Torque Converter
The 8HP45 transmission is fitted with a twin torsional damper torque converter. The damper enables a more direct engine connection for highly efficient power transmission when starting and shifting. The inner spring set is connected directly to the transmission, the outer spring set responds to torque from the engine via the lock-up clutch. When the lock-up clutch is applied, the hydro-dynamic circuit is by-passed and torque is transferred directly to the transmission input shaft. In comparison to conventional torque converters, the twin torsional damper applies the lock-up clutch earlier, therefore improving driving comfort at low engine speeds and increasing fuel efficiency by enabling the engine to be operated at lower RPM levels.
When the converter lock-up clutch is not applied, torque is transferred hydro-dynamically. Power from the engine is transmitted via the pump and turbine to the transmission input shaft. When the converter lock-up clutch is applied, the hydro-dynamic circuit is by-passed, torque is transferred directly via the lock-up clutch and the twin spring sets. The turbine inertia is coupled between the two spring sets which improves disengagement, helps reduce wear on the transmission and improve noise insulation.
The torque converter is the coupling element between the engine and the transmission and is located in the torque converter housing, on the engine side of the transmission. The driven power from the engine crankshaft is transmitted hydraulically and mechanically through the torque converter to the transmission. The torque converter is connected to the engine by a flex plate attached to the rear of the crankshaft.
The torque converter comprises a twin torsional damper, an impeller, a stator and a turbine. The torque converter is a sealed unit with all components located between the converter housing cover and the impeller. The two components are welded together to form a sealed, fluid filled housing. With the impeller brazed to the converter housing cover, the impeller is therefore driven at engine crankshaft speed.
The converter housing drive plate has four threaded bosses, which provide for attachment of the engine flex plate. The threaded bosses also provide for location of special tools which are required to remove the torque converter from the torque converter housing.
Lock-Up Clutch
The torque converter lock-up clutch is hydraulically controlled by an EPRS, which is controlled by the TCM. This allows the torque converter to have three states of operation as follows:
- Fully engaged
- Controlled slip variable engagement
- Fully disengaged.
The torque converter pressure valve reduces system pressure and guarantees the pressure needed for the torque converter. It also limits the maximum torque converter pressure, to prevent the torque converter from expanding.
The EPRS is operated by PWM (Pulse Width Modulation) signals from the TCM to give full, partial or no lock-up of the torque converter.
The lock-up clutch is a hydro-mechanical device which eliminates torque converter slip, improving fuel consumption. The engagement and disengagement is controlled by the TCM to allow a certain amount of controlled 'slip'. This allows a small difference in the rotational speeds of the impeller and the turbine which results in improved shift quality. The lock-up clutch comprises a piston and a clutch friction plate.
In the unlocked condition, the oil pressure supplied to the piston chamber is reduced and the pressure in the turbine chamber is allowed to push the piston back. In this condition the clutch plate are released and torque converter slip is permitted.
In the locked condition, the lock-up clutch spool valves are actuated by the EPRS. Pressurized ATF is directed into the lock-up clutch piston. The piston moves with the pressure and pushes the clutch plates together. As the pressure increases, the friction between the clutch plates increases, finally resulting in full lock-up of the clutch plates. In this condition there is direct mechanical drive from the engine crankshaft to the transmission planetary gear train.
The Transmission Idle Control feature is a standstill decoupling feature for the 8 speed transmission. When the vehicle comes to a standstill with the brakes applied and the TCS is in 'D' (drive) position, the converter is disconnected from the driveline so that only a slight residual load remains. This further reduces fuel consumption. Decoupling is by actuating clutch B in the transmission, and is dependent on load and output speed.