Transmission Cooler Lines Leaking? Here’s The Fix

Are the transmission cooler lines in your car leaking? What are the symptoms? Here's a simple guide that also explains how to fix the issue.

An automatic transmission has an optimal working temperature of between 160°F and 200°F (71°C to 93°C). Anything approaching 300°F (150°C) or higher means imminent failure.

The cooler (in the radiator) keeps the transmission fluid (ATF) cool. That is, within this range. Transmission cooler lines transport the fluid to and from it.

You won’t even think about your transmission cooling lines when everything’s working as it should. But if a leak develops, it won’t be long before things start to go wrong.

This guide is to help you understand transmission lines and what to do if they’re leaking.

Key Takeaways

  • Transmission cooler lines leak due to wear, damage, or improper installation.
  • Get new lines fitted at a trusted local shop.
  • New transmission cooling lines might cost between $250 and $350, including labor.

Table of ContentsShow

What Are Transmission Cooler Lines?

Transmission Cooler Lines

Transmission cooler lines, also referred to as ‘transmission cooling lines’ or just ‘transmission lines’, are precisely what they sound like.

They carry the transmission fluid through a cooler. Stock models are installed as part of the vehicle’s radiator.

Passing through the narrow channels and past the aluminum fins, the fluid cools. The flow then returns to the transmission housing and resumes its job.

The transmission cooler lines flow from the transmission housing to the radiator. The outbound and return pipes typically run next to each other underneath the car’s body.

They could be made of anything but usually come under one of the following categories:

  • Metal
  • Rubber
  • Braided steel
  • Braided nylon

Why Do Cars Have Transmission Cooler Lines?

Car Transmission Cooling Lines

The automatic transmission is one of the most complex parts on a car to deal with. It has numerous parts, all fitting together and relying on each other.

One of the main reasons for this complexity is the transmission’s position. Placed between the engine and the wheels, its job is to convert the engine speed into an efficient amount of torque.

This involves constantly-changing strains and pressures. Transmission fluid lubricates everything, but what happens when it gets too hot itself? It’ll start to burn and lead to extreme, dangerous internal temperatures.

That’s where the transmission cooler (in the radiator) comes into play. The transmission cooler lines take the ATF from the transmission to the cooler and back.

Aftermarket Transmission Cooler Lines

On some cars, you’ll find additional or aftermarket transmission coolers. These provide an extra bit of cooling power when you need it. The most common example is when towing.

Some brand-new vehicles might come with an extra transmission cooler if you opt for an off-roading or towing package. You also sometimes find them on used cars.

Aftermarket transmission coolers are usually installed next to the radiator or condenser. They could be before or after the circuit’s stock cooler (radiator).

Always be careful with aftermarket models on second-hand cars. They might have been installed incorrectly and are more likely to leak or cause other problems.

What Are Transmission Cooling Line Leaks?

Transmission Cooling Line Leaks

With a good understanding of transmission cooler lines, let’s look at why they leak.

Your transmission lines can only leak while the engine is on (and in the immediate moments afterward). The lines aren’t pressurized when the car isn’t running because the pump isn’t active.

When transmission cooling lines leak, transmission fluid drips out of the system. While a few splashes will coat your vehicle’s underside or engine bay, most will end up on the road.

That’s bad for your transmission, making it more likely to break or even seize. It’s also environmentally damaging.

Where Do Transmission Cooler Lines Leak From?

Here are some of the most likely places I’d expect transmission cooling lines to leak from:

  • Poorly-installed fittings (from aftermarket parts or improper earlier repair jobs)
  • Cheap clamps used with rubber-hose systems
  • Cracks in rubber or braided lines
  • Physical damage to metal pipes
  • Rusted metal lines

Symptoms Of Leaking Transmission Cooler Lines

I’ll run through a brief explanation of some signs of leaking transmission cooler lines.

Let me first point out that the leak symptoms below could be signs of almost any transmission fault.

A visual inspection is the only way to know if the lines are the problem. Most people will need professional help (and equipment) for this.

Caution lights and the existence of ATF underneath your vehicle are evident signs, but it pays to be certain.

Transmission Warning Lights And Messages

Transmission Overheating Light

The most apparent warning light that might pop up is the ‘transmission overheating’ icon.

The exact lights and messages depend on your car’s age, make, and model.

Take your car to a nearby mechanic to have them read the OBD codes. These might point you straight toward the transmission lines.

Transmission Fluid Leak Under Car

transmission fluid leak

Let’s assume that you have a leak in a transmission line. Where does all that ATF go?

It drips or sprays straight out, hitting your car or dropping straight to the ground.

Remember, your car must be switched on for the lines to constantly leak, but you might still be able to see small drips.

(Sometimes, the lines only noticeably leak when the engine’s on, and the system is pressurized. You’ll need help from a mechanic to find this problem.)

Low Transmission Fluid

Checking Transmission Fluid

It goes without saying, but a leaking transmission line means your ATF levels reduce.

When you check your transmission fluid level using the dipstick, you’ll see it’s gone down.

The issue might lie with a leaking transmission cooling line.

Dark Transmission Fluid

Transmission Fluid Color Chart

Transmission fluid burns when it gets too hot. No, literally. It doesn’t just get hot – it burns.

When you next check your transmission fluid, you’ll notice it’s a much darker shade of red. In severe cases, it might even turn brown or black.

If the problem is a leaky line, the system loses ATF all the time. If more leaks out, the rest of the system has to work harder. Eventually, it gets too hot to function properly.

This burnt fluid can’t do its cooling and lubrication jobs effectively. You’ll need new transmission fluid as well as a new line.

Burning Smell

man and bad smell inside car

As I mentioned above, low transmission fluid will result in what’s left burning.

This aroma is usually noticeable, and it isn’t particularly pleasant. You’re most likely to smell it when driving fast (for example, on the highway) or accelerating hard.

General Transmission Problems

modern automatic transmission

A transmission line leak leads to low ATF levels and, thus, overheating. In turn, the transmission itself starts to wear and get damaged. The hotter it gets, the worse this problem becomes.

As the temperature and damage increase, you’ll see transmission problems. These could manifest themselves in almost any way, but here are a few of the most common:

  • Unsettling noises (grinding, groaning, vibrating, squealing, etc.)
  • Shifting at random times
  • Heavy gear changes
  • Poor performance

How To Fix A Leaking Transmission Line

car technician holding the wrench

Repairing a leaking transmission line is straightforward. Unfortunately, it’s probably not what you want to hear.

You need to pop on down to your local mechanic and have them fix it.

Sure, it’s tempting to work on this fix yourself and save labor costs. But, as I’ve mentioned, an automatic transmission is complicated and expensive to repair.

Installing the lines incorrectly or using the wrong type will lead to transmission damage. That’s a car-crippling injury and a four-figure repair bill.

Spending a little more money to get this problem fixed by a trusted professional mechanic will pay off in the long run.

They’ll replace the affected part(s) and most likely do a transmission fluid flush.

It’s a good idea to have both lines replaced at the same time (this won’t cost much extra). That way, you know they’re both new and shouldn’t develop further faults for a long time.

A good mechanic will find out why the leak developed in the first place and take steps to prevent it in the future.

How Much Does It Cost To Repair A Leaking Transmission Line?

Handing Over Money Cost Price

This is very difficult to guesstimate, and you’ll get much more accurate answers by contacting local garages for quotes.

As a very broad answer, anticipate paying a few hundred dollars. I expect something between $250 and $350 for most standard cars.

However, it’s all subject to your make and model and the exact location and type of leak.

The shop’s labor rate, geographical location, and availability also come into play.

Rounding Up

Transmission cooler lines aren’t the easiest things to work with. Getting a repair wrong will make a relatively minor problem much worse.

In the same vein, catching it early means minimal damage to your transmission.

As soon as you notice any potential transmission problems, go to a local shop. I’d recommend a transmission specialist, since fast-fit technicians will likely subcontract the work out anyway.

Stay safe out there!

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!