Why Is There Transmission Fluid In My Coolant? (Here’s why)

Transmission oil mixing with coolant is a serious problem that needs immediate attention. If you’re lucky enough to catch this early, you can save yourself a ton of repair bills.

Why is there transmission fluid in my coolant? A faulty transmission cooler will allow coolant and transmission oil to mix. Transmission oil inside the coolant system can be flushed successfully however, coolant inside the transmission will mean a transmission rebuild.

In this post you’ll learn why transmission oil and coolant mix, what you can do to prevent further damage, how to save some money on the repair and and prevent this ever happening again.

What Color Is Your Coolant?


While having oil in the coolant is a common complaint, the root cause is usually a blow head-gasket. The gasket fails between the oil and coolant ports allowing engine oil to blow into the coolant mix. This results in a brown coffee mix with a cream head that can be clearly seen inside the coolant reservoir.

Transmission fluid inside the coolant system while still somewhat common, is much less so. The color of this concoction is more like a strawberry milkshake, a dull pink mix.

Transmission oil contamination

It’s important obviously to identify which type leak you’re dealing with. Simply observing the color inside your reservoir is a big clue, and you may already have your suspicions from other tell tale signs such as a mysterious loss of oil.

Faulty Transmission Cooler

Transmission cooler lines

The transmission cooler is as you know the source of your problem. Transmissions use oil to lubricate but also to cool the transmission. The oil gets hot 175 degrees F, especially when the vehicle is working hard like hauling a trailer or driving at highway speeds for long periods.

Without an oil cooler, the heat would cause components such as the oil pump and clutches to overheat and fail (over 200 degrees F), all expensive components.

How does the transmission cooler work?

The transmission oil pump creates the pressure needed to move the oil around the system. The oil carries the heat from the transmission via high pressure hoses to a cooler at the front of the vehicle. Ambient air blows across the fins of the cooler as your vehicle moves down the road. The cooled oil is then returned to the transmission to begin the cycle over.

Why does the transmission cooler fail?

Oil coolers are commonly integrated with the vehicles radiator, and that’s where the problems begin. The transmission cooler lines are piped to the radiator and run through an internal circuit that’s kept separated from the coolant. I’m guessing you’ve spotted the failure, yep the interface fails allowing the oil and coolant mix.

Two types of failure are common, either:

  1. Transmission oil leaks into the coolant, or
  2. Coolant leaks into the transmission

Although the root cause of the problem is identical, the subtle difference between which fluid migrates makes a difference to the repair bills and we’ll deal with the differences below.

Transmission Oil In The Coolant

Transmission oil

Transmission oil in the coolant is a major inconvenience, but it is by far preferable to the inverse. If you believe this is your problem go ahead and check your transmission oil level and color.

The fluid as you know should be red, any discoloration like a pink milky hue means you have a contaminated transmission and you’ll need to read the next subheading below – “What happens when coolant leaks into transmission”.

If however your transmission fluid is red, you’ll likely simply need to verify the fault – rupture of radiator to transmission cooler seal. It’s not uncommon for a radiator to transmission cooler to only leak under certain conditions. Such as, for example only when warn not when hot or cold.

This type detective work isn’t easy without the correct equipment, but it is important to do. You want to know you nailed the root cause, as getting it wrong could cost you needless expense. I recommend removing the radiator and sending it out for professional testing.

Expect to replace the radiator, as a leaking radiator is the only likely explanation for tranny fluid inside the coolant reservoir and it also happens to be a know fault, especially with some models.

Before fitting the new radiator, the whole system will need to be flushed. This is also a job for a pro shop, specialized kit and a de-greasing agents are required. The system will need to filled with fresh coolant and system bled.

What Happens When Coolant Leaks Into Transmission?

Transmission oil contamination

If you dip your transmission and find it is indeed a pink milkshake color, your transmission is likely contaminated and the news isn’t good I’m afraid, it will need a rebuild. Makes sense to begin by having all the facts, that means having the radiator removed and pressure tested by a professional.

With such an expensive repair bill ahead it’s better to be sure you can pinpoint the source of your problem.

A workshop may advise flushing the system with gallons and gallons of fresh transmission fluid, but in my experience this will only work short term, if at all.

The only solution is a transmission rebuild. The reasons are simple and obvious when explained. Coolant is as you know 50% water and water is near impossible to remove from the transmissions tiny passageways and torque converter.

Over time the water coolant mix attacks transmission components and causes problems like:

  • Corrodes steel components
  • Causes seals to swell and leak
  • Disintegrates the clutch disk materials
  • Shorts out electrical shift solenoids
  • Corrodes electrical connectors
  • Causes transmission pump scoring

In addition to a transmission rebuild, you’ll need to replace the radiator or consider plunging the the cooler ports on the radiator and instead fit an external cooler. This will prevent this ever happening again and is a relatively simple modification.

I don’t understand why manufacturers still use the integrated cooler system. This is a common failure and the damage caused is expensive to repair. But maybe I’m answering my own question here.

Is Coolant And Transmission Fluid The Same Thing?

Transmission oil

Coolant is designed to help cool the engine and is mixed with water and filled through the coolant reservoir. Transmission fluid on the other hand is an oil that is employed by the transmission to help lubricate and cool the transmission.

Mixing either of the fluids would cause serious damage, if you are in any doubt about where these fluids are filled, check your drivers manual.

The coolant is traditional filled through the radiator cap and on more modern vehicles through the coolant reservoir. The reservoir or cap will carry a radiator symbol or wording to indicate coolant type etc.

Transmission oil in coolant

The transmission fluid on the other hand is traditionally filled through the dipstick, but be mindful your car will likely have two dipsticks, one for the engine oil and a second for the transmission fluid.

What Happens If You Put Transmission Fluid In The Coolant Reservoir?

Mistakenly putting transmission fluid into the coolant reservoir is an unfortunately mistake but don’t beat yourself up, this is a fixable problem. If you spotted your mistake before starting the engine great, the problem should be a straight forward drain and refill.

If however the engine was started the job is a little bigger and will likely need to visit a shop for a procedure known as flushing. When the engine is started the contaminated fluid is pumped throughout the complete system. That includes:

  • Radiator – including transmission cooler core (if applicable)
  • Coolant reservoir
  • Thermostat housing
  • Water-pump
  • Engine – including alternator cooler, oil cooler, throttle body cooler, turbo charger cooler (if applicable)
  • Heater core

The shop uses a specialized flushing machine with a special formula to de-grease the coolant system.

Can Low Coolant Cause Transmission Slip?

Coolant is used to help control heat but not just engine heat. A vehicles coolant system is commonly employed to also help cool, throttle body’s, alternators, engine oil, turbo chargers and transmissions. A low coolant level may cause any of these components to misbehave.

Transmission oil as you know gets hot and needs cooling, low coolant will cause some transmission problems especially under heavy load.

That said, transmissions slip for a variety of reasons, common among them are:

  • Low fluid level
  • Old worn out fluid
  • Overheating transmission – Possibly caused by low coolant
  • Clutches worn
  • Contaminated fluid – Coolant or water mixed with tranny oil
Coolant system info graphic

Related Question

What temperature is too high for a transmission? A transmission starts to overheat and damage components above 210 degrees F. Overheating transmission oil is commonly caused by:

  • Towing a trailer
  • Low transmission oil level
  • Worn out transmission oil
  • Low coolant level

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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