Torque Converter Explaination - Part3

Hydraulic and electrical circuits

When hydraulic fluid is directed at the rear (Transmission Side) of the clutch disc, the disc moves forward (Towards the Engine) slightly on the turbine splines and engages its friction surface against the converter cover. This creates a solid direct drive unit from the engines crankshaft through the converter to the input shaft/drum assembly of the transmission. This direct physical connection acts the same as manual transmission clutch with the clutch pedal released.

To lock and unlock this direct drive physical connection we simply have to move the clutch disc forward (Towards the Engine) to engage it to the converter cover or rearward (Towards the Transmission) to disengage it from the converter cover. This is done by directing hydraulic fluid to the front (Engine Side) side of the clutch disc to disengage or to the rear side (Transmission Side) to move it forward to engage. This hydraulic fluid flows directly from the transmissions internal hydraulic pump through a TCC (Torque Converter Clutch) control valve. This TCC control valve is normally held in the disengaged position by a coil spring. To move the TCC control valve to the "engaged" position, hydraulic fluid must be applied to the end opposite the coil spring. This hydraulic pressure overcomes the coil spring force and moves the TCC control valve to it's "engaged" position. When the TCC control valve moves to its "engaged" position, hydraulic fluid directly for the transmissions internal pump, passes through the TCC control valve, to the rear side of the converter clutch disc. This fluid moves the disc forward where it forcefully engages the converter cover.

Normally the control valve stays in the disengaged position, held there by the coil spring. In this normal position, hydraulic fluid feeds directly from the transmission's internal pump, through the TCC control valve, where it is fed to the forward side of the clutch disc. This fluid moves the disc to the rear, disengaging the direct connection through the converter. Understand the fluid that actually moves the converter clutch disc flow directly from the transmissions hydraulic pump, through the TCC control valve, to either the front or rear side of the clutch disc depending on its position. This fluid will always be either applying the clutch or releasing the clutch depending on the TCC valve position. Whenever the vehicles engine is running, the converter is turning the transmissions internal hydraulic pump rotor at engine speed via the converters front drive hub. Whenever the engine is running, direct pump fluid will always be directed to one side or the other of the clutch disc by the TCC control valve. Remember this TCC control valve is spring loaded to the disengaged position and will stay there unless TCC control fluid is directed to the opposite end of the valve to move it to the engaged position.

The hydraulic fluid used to move the TCC control valve is not fed directly from the transmissions internal hydraulic pump. It's a regulated fluid that must travel through the transmissions valve body first. The valve body is the transmissions hydraulic management device. The TCC control fluid is only available to the TCC control valve after the transmission has shifted into second gear or higher. This prevents killing the engine when the transmission automatically down shifts when you slow down. If the converter clutch was still engaged when the vehicle came to a stop, it would kill the engine. The same thing would happen if you had a standard transmission and forgot to push in the clutch pedal when you came to a stop.

After the transmission has reached second gear or higher, regulated hydraulic fluid is directed through the valve body towards the TCC control valve. In the same passage as the TCC control valve is an electrically controlled solenoid. This solenoid dumps the TCC control fluid right into the transmission pan unless it it electrically energized. This prevents the TCC valve from moving into the engaged position until this solenoid is energized. When energized, a check ball inside the solenoid closes off this dump passage, stopping the dumping of the TCC control fluid into the pan. Once this fluid dumping stops, the passage that feeds fluid to the TCC control valve pressurizes, moving the TCC valve to the engaged position. This, of course, causes converter clutch lock up