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torque converter for automatic transmission


How torque converters work, and how to diagnose them
Copyright AA1Car

A torque converter is the donut-shaped object inside the bellhousing between the engine and transmission in automatic-equipped vehicles. It is actually three different things all rolled into one: a hydraulic slip clutch that allows the engine to idle while in gear; a reduction gear that multiplies engine torque; and a fluid coupling that transmits engine torque to the transmission.

torque converter components

To better understand how a torque converter works, let's look at the three basic components inside it: the impeller, turbine and stator wheels.

The impeller (also called the pump) is attached to the outer housing. This is the driving member in the converter. The blades on the impeller sling oil outward towards the turbine wheel as the converter spins. The converter housing is bolted to the flexplate on the end of the crankshaft so the impeller always pumps oil when the engine is running.

Facing the impeller is a second set of curved blades, the turbine wheel. This is the driven member in the converter. When oil from the impeller hits the turbine blades, the turbine wheel wants to spin. A splined shaft couples the turbine wheel to the transmission input shaft, so when the turbine wheel turns it drives the transmission.

Sandwiched between the impeller and turbine is a second wheel, the stator. The stator is a sort of control member. It multiplies torque by completing the oil flow circuit between the impeller and turbine. When the oil slings off the impeller and hits the curved blades of the turbine, it has to go somewhere. The stator redirects oil back towards the impeller where it gives the impeller an added boost. This helps the oil flow in a circular path (called "vortex" flow) for greater driving efficiency.


The stator wheel is mounted on the converter hub and floats on a set of one-way roller bearings. The one-way clutch allows the stator to rotate in one direction but not the other. The stator's blades curve in the opposite direction of the impeller and turbine blades. At idle, oil coming off the turbine blades hits the stator blades in such a way that the stator wants to turn the wrong way. It does not spin, however, because the one-way clutch holds it tight. As a result the oil slings back towards the impeller.

torque converter

As long as the impeller continues to turn faster than the turbine, torque will be multiplied and the converter will act like a giant reduction gear. The amount of multiplication is usually somewhere around two to two-and-a-half times engine output.

As the vehicle starts to move, however, the speed of the turbine wheel starts to catch up with that of the impeller. As the speed of the turbine approaches 90 percent of the speed of the impeller, the fluid dynamics inside the converter change. The fluid flows at a much steeper angle and now strikes the stator blades from the backside. This pushes the stator in the right direction and starts it turning. And as soon as the stator starts to spin, however, torque multiplication is lost and the converter locks up. The stator freewheels at the same speed as the turbine and impeller, and the three elements become a fluid coupling.


The torque converter is the fluid coupling between the engine and transmission. If the transmission is making noise in gear, but the noise goes away when it is shifted into neutral, the problem may be worn needle bearings in the torque converter. Needle bearings are used inside the torque converter to separate the stator from the impeller, the stator from the turbine, and the turbine from the converter housing.</P>

Torque converters contain a one-way clutch. If the clutch jams and locks the stator (which normally keeps the stator tuning only one way), the converter cannot circulate the fluid properly between the impeller and turbine. The resulting turbulence inside the unit can cause a lack of power at highway speeds and may cause the engine to overheat during cruise conditions. A locked one-way clutch will also cause the fluid to run dangerously hot, which can damage the transmission and lead to transmission failure. If the one-way clutch fails to hold its position and freewheels in both directions, the torque converter cannot multiply torque normally and the vehicle will accelerate slowly (like starting out in 2nd gear). The only cure for either condition is to replace the torque converter.


A stall test can be used on many older transmissions to check its ability to hold torque and the operation of the converter one-way clutch. Some manufacturers do not recommend using a stall test because it stresses the transmission, so if you use this procedure, do not do it for more than five seconds at a time. Do not perform this test on vehicles that are equipped with a traction control and/or ABS systems. The inputs of a partially opened throttle, brakes applied, and no detectable movement by the wheel speed sensors will at best set a code, and at worst cause damage to the vehicle. Also, on electronically controlled transmissions that use vehicle speed sensor to determine shift points be prepared for a code to be set if you perform a stall test

Before conducting a stall test, check the fluid level and condition. Chock the wheels and set the parking brake. Start the engine and place the transmission in drive while holding the brake pedal under firm pressure so the vehicle does not move. Then push the accelerator to the floor while holding the brakes on. Note the maximum rpm the engine reaches. This is the stall speed. If it is lower than specifications, the torque converter one-way clutch is slipping. If the stall speed is higher than specifications, the transmission is slipping. Possible causes include a low fluid level, restricted fluid filter, a sticking pressure regulator valve, slipping clutches, bands, shaft splines or one-way clutch.


Most late-model automatics have some type of lockup torque converter to improve fuel economy. If the lockup fails to engage, there will be some slippage and fuel economy will drop. Causes here include a bad lockup solenoid, incorrect sensor input information to the transmission controller or PCM (typically a speed sensor) and hydraulic control problems. If the lockup fails to release, the engine may shudder and die when coming to a stop. Causes here include a faulty lockup solenoid, a sticking lockup valve, grounded lockup solenoid wiring or a missing lockup solenoid spacer plate screen.

Lockup shudder is another complaint that may be encountered. This refers to a vibration that is felt just before or after lockup occurs. This kind of problem can be hard to diagnose because the cause may be the torque converter, transmission or engine. Bad motor mounts, engine misfire, a bad CV-joint or U-joint, etc. can all cause vibrations that may be felt as a shudder throughout the drivetrain.

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