How Transmission Fluid Gets Into the Torque Converter

Transmission fluid is pushed into the torque converter by the transmission’s front (main) pump, which draws fluid from the pan and sends a regulated “converter charge” stream through internal passages in the stator support/input shaft. The fluid fills the converter, circulates through the impeller, turbine, and stator, exits via an outlet passage to the cooler, and returns to the transmission and sump. This closed-loop flow provides both hydraulic coupling and cooling, and is managed by valves and, in modern units, electronically controlled solenoids.

The Basic Hydraulic Supply

In a conventional automatic transmission, a mechanically driven front pump (typically a crescent gear or vane pump) is bolted to the case at the bellhousing. It is driven by the engine through the torque converter cover/hub. When the engine turns, the pump pulls ATF through the pickup and filter, creates line pressure, and the pressure regulator and converter-charge circuits meter part of that pressure to feed the torque converter continuously.

The following components coordinate to move fluid into, through, and out of the torque converter:

• Front (main) pump: Creates flow and pressure from fluid in the pan/sump via the filter.
• Pressure regulator valve: Sets line pressure and diverts a portion to converter charge pressure.
• Converter charge/limit valves: Control how much pressure and flow enter the converter to ensure cooling and lubrication.
• Stator support/input shaft passages: Hollow shafts and drilled galleries route fluid into the converter and return it out.
• Converter outlet and cooler circuit: Hot fluid leaves the converter, runs through the external cooler/heat exchanger, then returns to the transmission.
• Lube and return circuits: After cooling, fluid lubricates bearings/gears and drains back to the pan for recirculation.

Together, these parts ensure the converter is always filled and circulating fluid whenever the engine is running, allowing hydraulic torque multiplication, lock-up control, and effective heat removal.

Step-by-Step Flow Path

While details vary by transmission family (RWD longitudinal, FWD transaxle, hybrid with auxiliary pumps), the fundamental flow path is similar across most modern automatics:

1. ATF sits in the transmission pan (sump).
2. The front pump draws fluid through the pickup and filter.
3. The pressure regulator sets line pressure; a dedicated circuit meters “converter charge” flow.
4. Converter feed oil travels through the stator support/input shaft’s internal passages into the torque converter.
5. Inside the converter, feed oil enters the impeller (pump) section, is flung to the turbine, and redirected by the stator.
6. Fluid exits the converter via an outlet passage.
7. The outlet directs fluid through the transmission cooler (in-radiator or external heat exchanger).
8. Cooled fluid returns to the transmission, often through a lube circuit, and eventually drains back to the pan.

This continuous loop keeps the converter full, transmits torque hydrodynamically, and manages heat generated under load and during slip.

What Happens During Lock-Up

Most modern converters include a torque converter clutch (TCC) that mechanically links the converter cover to the turbine at cruise to improve efficiency. Hydraulic control valves and solenoids direct “apply” and “release” oil to the clutch. During TCC operation, the transmission may alter converter flow direction or routing to maintain cooling while applying the clutch.

Common operating states and how fluid routing is handled:

• TCC off (open converter): Continuous flow feeds the converter, exits to the cooler, and returns to the sump/lube circuits.
• TCC applying (modulating): Solenoids and valves proportion apply and release pressures to engage the clutch smoothly; flow is maintained for cooling and clutch control.
• TCC on (locked or controlled slip): Converter flow may be rerouted (e.g., through the clutch circuit) to keep the clutch face cool while minimizing unnecessary circulation losses.

The result is efficient cruising with reduced heat and improved fuel economy, while preserving lubrication and cooling inside the converter.

Design Variations and Modern Considerations

Inlet/Outlet Passages

Many units feed the converter through the hollow stator support and return fluid via separate drilled galleries; others route feed and return through specific channels in the input shaft. Layout differs among manufacturers (e.g., GM, Ford, ZF, Aisin/Toyota), but the principle—regulated charge into the converter, controlled outlet to the cooler—remains consistent.

Stop-Start and Hybrid Systems

With engine stop-start, some transmissions use an auxiliary electric pump to maintain converter and clutch feed when the engine is off at a stop. Hybrids may use electric pumps exclusively at low or zero engine speed, ensuring the converter and lubrication circuits remain active.

Cooler Bypass and Thermostats

Many late-model transmissions incorporate cooler bypass valves or thermostats to speed warm-up and prevent over-cooling, as well as check valves to maintain prime and direct proper flow on start-up and during temperature changes.

Symptoms When the Converter Isn’t Getting Fluid

If converter charge flow is inadequate—because of low fluid, a clogged filter, pump wear, stuck valves, misrouted cooler lines, or a failed auxiliary pump—observable symptoms can develop quickly.

• Delayed or slipping engagement when shifting into Drive/Reverse, especially cold.
• Overheating, dark/burnt ATF, or frequent thermal derate modes.
• Shudder during lock-up or inability to achieve/hold TCC engagement.
• Whine or growl from the pump/front end of the transmission.
• Diagnostic trouble codes for pressure, TCC performance, or converter clutch slip.

Addressing fluid level/quality first, then inspecting the filter, cooler flow, pump pressure (with a gauge), and relevant valves/solenoids typically isolates the cause.

Maintenance and Service Tips

Keeping the converter’s charge circuit healthy comes down to correct fluid, good filtration, and proper assembly practices during service or rebuild.

• Use the exact OEM-specified ATF; friction and viscosity profiles affect charge, TCC control, and cooling.
• Maintain proper fluid level; both low and overfilled conditions can aerate fluid and starve the converter.
• Replace the filter and inspect the pan for debris on schedule or when symptoms appear.
• When reinstalling a transmission or converter, always seat the converter fully on the pump and pre-fill it if specified; misalignment can damage the pump tangs and kill charge flow.
• Prime the system after overhaul; verify cooler flow and check for thermostatic/bypass valve function.
• Confirm cooler line routing and any check valves/thermostats are installed in the correct orientation.

These steps preserve converter charge flow, reduce heat, and extend transmission life.

Summary

The torque converter is supplied by the transmission’s front pump, which sends regulated “converter charge” fluid through internal shafts and passages into the converter, where it circulates, exits to the cooler, and returns to the transmission. Control valves and—on modern units—electronic solenoids manage this flow for performance, cooling, and torque converter clutch operation. Proper fluid, correct assembly, and sound cooling circuit function are essential to keep the converter fed and the transmission reliable.

Does the transmission and torque converter share fluid?

How does the torque converter work? The torque converter is, essentially, a fluid pump. It shares fluid with the transmission that is pumped outward toward the outside of the torque converter. Inside is the torque converter is an impeller.

How much transmission fluid stays in the torque converter?

Torque Converter Fluid Capacity
Size of Converter	# of Quarts
9″	2.5
8″	2
7″	2

Does transmission fluid affect the torque converter?

Failure of Internal Components: Components like bearings, seals, and vanes can wear out or become damaged due to age, leading to reduced performance. Incorrect Fluid Type: Using the wrong type of transmission fluid can adversely affect the torque converter’s operation, potentially leading to damage over time.

How does transmission fluid get in the torque converter?

The transmission fluid is drawn from the pan through a filter by the pump that is behind the torque converter. The fluid level in the pan and transmission body is critical.