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The front section is called the torque converter. In replacing the traditional clutch, it performs three functions:
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 It acts as a hydraulic clutch (fluid coupling), allowing the engine to idle even with the transmission in gear. <p>
 It allows the transmission to shift from gear to gear smoothly, without requiring that the driver close the throttle during the shift. <p>
 It multiplies engine torque making the transmission more responsive and reducing the amount of shifting required.

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The torque converter is a metal case that is shaped like a sphere that has been flattened on opposite sides and is bolted to the rear of the engine's crankshaft.
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The case contains three sets of blades. One set is attached directly to the case forming the impeller or pump. Another set is directly connected to the output shaft, and forms the turbine. The third set (stator) is mounted on a hub which, in turn, is mounted on a stationary shaft through a one-way clutch. Rollers are wedged into slots, preventing backward rotation. When the rollers are not in the slots, the stator turns in the same direction as the impeller. The pump, which is driven by the converter hub at engine speed, keeps the torque converter full of transmission fluid at all times. Fluid flows continuously through the unit to provide cooling.
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A fluid coupling will only transmit the torque the engine develops; it cannot increase the torque. This is one job of the torque converter. The impeller drive member is driven at engine speed by the engine's crankshaft and pumps fluid, to its center, which is flung outward by centrifugal force as it turns. Since the outer edge of the converter spins faster than the center, the fluid gains speed. Fluid is directed toward the turbine driven member by curved impeller blades, causing the turbine to rotate in the same direction as the impeller. The turbine blades are curved in the opposite direction of the impeller blades.
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In flowing through the pump and turbine, the fluid flows in two separate directions. It flows through the turbine blades, and it spins with the engine. The stator, whose blades are stationary when the vehicle is being accelerated at low speeds, converts one type of flow into another. Instead of allowing the fluid to flow straight back into the pump, the stator's curved blades turn the fluid almost 90° toward the direction of rotation of the engine. Thus the fluid does not flow as fast toward the pump, but is already spinning when the pump picks it up. This has the effect of allowing the pump to turn much faster than the turbine. This difference in speed may be compared to the difference in speed between the smaller and larger gears in any gear train. The result is that engine power output is higher, and engine torque is multiplied.
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As the speed of the turbine increases, the fluid spins faster and faster in the direction of engine rotation. Therefore, the ability of the stator to redirect the fluid flow is reduced. Under cruising conditions, the stator is eventually forced to rotate on its one-way clutch and the torque converter begins to behave almost like a solid shaft, with the pump and turbine speeds being almost equal.
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In the late 70's, Chrysler Corporation introduced an automatic transmission, featuring what is called a "lock-up" clutch in the transmission's torque converter. The lock-up is a fully automatic clutch that engages only when the transmission shifts into top gear or when needed based on a predetermined demand factor.
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The lock-up clutch is activated by a piston. When engaged, the lock-up clutch gives the benefits of a manual transmission, eliminating torque converter slippage. In the engaged position, engine torque is delivered mechanically, rather than hydrodynamically (through fluid). This gives improved fuel economy and cooler transmission operating temperatures.
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In the early 80's, Ford introduced what is known as the Automatic Overdrive Transmission (AOT). Essentially, this transmission uses a lock-up torque converter, by offering an additional refinement. The transmission is a four-speed unit, with fourth gear as an overdrive (0.67:1). Torque is transmitted via a full mechanical lock-up from the engine, completely bypassing the torque converter and eliminating hydraulic slippage.
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In third gear (1:1 ratio), engine power follows a "split-torque" path, in which there is a 60% lock-up. Sixty percent of the power is transmitted through solid connections and 40% of the engine power is delivered through the torque converter.
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<size=4>Computer controlled converter clutch</size>
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With the use of microcomputers governing the engine fuel and spark delivery, most manufacturers change the converter clutch electronic control to provide the grounding circuit for the solenoid valve through the microcomputer, rather than the governor pressure switch. Sensors are used in place of the formerly used switches and send signals back to the microcomputer to indicate if the engine is in its proper mode to accept the mechanical lock-up of the converter clutch.
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Normally a coolant sensor, a throttle position sensor, an engine vacuum sensor and a vehicle speed sensor are used to signal the microcomputer when the converter clutch can be applied. Should a sensor indicate the need for the converter clutch to be deactivated, the grounding circuit to the transmission solenoid valve would be interrupted and the converter clutch would be released.
 
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