What Is a Torque Converter Filled With?

A torque converter is filled with automatic transmission fluid (ATF). This specialized hydraulic oil circulates through the converter to transmit power, multiply torque at low speeds, lubricate internal parts, and dissipate heat. In most conventional automatics—and in some CVTs that use a torque converter—the unit is fully flooded with the manufacturer-specified fluid while the engine runs, with the transmission’s pump constantly moving fluid through the converter, cooler, and back.

How the Fluid Works Inside the Converter

Inside the torque converter, an engine-driven impeller (pump) slings ATF outward to drive a turbine connected to the transmission input shaft, while a stator redirects flow to increase torque at low speeds. The same fluid also supports a lock-up clutch that mechanically links engine and gearbox at cruising speeds to reduce slip and improve efficiency.

The following points outline what ATF actually does in this hydrodynamic coupling:

• Transmits power: Fluid flow between the impeller and turbine transfers engine torque without a rigid mechanical connection.
• Multiplies torque: The stator redirects returning fluid to boost turbine torque during launch and low-speed operation.
• Lubricates components: ATF reduces wear on bearings, thrust washers, and the lock-up clutch.
• Controls friction: Additives tailor grip for smooth clutch lock-up and release without shudder.
• Manages heat: Continuous circulation carries heat to the transmission cooler to prevent overheating.

Together, these roles let ATF function as both a hydraulic medium and a thermal/lubrication agent, enabling smooth takeoff, efficient cruising, and long component life.

Why the Exact Fluid Matters

Not all ATFs are interchangeable. Modern transmissions rely on precise fluid chemistry for shift quality, durability, and clutch control. Using the wrong fluid—or mixing types—can cause shudder, slipping, or overheating.

Key characteristics of the specified fluid include:

• Viscosity and shear stability: Maintains consistent flow and pressure across temperatures and miles.
• Friction modifiers: Calibrated for the converter’s lock-up clutch engagement characteristics.
• Oxidation and thermal resistance: Resists breakdown under high heat and shear inside the converter.
• Anti-foaming and anti-wear additives: Ensures stable hydraulic action and protects components.
• Compatibility: Matches seals and materials used by the transmission manufacturer.

OEM specifications—such as Dexron VI, Mercon LV/ULV, Toyota WS, ZF Lifeguard, Honda DW-1, or dedicated CVT fluids—are engineered for particular hardware and control strategies; always use the listed fluid.

Service Implications and Signs of Trouble

While some automatics are labeled “filled for life,” real-world heat and duty cycles often warrant periodic service. Follow the vehicle manufacturer’s interval and procedure, especially for sealed units that require temperature-based level checks.

Watch for these indicators that the converter’s fluid may need attention:

• Lock-up shudder or vibration at steady cruise.
• Delayed engagement when shifting into Drive or Reverse.
• Excessive slip, surging, or poor acceleration off the line.
• Overheating warnings, dark/burnt-smelling fluid, or metallic debris.
• Fluid leaks leading to low level and aeration (foaming).

Addressing fluid condition early—using the correct ATF and proper fill procedures—can restore smooth operation and prevent costly damage.

Note on CVTs and Hybrids

Many CVTs use a dedicated CVT fluid with different friction properties; some include a torque converter that is filled with that CVT fluid, not conventional ATF. Most dual-clutch transmissions (DCTs) do not have torque converters, though some hybrid systems do.

Summary

A torque converter is filled with automatic transmission fluid, which functions as the hydraulic medium for power transfer, torque multiplication, lubrication, and cooling. The exact, OEM-specified fluid is critical to performance and longevity, particularly for the lock-up clutch. Proper fluid type, level, and condition are central to smooth operation and avoiding shudder, slip, or overheating.

What fluid is in a torque converter?

A torque converter contains automatic transmission fluid (ATF), sometimes called torque fluid, which serves as the hydraulic fluid that transfers power from the engine to the transmission. This fluid is crucial for the torque converter’s operation, as it creates the friction and hydraulic pressure needed to spin the transmission’s internal components, allowing the vehicle to move.
 
How the Fluid Works

1. Power Transfer: The engine spins the impeller (or pump), which is directly connected to the engine’s crankshaft. 
2. Fluid Circulation: As the impeller spins, it pushes transmission fluid outwards with great force. 
3. Turbine Rotation: This fluid then strikes the blades of the turbine, causing the turbine to spin. 
4. Transmission to Gears: The turbine is connected to the transmission’s input shaft, so as the turbine turns, it transfers engine power to the transmission’s gears. 

Why the Fluid is Necessary

• Lubrication: The fluid lubricates the internal components of the torque converter. 
• Hydraulic Action: The fluid’s viscosity and the way it’s directed creates the hydraulic pressure needed to transmit torque. 
• Damping: It allows the vehicle to idle without lurching forward by absorbing the engine’s initial torque. 
• Cooling: The fluid also helps to cool the torque converter and transmission components, especially under heavy loads. 

Important Considerations

• Viscosity: Opens in new tabThe fluid’s viscosity is important; it needs to be viscous enough to transfer power effectively but not so viscous that it clogs the delicate internal parts. 
• Correct Fluid Type: Opens in new tabIt’s essential to use the specific type of transmission fluid recommended by the vehicle’s manufacturer. 
• Fluid Level: Opens in new tabThe fluid level must be sufficient for the torque converter to function correctly; low fluid levels can lead to transmission problems. 

What is inside a torque converter?

A torque converter contains an impeller connected to the engine, a turbine linked to the transmission’s input shaft, and a stator to redirect fluid and multiply torque. These components are housed in the converter’s casing and operate within a bath of transmission fluid. Many modern torque converters also feature a lock-up clutch that engages to create a direct mechanical connection at higher speeds, eliminating slippage.
 
This video provides an overview of the internal components of a torque converter: 41sTransmissions America (John and Eddie) YouTube · May 8, 2023
Here are the main components and their functions:

• Impeller (or Pump): This is a wheel with curved fins that is welded to the converter’s housing and connected to the engine’s flywheel. As the engine spins, the impeller flings transmission fluid outward with centrifugal force, transferring power. 
• Turbine: Located opposite the impeller, this component is connected to the transmission’s input shaft. The high-pressure fluid from the impeller strikes the turbine’s angled blades, causing it to spin and drive the transmission. 
• Stator: Positioned between the impeller and the turbine, the stator is equipped with a one-way clutch or sprag clutch. It redirects the fluid returning from the turbine back to the impeller, increasing its pressure and multiplying the engine’s torque, especially at low speeds. 
• Lock-Up Clutch: Found in many modern converters, this clutch assembly is attached to the turbine and can be hydraulically engaged. When engaged, it locks the turbine directly to the converter housing, creating a direct drive to the transmission and improving fuel efficiency by eliminating slippage. 
• Transmission Fluid: This incompressible fluid acts as the medium for transferring power between the impeller and the turbine, similar to how air transfers power between two fans. 
• Bearings and Seals: Various bearings and seals within the torque converter ensure smooth rotation of its components and prevent fluid from escaping. 

This video explains the basic operation of a torque converter: 44sSabin Civil EngineeringYouTube · Apr 13, 2018

How to fill up a torque converter?

To fill a torque converter, pour a quart or so of transmission fluid into it, then spin the converter to help it fill, repeating until you see bubbles or can’t add any more fluid. Afterward, install the converter properly into the transmission, ensuring it seats fully by listening for clicks and checking the depth. Finally, complete the installation of the engine and transmission, then check and adjust the transmission fluid level while the engine is running.
 
Before Installation

1. Prepare the Converter: Remove any packaging and, if applicable, spin the old converter a few times to drain old fluid. 
2. Add Fluid: Pour transmission fluid into the converter’s opening until it starts to bubble. 
3. Spin and Repeat: Rotate the converter by hand to encourage the fluid to circulate and the converter to fill. Continue adding fluid and spinning until you can’t fit any more in without it bubbling out. 

During Installation

1. Align and Insert: Carefully slide the torque converter into the transmission. 
2. Seat the Converter: Rotate the converter as you push it in to help it engage the pump drive, splines, and input shaft, listening for a distinct “click” or “pop”. It should feel fully seated, and the distance between the converter’s mounting flange and the bell housing should be about one inch. 
3. Secure the Converter: Once fully seated, install the bolts that secure the torque converter to the flex plate. 

After Installation

1. Connect Components: Reinstall the transmission, engine, and any other removed components in reverse order of their removal. 
2. Fill the Transmission: Before starting the engine, add the proper amount of transmission fluid through the dipstick tube. 
3. Check the Level: Start the vehicle, let it warm up, and then shift through all the gears, pausing in each. With the engine still running and the transmission in Park, check the fluid level and add more until it drips out of the fill plug. 

Are torque converters filled with ATF?

A torque converter is filled with Automatic transmission fluid (ATF). If the fluid contains debris, grime, or a black sludge, it is contaminated, and this is a sign that the transmission is damaged. Also, dirty fluid can impact the components of the torque converter such as the stator’s bearings and turbine’s fins.