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Transmission Cooler Line Leak at Radiator (Complete Guide)

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Last Updated on: 7th September 2023, 12:47 am

The performance of your car’s transmission system largely depends on a critical element known as the transmission cooler lines.

These lines have the task of ferrying fluid from the transmission to the radiator, where it undergoes cooling before it’s cycled back into the system.

A leaking transmission cooler line, particularly at the radiator, can lead to serious issues like the failure of the transmission, overheating, gear slipping, trouble with gear changes, among others. Fortunately, there are clear indicators to warn you.

In this article, we’ll examine the reasons behind and symptoms of a transmission fluid leak at the radiator, as well as methods for addressing it.

Additionally, we’ll shed light on the seriousness of this issue and other pertinent details that you need to be aware of.

What Are Transmission Cooler Lines?

Transmission cooler lines are the tubes responsible for carrying transmission fluid from the transmission system to the radiator.

This fluid is subsequently cooled down by the radiator, before being channeled back to the transmission for the cycle to be repeated.

These lines are typically constructed from materials such as stainless steel, rubber, or a woven fabric and are attached to both the transmission and the radiator using particular fittings.

The following image illustrates three variations of these lines.

examples of transmission cooler lines

Is a Transmission Cooler Line Leak Serious?

Neglecting a leak in the transmission cooler line can lead to severe consequences.

When the level of transmission fluid dips significantly, it can result in the gearbox overheating and undergoing potential damage.

The ability of the transmission to switch gears correctly can also be compromised, which could inflict further harm to the transmission system.

In certain situations, a leak in the transmission cooler line could even result in total transmission failure.

Signs of Transmission Cooler Line Leaks

Below are several indicators that can suggest a leakage in your transmission cooler line.

  • Leakage of transmission fluid: A clear sign of leakage is a pool of transmission fluid, typically red or pink, appearing under your vehicle near the radiator.
  • Burning odor: A peculiar burning smell originating from the transmission or radiator can also be an indication.
  • Issues with transmission: The transmission may start slipping or show issues with gear shifting.
  • Change in coolant color: The mixing of transmission fluid and coolant in the radiator can result in a distinct “strawberry milkshake” discoloration.
  • Tendency to overheat: If the radiator is leaking coolant or mixing it with transmission fluid, you might observe frequent overheating of the engine.

The accompanying image shows rusted transmission coolant lines. Check for fluid leaks around these lines and their connections.

rusted tranmission cooler lines

Can Transmission Fluid Leak Into the Radiator?

Indeed, a leak in the transmission cooler lines can result in transmission fluid seeping into the radiator. This occurrence may be indicated by a change in the color of the radiator fluid or a burnt smell.

Furthermore, the transmission fluid can harm the radiator, potentially leading to more leaks or even total radiator failure.

Can a Line Leak Lead to Overheating?

Yes, a leak in the transmission cooler line can result in overheating. When the level of the transmission fluid becomes too low, it can cause the gearbox to overheat and sustain damage.

If the transmission fluid leaks into the radiator, it can lead to the overheating of the coolant fluid, which could potentially cause the engine to fail.

How to Detect Leaks in Coolant Lines

There are a few signs that can suggest a leak in your coolant lines, such as the following.

One noticeable sign is a pool of coolant or transmission coolant fluid under the car, typically situated beneath the engine bay or radiator.

A burning smell emanating from the engine area. You might observe that the engine is operating at a high temperature or that the temperature gauge is displaying a higher reading than usual.

For accurate diagnosis, it’s necessary to examine the transmission lines and the radiator for potential leaks or issues to ascertain if the problem is due to transmission fluid lines or coolant hoses.

How to Fix Transmission Cooler Line Leak at Radiator

To repair a leak in the transmission cooler line at the radiator, you must first identify the source of the leak.

This can be achieved by either visually inspecting the lines or utilizing a pressure tester to detect leaks.

Once the leak has been located, you can then replace the faulty line or fitting.

In certain instances, it might be necessary to substitute the whole transmission cooler line, or in extreme cases, the heater matrix or the radiator.

Transmission Cooler Line Leak After Radiator Replacement

It’s not unusual for a transmission cooler line leak to develop after a radiator replacement.

This could be due to improper radiator installation or harm to the transmission cooler lines during the process of radiator replacement.

To rectify this, inspect the transmission cooler lines and the radiator, and repair or replace any lines found to be damaged or leaking.

What If Transmission Fluid Seeps into the Radiator

If transmission fluid, also known as gearbox fluid, infiltrates the radiator, it can inflict damage on the radiator, leading to additional leaks or even complete radiator or engine failure.

Furthermore, the transmission fluid can discolor the coolant or give it a burnt odor.

If the fluid level drops too significantly, the transmission fluid can also cause harm to the engine and transmission.

In extreme cases, this can lead to complete transmission failure or overheating and failure of the engine.

Why are Transmission Lines Connected to the Radiator?

Transmission lines are linked to the radiator to cool down the transmission fluid. As the fluid circulates through the transmission, it can heat up due to friction and other influences.

The radiator assists in dispersing this heat, maintaining the transmission fluid at the appropriate temperature to ensure the correct operation of the transmission.

How are Transmission Cooler Lines Linked to the Radiator?

The connection between the transmission cooler lines and the radiator is facilitated by fittings.

These fittings are typically positioned on both the transmission and the radiator (as depicted in the following image).

The design of these fittings is aimed to be snug and secure to inhibit leaks, but occasionally, they can fail.

transmission cooler line fitting connecting to the radiator outlined with a red circle

How Do You Stop a Transmission Line From Leaking?

In order to halt a leak in the gearbox line, it’s necessary to replace the faulty line or fitting. Sometimes, you might even need to replace the entire hose.

Regular inspection and maintenance of the transmission cooler lines is vital to prevent the onset of leaks.

Cost to Replace or Repair Transmission Cooling Lines

The expense involved in replacing or repairing transmission cooling lines can differ based on the specific vehicle model and the severity of the damage.

Usually, the cost for most transmission cooler line repairs falls within the range of $100 (roughly £75 or €85) and $500 (around £365 or €415).

However, there could be instances where the repair cost is higher, particularly if the transmission or radiator has also suffered damage.


A leak in the transmission cooler line at the radiator can pose a serious issue that can result in damage to both the transmission and the engine.

It’s crucial to be vigilant for signs of a leak such as a pool of transmission fluid or a burning odor, and get any leaks fixed promptly.

Routine maintenance and scrutiny of the vehicle can assist in averting leaks before they occur.

If you suspect that your transmission cooler line is leaking, it’s advisable to get your vehicle checked by a skilled mechanic.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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