Service Information

Volt

Transmission General Description

The MKA 4ET50 is a fully automatic, front wheel drive transaxle, variable-speed, electronic-controlled transmission. It consists primarily of a torque damper assembly, an integral main and auxiliary fluid pump and housing, 1 planetary gear set, 2 friction (rotating) and 1 mechanical (stationary) clutch assemblies, a hydraulic pressurisation and control system, and two internal electric motors. Drive motor with generator A (1) is 55 kW and drive motor with generator B (2) is 111 kW.

The torque damper contains internal torsional springs, fluid, input shaft and a clutch. The torque damper acts as a spring coupling to smoothly transmit power from the engine flexplate to the transmission. The damper provides a mechanical direct drive coupling of the engine to the transmission. The clutch is applied during engine starting and stopping events.

The planetary gear set provides the Electrically Variable forward mode ratios and reverse. Changing ratios is fully automatic and is accomplished through the use of a transmission control module (TCM) located inside the transmission and the drive motor generator power inverter module located under hood. Both the TCM and the drive motor generator power inverter module receives and monitors various electronic sensor inputs and uses this information to vary the torque output at the optimum time based on throttle position.

The TCM commands shift solenoids and variable bleed pressure control solenoids to control torque timing and feel. All the solenoids, including the TCM, are packaged into a self-contained control solenoid valve assembly.

The main hydraulic system primarily consists of a gerotor-type pump, control valve body assemblies, damper housing and case. The pump maintains the working pressures needed to stroke the clutch pistons that apply or release the friction components. These friction components, when applied or released, support the automatic ratios qualities of the transmission.

The hydraulic system also uses a secondary system and consists of an auxiliary pump located inside of the transmission. This is a gerotor-type pump, electric three phase High Voltage motor, electric harness, and control module. The control module is located inside of the drive motor generator power inverter module under the hood. This pump maintains working pressures when the engine is off.

The friction components used in this transmission consist of 3 multiple disc clutches. The multiple disc clutches combine with a planetary gear set and electric motor to deliver the different ratios, forward and reverse. The gear set then transfers torque through the final drive assembly.

The transmission may be operated in any of the following gear ranges:

P (PARK): This position locks the wheels and prevents the vehicle from rolling either forward or backward. PARK is the best position to use when starting the vehicle. Because the transmission utilises a selector lock control system, it is necessary to fully depress the brake pedal before shifting out of PARK. For safety reasons, use the parking brake in addition to the PARK position.

R (REVERSE): This position allows the vehicle to be operated in a rearward direction. This is done by the Drive Motor with Generator B and is electric.

N (NEUTRAL): In this position, the propulsion system does not connect with the wheels.

D (DRIVE): Drive range should be used for all normal driving conditions for maximum efficiency and fuel economy. Drive range allows the transmission to operate in electrically variable modes providing various ratios and output torque.

L (Low): This position is used for the "feel" of engine braking and can be used for the slowing of the vehicle once the throttle is lifted. While in the position the vehicle will slow more quickly and use the B motor to more aggressively collect energy during a regenerative braking event.