Chapter 17

Torque Converters

Objectives (1 of 2)

• Explain the function of the torque converter in a vehicle equipped with an automatic transmission.

• Explain how the torque converter is coupled between the crankshaft and the transmission.

• Identify the three main elements of a torque converter and describe their roles.

• Define torque multiplication and explain how it is generated in the torque converter.

Objectives (2 of 2)

• Define both rotary and vortex fluid flow and explain how each affects torque converter operation.

• Describe the overrunning clutch, lockup clutch, and variable pitch stators.

• Outline torque converter service and maintenance checks.

• Remove, disassemble, inspect, and reassemble torque converter components.

Shop Talk

• Torque converters can be confused with fluid couplings because both use similar operating principles.

• The most fundamental difference is that torque converters use curved blades, while fluid couplings and fluid flywheels use straight pitch blades.

• Torque converters also use stators and have the ability to multiply torque, neither of which is characteristic of fluid couplings.

Torque Converters

• Automatic transmissions use a torque converter to couple the engine to the transmission.

• The torque converter:– Transmits the twisting force or torque delivered to it

by the engine crankshaft

– Multiplies engine torque when additional power is needed

• The amount of torque transferred from the engine to the transmission by the torque converter is directly related to engine rpm.

Torque Converter Construction

• Truck torque converters can be:– Serviceable

• Many torque converters are welded together. A normal service shop cannot disassemble them for servicing and repair.

– Non serviceable• The only types of torque converters (T/Cs) that

are readily serviceable are the types that are bolted together.

Flex Plate

• The flex plate carries the starter motor ring gear.

• The combined mass of the torque converter and flex disc acts like a flywheel to smooth out the power pulses produced by the engine.

• The flex plate also allows for a slight alignment tolerance between the engine and torque converter assembly.

Components

• The torque converter has three main components.– Impeller

– Turbine

– Stator

– Optional lockup clutch

Exterior

• The exterior of the torque converter shell is shaped like two bowls facing each other. – They are either welded or

bolted together.• A pilot shaft supports the

weight of the torque converter at the front.

• At the rear of the torque converter shell is the pump drive hub with notches or flats which are used to drive the transmission pump.

• Curved blades rotate as a unit with the shell at engine speed. It starts the transmission oil circulating within the torque converter shell.

• The impeller is positioned with its back to the transmission housing; the turbine is positioned with its back to the engine.

• The hub of the turbine is splined so that it can drive the turbine (transmission input) shaft. The turbine shaft transfers engine torque to the transmission gearing.

• The turbine blades are designed to have a greater curve than the impeller blades.

• This helps reduce oil turbulence between the turbine and impeller blades–turbulence that would slow impeller speed and reduce the converter’s efficiency.

• A fundamental law of hydraulics states, “The more the moving stream of fluid is diverted (changed), the greater the force it places on the curved reaction surface.”

• As oil in the torque converter moves around the turbine blades, it pushes against the blades and transmits additional force.

Principles of Operation

• As the pump impeller rotates, centrifugal force throws the oil outward and upward.

• The faster the impeller rotates, the higher the centrifugal force.

• Fluid under pressure is continuously delivered through the converter hub.

• It is important to note that the oil pump delivering the fluid is driven by the engine.

• A seal or combination of seals prevents fluid from being lost from the system.

• Oil leaving the turbine is directed to an external oil cooler and then to the transmission oil sump.

• At idle there is insufficient centrifugal force within the torque converter to move the truck.

• As impeller speed increases the centrifugal force of the oil directed against the turbine blades becomes great enough to move the vehicle.

Shop Talk

• There can be a mechanical connection between the impeller and turbine by the use of a lockup clutch. – A lockup clutch eliminates slippage between

the impeller and turbine at certain speeds.– This helps to reduce the heat generated in the

fluid and improves fuel mileage.

Types of Oil Flow (1 of 2)

• The two types of oil flow that occur within the T/C are:– Rotary

• Rotary flow describes the centrifugal force applied to the fluid as the converter rotates on its axis.

– Vortex• Vortex flow is the

circular flow that occurs as the oil is forced from the impeller to the turbine and then back to the impeller.

Types of Oil Flow (2 of 2)

• If a toy pinwheel were held at arm’s length and swung in a large circle, air movement at the outer circle would produce rotary flow, while the small circles cut by the pinwheel’s propeller vanes would produce vortex flow.

• The point when the speed of the turbine approaches the speed of the impeller is referred to as the coupling point.

Split Guide Rings

• Fast moving oil exits the impeller blades, striking the turbine blades with considerable force.

• It then has a tendency to be thrust back toward the center of both impeller and turbine.

• To control this fluid thrust and the turbulence that results, a split guide ring is located in both the impeller and turbine sections of the T/C.

• The guide ring suppresses turbulence, allowing more efficient operation.

Stator (1 of 3)

• The stator is the key to torque multiplication. It redirects the oil leaving the turbine back to the impeller.

• The stator then redirects the fluid flow so that the oil reenters the impeller, moving in the same direction as the impeller.

• The kinetic energy remaining in the oil now helps rotate the impeller with more force, multiplying torque.

Stator (2 of 3)

• The roller clutch is designed with an inner race, rollers, accordion (apply) springs, and outer race.

• Around the inside diameter of the outer race are several cam-shaped pockets. – The rollers and accordion

springs are located in these pockets.

• As the vehicle begins to move, the stator stays in its stationary or locked position.

Stator (3 of 3)

• Locking mode takes place when the outer race attempts to rotate counterclockwise.

• The accordion springs force the rollers down the ramps of the cam pockets.

• As vehicle road speed increases, turbine speed approaches impeller speed.

• Oil exiting the turbine vanes now strikes the back face of the stator, causing the stator to rotate in the same direction as the turbine and impeller, unlocking the clutch.

Variable Pitch Stator

• Each of a series of movable stator vanes has a crank rod fitted into a circular groove in the hydraulic piston.

• The movement of a hydraulic piston varies the angle of the stator vanes.

Lockup Clutches

• A lockup torque converter eliminates the 10 percent slip that takes place at the coupling phase of operation.

• The engagement of a clutch has the advantage of improving vehicle fuel economy, lowering overall engine emissions, and reducing torque converter operating heat and engine speed.

Caution

• The torque converter is free to move forward when the transmission is disconnected from the engine.

• To ensure that the torque converter does not separate from the transmission while the transmission is being removed from the vehicle, install a retaining strap to hold the T/C in position.

Lockup Clutch Back Plate

• The back plate should be flat to within 0.006 inch.

• Inspect the football key slot for evidence of wear from movement of the lockup back plate and football key.

• Excessive wear or elongation of the football key slot will require component replacement.

• Inspect the back plate for cracks, from the key slot to the ID of plate.

Turbine Assembly

• Inspect the turbine assembly for cracked or broken vanes and signs of overheating.

• If the turbine assembly is taken apart, inspect the rivet holes for signs of wear or elongation.

• The turbine assembly hub should be checked for stripped, twisted, or broken splines.

Lockup Clutch Piston

• Inspect the seal ring groove.

• Check the lockup pinholes.

• Inspect the friction surface for wear, scoring, scratches, signs of overheating, and flatness. – It should be flat to within

0.003 inch TIR.

• Inspect the seal surface.

Inspecting the Stator Assembly

• Inspect the stator assembly. • Inspect the rivets for cracks

or loose fit. • Check the stator cam roller

pockets.• Inspect the stator thrust

bearing race surface.• Check the stator freewheel

roller surface, thrust bearing surface, and roller bearing surface.

• Inspect the stator freewheel rollers.

Pump/Impeller

Pump (Impeller) Hub (1 of 2)

• Check the seal ring grooves on the pump hub for cracked edges, nicks, burrs, and sharp edges.

• Inspect the front seal surface for scoring, scratches, nicks, and grooves. – No rework of any

irregularities noted is allowed on the seal surface.

– The use of a crocus cloth or light honing on this surface could promote leakage past the front seal.

Pump (Impeller) Hub (2 of 2)

• There should be no signs of cracks, scoring, metal transfer, or heat damage on the pump drive flange.

• Inspect the snap ring groove for burrs, cracks, and nicks.

• Check the bearing race surface.

• Inspect the roller bearing bore.

• Inspect the gasket surface. • Inspect the pump hub for

pulled, stripped, or crossed threads.

Summary (1 of 2)

• Automatic truck (and passenger car) transmissions use a type of fluid coupling known as a torque converter to transfer engine torque from the engine to the transmission.

• A flex plate, sometimes called a flex disc, is used to connect the torque converter to the crankshaft.

• Transmission oil is used as the medium to transfer energy from the engine-driven impeller to the turbine, which in turn drives the transmission.

Summary (2 of 2)

• Two types of oil flow take place inside the torque converter.– Rotary flow and vortex flow

• A converter lockup clutch enables a mechanical coupling of the engine and transmission.