A lockup torque converter eliminates the 10 percent slip that takes place between the impeller and turbine at the coupling stage of operation. The engagement of a clutch between the engine crankshaft and the turbine assembly has the advantage of improving fuel economy and reducing torque converter operational heat and engine speed.

There are two types of lockup torque converters. The centrifugal lockup clutch (CLC) was installed in Ford Motor Company C-5 automatic transmissions during the early 1980s. The piston lockup clutch (PLC) is the type installed in most automatic transmissions.

Centrifugal Lockup Clutch (CLC)

The principal part added to the torque converter is the clutch and damper assembly located between the torque converter shell and turbine assembly.

The clutch and damper assembly looks like a manual transmission clutch disc. At its center is an internally splined hub meshing with an externally splined hub on the turbine assembly. The clutch and damper assembly drive the turbine when the lockup clutch is engaged. Outward from the internally splined hub is the coasting one-way overrunning clutch. This sprag-type clutch connects the clutch and damper assembly to the turbine hub when the engine is driving the vehicle. When the vehicle is decelerating or coasting, the coasting clutch overruns and disconnects the turbine from the torque converter shell. This clutch is considered a safety factor during an emergency stop because the engine will not stall.

Midway from the center of the clutch and damper assembly are six coil torsional springs that absorb the shock associated with the torque converter locking action. Collectively, the torsional coil springs are called the absorption unit. Placed around the outside edge of the clutch and damper assembly are several shoe assemblies called centrifugal clutch shoes. Each shoe is mounted on a spring sensitive to centrifugal force and faced with a friction material pad.

With the vehicle stopped, the torque converter shell drives the impeller using fluid to rotate the turbine and turbine shaft hydraulically. As turbine speed and centrifugal force increase the centrifugal clutch, the shoes are thrown outward and expand to contact the inside diameter of the torque converter shell. Power flows from the inside surface of the torque converter shell to the expanded centrifugal clutch shoe, clutch, and damper assembly to drive the turbine hub and turbine shaft.

In the lockup mode, there is no hydraulic operation. The drive is strictly mechanical. If the driver requires an increase in vehicle speed, increased impeller speed raises vortex flow and produces some torque multiplication. A friction-modified automatic transmission fluid permits the centrifugal clutch shoes to slip when extra torque is needed. When the demand for torque has been satisfied, the centrifugal clutch shoes resume driving the interior of the torque converter shell, reestablishing lockup torque converter action.

Piston Lockup Clutch (PLC)

The other type of lockup torque converter has a piston clutch located between the front of the turbine and the interior front face of the shell. Its main components are a piston plate and damper assembly and a clutch friction ring. The friction ring is bonded to the piston plate and damper assembly, in many cases. The second part of the damper assembly is made of several coil springs designed into the piston plate to transmit driving torque and absorb shock.

In piston-type lockup torque converters, the front section of the turbine shaft is drilled lengthwise allowing fluids to be supplied to and drained from the chamber between the front side of the piston plate and damper assembly and torque converter shell.

Whereas a centrifugal lockup clutch is operated by engine speed, the piston lockup clutch is controlled by hydraulic valve action or, more precisely, by computer. The computer control is superior because information about the engine, fuel, ignition, vacuum, and operating temperature is fed into the computer so engagement is closely monitored to take place at exactly the right time as it relates to engine operation.

To provide for piston clutch control, Chrysler adds a three-valve module to its standard transmission valve body. The lockup valve is controlled by fluid pressure produced by the transmission's governor assembly called governor pressure. When vehicle road speed is high enough, governor pressure forces the lockup valve to move against coil spring tension. The moving over of the lockup valve permits fluid pressure to move to the fail-safe valve. The purpose of the fail-safe valve is to prevent lockup clutch engagement until the transmission is in third gear. Third gear fluid pressure moves the fail-safe valve, which allows fluid pressure to flow to the switch valve. The purpose of the switch valve is to direct fluid pressure, called line pressure, between the turbine shaft and the stator support to fill the torque converter. When the torque converter is filled, fluid flows out from the space around the periphery of the impeller and turbine to the rear of the piston plate and damper assembly. This fluid flow acts on the rear surface of the piston plate and moves it forward to bring the piston plate and friction ring into contact with the torque converter shell. When the piston plate engages, the clutch friction ring forms a fluid pressure seal with the torque converter shell. This seal stops fluid pressure from leaking to the turbine shaft. A fluid leak in this area decreases the fluid pressure that keeps the clutch engaged. The fluid in the torque converter does not circulate but remains there to act as the torque converter coolant and lubricant throughout the lockup engagement.

Forced Disengagement

While in the lockup mode, the driver might want to accelerate rapidly. By opening the throttle, pressure increases, stroking the fail-safe valve to block line pressure to the lockup valve. Spring tension moves the switch valve, directing fluid pressure to the front (disengaged) side of the piston plate and damper (assembly) side of the piston plate and damper assembly. The torque converter then returns to conventional operation.