator, which limits the airflow through it. Still, with a good location on the side of the bumper and a thermostat, nothing beats a standalone cooler.

Radiator Cooler Plus a Stacked Plate Cooler

A radiator cooler plus a stacked plate cooler (transmission fluid radiator) is the best solution for those who want to improve the cooling properties of the standard coolant radiator system. The way it works is that you install a standalone transmission fluid radiator and feed it through the standard radiator cooler. In other words, the transmission fluid flows into the radiator cooler (coolant radiator) for the initial cooling, just like it does in the standard OEM system.

Then, when the cool transmission fluid exits the radiator, it goes into an external transmission cooler radiator for additional cooling before it returns back to the transmission. So essentially, the transmission fluid is cooled twice as it flows through the integrated cooler and the aftermarket external one. Another positive side of this system is that the coolant warms the fluid up to operating temperatures quicker, while the external one keeps the fluid cool under high loads.

Is a Transmission Cooler Necessary?

Yes, a transmission cooler is necessary. The transmission fluid is essentially the same as engine oil, only denser, and when it overheats, it loses its lubricating properties. Now, even with a transmission cooler, the fluid can reach temperatures of 350F (175C), and without a cooler, the fluid will begin burning within minutes.

Do All Vehicles Have Transmission Coolers?

No, not all vehicles have transmission coolers. Almost no manual transmission vehicle comes with a transmission cooler from the factory. However, all automatic cars do have transmission coolers, although there might be some exceptions.

What Is the Difference Between an Oil Cooler and a Transmission Cooler?

An oil cooler and a transmission cooler essentially do the same job; they both cool oil. However, an oil cooler refers to external engine oil coolers/radiators, which are a popular addition to performance cars. A transmission cooler, on the other hand, looks the same and does the same job, but instead of engine oil, it cools transmission oil/fluid.

Does a Transmission Cooler Improve Performance?

No, a transmission cooler doesn’t improve performance directly. However, if you are racing the vehicle, the transmission could be prone to overheating, after which the car can enter limp mode. And a transmission cooler can help prevent that and extend your transmission’s life.

Do Transmission Coolers Use Coolant?

No, transmission coolers don’t use coolant directly, or in other words, coolant doesn’t run through transmission cooler lines. However, factory transmission coolers often sit inside the radiator, and the coolant both cools and warms up the transmission fluid by flowing over the pipes that hold it.

Where Is the Transmission Cooler Located?

The factory transmission cooler is located inside the radiator, so you can’t really see it. However, the external transmission coolers, if your car has one, can be in front of the radiator, inside the wheel wells, next to the radiator, under the car, etc. Essentially, anywhere with good airflow.

How Much Does It Cost To Install an Additional Transmission Cooler?

Installing an aftermarket transmission cooler can cost anywhere from $100 to $800, depending on your car. The biggest issue with installing a new cooler is figuring out where to mount it inside the bumper, and that can call for a lot of fabrication, experimenting, and consequently man hours.

Can a Transmission Cooler Cause Engine to Overheat?

Yes, a transmission cooler can cause an engine to overheat, but only if it fails. Since factory coolers sit inside the radiator, there will be some heat transfer from the transmission fluid to the coolant, but not enough to overheat the engine. However, if the cooler inside the radiator cracks or starts leaking, transmission fluid will get into the coolant, which can lead to overheating very quickly.

Conclusion

In the end, the transmission cooler flow direction is difficult to tell without proper investigation, temperature readings, or hopefully markings on the cooler itself. But as a rule of thumb, the top line on the cooler flows from the transmission, and the bottom line flows back into the transmission.

To help you out, you can check out our chart with all the popular transmissions and what the two cooler hose attachments do on them, feed or return into the cooler. And hopefully, our diagram will help you understand how different transmission cooler systems work.

Here are some articles that you might enjoy:

Is the Trunk of a Car Cooler Than the Backseat?

Intercooler vs Radiator: What Are the Differences?

Transmission Cooler Lines Leaking At Radiator – How To Fix

7.3 IDI vs 7.3 Powerstroke Diesel Engine: Which Is Better?