Torque Converter: Description and Operation

DESCRIPTION
The torque converter provides fluid coupling and torque multiplication and acts as a shock absorber. It couples the engine to the gear train as a fluid coupling or fluid clutch, providing hydraulic drive or coupling between engine and gear train. In certain operating conditions, it multiplies torque, providing extra reduction to match engine output to the driveshaft. It also absorbs the shock of gear shifting in the drive train.

CONSTRUCTION
The A4LD 10-1/4 inch converter resembles other converters, but has a hydraulic piston plate added. This feature changes the hydraulic coupling to a more efficient mechanical coupling.

OPERATION
1. Converter clutch application is based on both engine speed and vehicle speed by combined transmission hydraulics and on-board computer electronic controls. When this clutch engages there is a mechanical connection between the engine and rear wheels. This feature is provided to improve both driveline efficiency and fuel economy. Whenever the clutch is engaged, the vehicle may respond in ways similar to driving with a manual transmission. This is normal and should not be considered as adverse or indicating need for servicing.
2. Converter clutch engagements and disengagements are scheduled hydraulically but can be overridden electronically. The converter clutch is inhibited from engaging when engine coolant temperature is below 128° F or above 24O° F, during heavy brake application, at closed throttle, during heavy or wide open throttle acceleration, during quick tip-ins or tip-outs, or when actual engine speed is below a certain value at lower vacuums. During these modes, no current flows through the solenoid.

Hydraulics With Converter Clutch Unlocked, 1988 Thunderbird And 1988-1989 Mustang:

Hydraulics with converter clutch unlocked, (1988-1990 Bronco II, 1988-1991 Aerostar And Ranger, 1990-1991 Mustang, And 1991 Explorer):

3. When the converter clutch shuttle valve is resting on the plug, line pressure is directed through the shuttle valve and to the torque converter in a flow path that pushes the lockup piston off. When line pressure on the spring end of the converter clutch shuttle valve is exhausted, line pressure on the plug end of the valve forces the valve to move and compress the spring. Governor pressure acting on the converter clutch shift valve has not yet moved the valve to the upshifted position. Line pressure is therefore acting on the spring end of the converter clutch shuttle valve. The torque converter is therefore unlocked.

Hydraulics With Converter Clutch Locked, (1988 Thunderbird And 1988-1989 Mustang):

Hydraulics With Converter Clutch Locked. (1988-1990 Bronco II, 1988-1991 Aerostar And Ranger, 1990-1991 Mustang, And 1991 Explorer):

4. As vehicle speed increases, governor pressure increases and the converter clutch shift valve moves to the upshifted position. Oil on the spring end of the converter clutch shuttle valve now drains to exhaust at the converter clutch shift valve. Line pressure is now directed through the shuttle valve to the converter in a flow path that pushes the piston on.
5. If the brakes are now applied, or the vehicle is operated in any of the other inhibit modes, current will not flow through the solenoid.

Hydraulics With Converter Clutch Unlocked But Electronically Inhibited, (1988 Thunderbird And 1988-1989 Mustang):

Hydraulics With Converter Clutch Unlocked But Electronically Inhibited, (1988-1990 Bronco II, 1988-1991 Aerostar And Ranger, 1990-1991 Mustang, And 1991 Explorer):

6. With no current to the solenoid, line pressure can flow through the solenoid valving and enter the lockup inhibition passage. Line pressure in the inhibition passage forces the shuttle ball to take its position. The shuttle valve moves against the plug and the converter unlocks. Since this is a hybrid system, it will often be necessary to check both the hydraulic and electronic portions of the system.

Component Functions

Exploded View Of Locking Torque Converter:

1 The converter cover (1) is bolted to the engine flywheel and is driven at engine speed. It has a hub that pivots in the crankshaft. Thus the cover transmits engine power into the converter.
2. The impeller (2) is the pumping member for fluid coupling and torque multiplication. It is welded to and driven by the cover. Its rear hub drives the transmission oil pump.
3. The turbine (3) is the driven member of the converter. It is driven by fluid from the impeller and is splined to the input shaft.
4. The reactor (4), also called the stator, causes hydraulic reaction during torque multiplication. It includes a one-way clutch to hold it stationary only when reaction is required.
5. The piston plate clutch and damper assembly (5) has friction material on the outer portion of the plate and a spring cushioned damper assembly. It transmits engine power to the turbine from the converter cover.
6. The thrust washers (6) and roller bearing (7) provide bearing surfaces for the converter components that turn against each other.

Torque Multiplication
1. Torque multiplication occurs from stall (engine turning but turbine stopped) up to engagement of the converter clutch.
2. The impeller is driven at engine speed. Fluid is supplied from the valve body to the spaces between the vanes. Centrifugal force causes the fluid to be set in motion and forced out of the openings around the inner ring. The fluid is directed toward the turbine vanes. Striking the turbine vanes, the fluid causes it to turn in the same direction as the impeller. The fluid flows around the turbines' inner ring and is directed to the reactor. The reactor is held stationary by its one-way clutch. Its vanes direct the fluid back to the impeller in a direction that assists the impeller. The impeller again accelerates the fluid and returns it to the turbine with increased energy.
3. Torque multiplication comes from a reduced turbine speed and reuse of the velocity of the fluid returning from the reactor to the impeller. As turbine speed increases in relation to the impeller, there is less energy in the returning fluid, and therefore less torque multiplication.

Operation And Power Flow

Torque Converter Power Flow With Hydraulic Input:

1. Power flow through the torque converter when the clutch is not active is as shown in the Torque Converter Power Flow With Hydraulic Input image. The engine drives the converter cover (1).
2. The converter cover drives the impeller (2).
3. The impeller drives the turbine hydraulically (3).
4. The turbine drives the input shaft (4).
5. The input shaft transmits power into the gear train (5).

Torque Converter Power Flow With Mechanical Input:

6. Power flow through the torque converter when piston plate clutch is engaged is as shown in the Torque Converter Power Flow With Mechanical Input image. The clutch is engaged when hydraulic pressure from the valve body is applied to the clutch plate from the turbine side of the plate.
7. The engine drives the converter cover (1).
8. The converter cover drives the piston plate clutch (2).
9. The clutch drives the turbine (3).
10. The turbine drives the input shaft (4).
11. The input shaft transmits power into the gear train (5).