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Transmission Fluid Leak: What You Need to Know & How to Fix It

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Your car’s transmission manipulates rotational force from the engine and transfers that force to the drive wheels. Without a functional transmission, your vehicle might run, but it won’t move under its own power.

It goes without saying that the transmission is an essential part of your car⁠—and it’s also very expensive. In most cases, only the engine costs more than the transmission to replace. So, you want to make sure there’s always enough fluid to cool and lubricate the transmission’s internal components.

Leaks can starve the transmission of the fluid it so desperately needs, resulting in extensive damage and a massive repair bill. You’ll want to fix any leaks right away to avoid major problems down the road.

Trans leak 1 1
Your car’s transmission manipulates rotational force from the engine and transfers that force to the drive wheels.

What Causes a Transmission Leak?

You’re most likely to notice a transmission fluid leak when parked. Droplets of fluid underneath the transmission are a telltale sign of a leak. Automatic transmission fluid is typically red in color when it’s relatively fresh and clean. Manual transmission fluid (or gear oil) is semi-transparent or honey-colored.

But what exactly causes a transmission fluid leak in the first place? Unfortunately, the answer to that question isn’t straightforward. A transmission fluid leak can stem from a variety of sources, ranging from a worn pan gasket to a cracked housing.

Some of the most common causes of a transmission fluid leak include:

Trans pan 1
An example of a leak from the transmission pan gasket.

Worn Transmission Pan Gasket

Most automatic transmissions have a pan that acts as a reservoir for fluid. A gasket provides a seal between the pan and the transmission. That gasket can eventually wear out, resulting in a fluid leak.

Bad Output Shaft (Axle) Seal

Output shaft seal leaks are a common problem. On vehicles that have a rear-wheel drive bias layout, the output shaft seal is at the rear of the transmission extension housing. The seal prevents fluid from leaking out between the transmission and the driveshaft.

Vehicles with a front-wheel drive bias layout usually have two output shaft seals (also known as axle seals). The seals prevent fluid from leaking out between the transmission (also known as a transaxle) and the constant velocity (CV) axles.

Faulty Transmission Cooler Lines

Automatic transmissions have lines that run to a dedicated cooler, which is usually located inside the radiator. These lines can develop leaks, often from the crimp area, leading to a pool of fluid under your car.

Worn Fill Tube Seal

Some automatic transmissions have a dipstick tube. At the base of the tube, there’s a seal that can develop leaks.

, Transmission Fluid Leak: What You Need to Know & How to Fix It
If the source of the leak is not apparent, you’ll want to clean the transmission with a suitable solvent.

Damaged Front Pump (Torque Converter) Seal

If you find transmission fluid leaking from the bell housing area between the engine and automatic transmission, you’re probably dealing with a bad front pump seal (also known as a torque converter seal). The seal, which is usually located in the transmission pump, fits over the snout of the torque converter.

Cracked or Porous Transmission Housing

Although not as common as some other issues, it’s possible for a fluid leak to develop from a cracked or porous transmission housing. When the housing is damaged, the fix is usually to replace (rather than rebuild) the transmission.

How to Fix a Transmission Leak

Before you can fix a transmission leak, you must first find out where the leak is coming from. Safely raise and support the vehicle using a jack and jack stands, and take a look. If the source of the leak is not apparent, you’ll want to clean the transmission with a suitable solvent. Then you can run the vehicle and recheck for leaks.

If you still can’t pinpoint the leak, you can spray the suspect area with talcum powder. Run the vehicle, then recheck the transmission for leaks. In most cases, you will see the flow pattern of the leak in the talcum powder.

You can also use a black light to pinpoint leaks. First, you add fluorescent dye to the transmission, then operate the vehicle. Recheck the transmission using the black light—any leaks should appear bright yellow due to the dye mixed in with the fluid.

Once you know where the leak is coming from, you can replace the necessary component (s) to fix the problem. It’s a good idea to consult a repair manual or an online repair database for instructions on fixing your particular application.

man capturing the details of a car transmission 1
If ignored, a leak can cause the transmission to run low on fluid, resulting in severe damage.

Is it Safe to Drive with a Transmission Leak?

You should always fix a transmission leak as soon as possible. A leak can starve your transmission of the fluid that cools and lubricates vital internal components. If ignored, a leak can cause the transmission to run low on fluid, resulting in severe damage. Then, the transmission will either need to be rebuilt (if possible) or replaced.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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