Torque Converter
A torsional damper torque converter is used on 8HP70 transmissions.
The torsional 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 through 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 torsional damper applies the lock-up clutch earlier. This improves driving comfort at low engine speeds and increases fuel efficiency by enabling the engine to be operated at lower Revolutions Per Minute (RPM) levels.
When the converter lock-up clutch is not applied, torque is transferred hydro-dynamically. Power from the engine is transmitted through 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 through the lock-up clutch and the tandem spring sets. The turbine inertia is coupled between the 2 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 drive plate attached to the rear of the crankshaft.
The torque converter comprises a 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 2 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 4 threaded bosses, which provide for attachment of the engine drive 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 Electronic Pressure Regulating Solenoid (EPRS), which is controlled by the TCM .
This allows the torque converter to have 3 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 Pulse Width Modulated (PWM) 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 comprising of friction and steel plates.
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 plates 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 DRIVE (D) the converter is disconnected from the driveline. This results that that only a slight residual load remains, which further reduces the fuel consumption. Decoupling is by actuating clutch B in the transmission, and is dependent on load and output speed.