Automatic Transmission Torque Converters

A torque converter is a fluid coupling device that transmits engine power to the automatic transmission. It serves the same purpose as the clutch in a vehicle with a manual transmission, in that it allows the engine to continue running when the vehicle is brought a stop while in gear, except the torque converter does this without requiring any action on the part of the driver. The torque converter, along with the transmission components that enable it to shift by itself, are what make the transmission automatic. Torque converter function is analogous to 2 electric fans positioned face-to-face, with only one turned on. Airflow from the powered fan will turn the blades of the other fan without any mechanical connection. The impeller inside the torque converter is attached to the engine’s crankshaft, and like the powered fan, provides force, except the medium inside the torque converter is transmission fluid instead of air. The torque converter turbine, which is attached to the transmission input shaft, is like the non-powered fan.

The torque converter housing is bolted to a flexplate, which is bolted to the engine crankshaft, and the converter is housed inside the forward part of the automatic transmission case. The impeller, sometimes called the pump, is in the rear of the housing, facing the engine and opposite from the turbine in the front of the housing. Another component called a stator is located between the impeller and turbine. Both the impeller and turbine have many curved blades. When the engine is running the impeller blades direct fluid at the turbine blades, turning the transmission and moving the vehicle. Fluid enters the turbine at the outside and exits at the center. However, if it were allowed to flow unimpeded upon exit, it would contact the impeller in the opposite direction, hindering impeller rotation. Instead, the stator redirects the fluid, so it contacts the impeller in the same direction it’s rotating, in effect increasing the force being applied to the impeller by the engine.

There are 3 stages of torque converter operation. The first stage is Stall, which occurs at engine idle speed and just off idle when the vehicle begins to move. At idle the impeller isn’t turning very fast and so isn’t applying fluid with much force, so the turbine doesn’t turn, and the vehicle won’t move as long as the brake is applied. In the fan analogy, this is akin to holding the blades of the non-powered fan still – the powered fan will continue to run. When the driver releases the brake pedal and presses on the gas pedal, the impeller turns much faster and there is a great difference between impeller and turbine rotational speeds.

This is when torque multiplication is greatest and needed the most, to get the stationary vehicle moving. During the Acceleration stage torque multiplication continues but at a reduced amount as turbine speed catches up to impeller speed. The Coupling stage is when turbine speed is almost the same as impeller speed, however there is still some degree of slippage, which creates heat and inefficiency. To eliminate this inefficiency and increase fuel mileage at cruising speed, a torque converter clutch is used to lock the turbine to the impeller. Torque converters are generally very reliable components and should last the life of the vehicle. However, if the vehicle is used for towing, hauling, and other types of severe use that can be hard on transmission components and transmission fluid, and/or if regular maintenance like fluid changes are neglected, then the torque converter and other transmission components may fail prematurely.

Torque converter failure symptoms include stall speed that is higher or lower than normal, slipping, overheating, shuddering, noise, and fluid leakage. Since symptoms like slipping and overheating can be caused by a low fluid level, the fluid level should always be checked first. The torque converter seal can be the source of a leak. The impeller, turbine and stator rotate on bearings and this can be the source of noise if they are defective. Shuddering can be caused by a defective torque converter clutch. Whatever kind of torque converter problems you’re experiencing, we have the right torque converter and related components you may need to restore proper automatic transmission operation.