Automatic gearbox - JF403E (Operation): Description and operation

OPERATION

Torque converter

The torque converter consists of an impeller, stator and turbine. The impeller is driven by the engine, while the turbine drives the transmission. The stator is situated between the impeller and turbine on a one-way clutch. Power is transmitted by the impeller picking up oil and throwing it out into the turbine, thereby causing it also to rotate. The stator redirects the oil thrown back by the turbine so that it re-enters the impeller in the same direction of rotation as the impeller, and at the best possible angle for efficient power transmission.

The one-way clutch prevents the stator from moving backward, so that this accurate redirection of oil can be achieved. When the engine is idling the impeller throws out very little oil. Thus the turbine is not forced to turn, and the power is not transmitted to the transmission.

As engine speed increases, the impeller throws out more oil. The turbine begins to turn and picks up speed as the engine speed rises.

As the speed of the turbine increases the oil is thrown against the back of the stator, causing it to turn in the same direction.

When turbine speed approaches impeller speed, centrifugal force in both units is almost equal and all three components move at nearly the same rate. This is called ’Coupling Point’.

The torque multiplication or drive ratio varies until a one to one coupling point is reached. Remember, the engine is connected to the impeller and the transmission input shaft to the turbine.

To achieve the power required to climb a hill, the driver depresses the accelerator pedal and the torque converter reacts by increasing the torque multiplication to a maximum of 3.027:1. When driving on a flat road at cruising speed, the power required is not great and therefore, the torque converter stays at one to one.

Torque converter lock-up system

With a conventional torque converter there is always a certain amount of slip between the impeller and turbine. This can contribute to a reduction in fuel economy especially during high speed cruising conditions. This is eliminated in the transmission by a torque converter lock-up mechanism. Lock-up is achieved by a lock-up piston incorporating a friction lining. When the piston is applied by hydraulic pressure the friction lining contacts the impeller eliminating any slip between the turbine and impeller. This system is controlled by throttle pressure, and by a lock-up control solenoid valve being activated by the ECU.

When throttle opening, vehicle speed, coolant temperature and engine speed exceed certain limits oil between the converter cover and lock-up piston is discharged, converter pressure enters through the inlet and exerts pressure on the lock-up piston engaging the lining with the impeller. The effect is to by-pass the converter, thereby placing the torque converter in direct drive.

When throttle opening, vehicle speed, coolant temperature and engine speed fall below certain limits converter pressure enters via the inlet and moves the lock-up piston away from the impeller disengaging the lining. The lock- up mechanism is now released and the impeller and turbine can rotate at different speeds.

A refinement to the torque converter lock-up system is the reduction in the harshness or shock during lock and un-lock. The ECU operates the lock-up solenoid which gradually varies the fluid pressure to the lock-up control valve, this causes the lock-up piston to move slowly, engaging and disengaging the lock-up clutch smoothly.

In order to promote engine warm-up at low temperatures, the lock-up is inhibited when the transmission fluid temperature is below 10 °C.