Although the torque converter clutch is not part of the engine, it can make the car feel like the engine has a problem. The torque converter clutch is also known as a lock up converter. The lockup clutch has many purposes and is part of the transmission. A regular converter in an automatic transmission is made to slip at idle so that the car does not move. As the accelerator is pressed the RPM's raise and the torque converter will start to lockup or engage. When the converter engages the car will move.
Even though the torque converter is engaged it never completely engages the way a manual transmission car clutch will. It will always slip a little, which is not very efficient. This is why a manual transmission car gets better gas mileage and has more power than an automatic transmission car. To increase power and mainly fuel mileage, a TCC is added to an automatic transmission.
The torque converter clutch is an electronic clutch that will engage the engine and drivetrain 100 percent. The TCC is made to engage usually only in the top gear when the car is cruising, not accelerating. The lock up converter will disengage when the accelerator is pressed hard enough. Over time the clutch will wear out especially if the vehicle ever tended anything and was operated in overdrive (top gear). This will wear the clutch out prematurely. If you have a tachometer you will see the RPM's drop a few hundred when it engages.
If the TCC is bad one of three things will usually happen. First, the vehicle will tend to rev up and down as it engages and disengages, not being able to keep the clutch clutch locked up. Second, the converter clutch will engage randomly in gears that it should not even engage in usually. Third the clutch does not want to disengage. As you slow down the vehicle will not downshift and if you try to accelerate the RPM's will be so low that the vehicle will have difficulty moving quickly. In extreme cases the lockup converter will try to keep the vehicle moving as you try to stop at a stop light, or will hang out and die.
Many people think that this problem is coming from their engine because it causes the car to stall. The check engine light will not always show an error for the TCC. Most lockup converters are controlled by a wire or wires that plug into the transmission. It is possible on some models to simply disconnect this plug. This is not a problem and will only cause you to lose a little gas mileage. The RPM's will run slightly higher than before but will not harm anything or be an issue.
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