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Lock-up Torque Converters

Automatic transmissions have always had the reputation for being less fuel efficient than a comparable stick-shift transmission. Automatic transmissions utilize a torque converter to connect and disconnect the engine from the drive wheels, but by design, they never really "lock" the engine to the transmission input-shaft in a true 1:1 ratio, the way a clutch does in a standard transmission. This "slippage" that occurs within the torqueconverter is transformed into heat. At lower engine Rpm's, the slippage is greater (and the heat generated is also greater), and the converse is also true; As engine Rpm's increase, the torque converter gets closer to "locking up"(closer to a 1:1 ratio) and the heat generated by the Torque converter is reduced.
Today's automobiles are designed with more passenger comfort and higher fuel efficiency in mind. Higher final drive ratio's (Lower numerically), make the engine turn more slowly at a given driving speed, and Over-drive features of the new generation automatic transmissions, have drastically reduced the speed the engine turns in relation to cruising speed. This presents problems with heat generated by the torque converter; transmissions overheat, and fuel is wasted. This is why lock-up torque converters have come into play. A clutch has been added to the torque converter to mechanically "lock" the engine to the transmission input shaft, eliminating the "slippage" and gaining the fuel mileage that normally lost in an automatic transmission.

The clutch in a lock-up torque converter is hydraulically applied (Some are centrifugally applied). This application can be controlled by a combination of hydraulic valves, pressure switches, electric solenoids, computer signals, etc. Most lock-up converters can be "felt" applying and releasing by cruising at a speed above 50mph at a light throttle setting (not accelerating or decelerating), and gently pushing on the brake pedal (just enough to turn on the brake lights, but not enough to apply the brakes), and releasing the brake pedal SLOWLY. As you push the brake pedal down, you will feel a subtle "downshift", and as you release the brake pedal (sometimes there is a slight delay), you should feel a subtle "upshift". If you feel this happening, then your lock-up torque converter is probably working correctly.

Make sure that your TCC (torque converter clutch) is working, so you won't overheat your transmission (severely shortening the life of the transmission), and throw away a bunch of money on fuel. Also, you might note that it is illegal to modify your automobile in any way that might alter it's fuel mileage or smog emissions, so don't look for a way to disable your TCC!

Torque Converter Clutch Application

The clutch in a lockup torque converter is quite a bit different from any "typical" clutch seen in other applications. It is made up of a spring steel, with clutch lining on the outer diameter of one face, that mates with a corresponding "flat spot" on the inside of the torque converter housing. The torque converter housing is directly bolted to the engine flexplate (some people call it the flywheel), so it is obvious that the converter housing turns at exactly engine speed. The spring steel clutch is splined to input shaft (it is actually splined to the turbine, which is then splined to the input shaft), so it is also obvious that the clutch turns at exactly turbine, or transmission input shaft speed.

Fluid enters the torque converter through the input shaft, where it is "dumped" in behind the spring steel clutch, inbetween the clutch, and the converter cover, creating a highpressure area behind the clutch, forcing the clutch to spring away from the converter cover, and keeping the converter "unlocked". Fluid then enters the rest of the converter, leaking past the "gap" between the clutch, and the cover and does its normal job within the "pump", "turbine", and "stator", before exiting through the hub, and on to the cooler passage.

When conditions are right for a "lockup" to occur, fluid is reversed, by the lockup valve; Fluid enters the converter through the hub area, does it's job within the "pump", "turbine", and "stator", and creates a high pressure area on the opposite side of the clutch, forcing the clutch against the converter cover. The clutch material "grabs" the converter cover; The clutch, which is splined to the input shaft, becomes "one" with the converter cover, allowing no slippage between the two; .....lockup.