GM 6-Speed 6T40 MHH Hydra-Matic eAssist Automatic Transmission

The special MHH eAssist version of the 6T40 transmission uses electrification to increase fuel economy. The added electric power provided by the eAssist system allows for higher gearing which improves steady state efficiency without compromising acceleration, performance or driveability. The eAssist system provides electric assistance at cruising speeds, allowing the driver to accelerate lightly or ascend mild grades and minimize transmission downshifting.

An auxiliary, electric oil pump is added to the 6T40 to keep the transmission launch clutch primed when the engine shuts off at a stop. That keeps the transmission ready to perform when the driver accelerates for a seamless, uncompromised driving experience.

MHH Gen 2 For eAssist Applications

The special MHH version of the 6T40 transmission uses light electrification to increase fuel economy. Significant internal changes to clutch controls and hardware provide reduced spin losses, while improving shift response and time. The added electric power provided by the eAssist system allows for higher gearing to improve steady state efficiency without impacting acceleration performance or driveability. The eAssist system’s capability of providing some electric assistance at cruising speeds allows the driver to accelerate lightly or ascend mild grades without the transmission downshifting.

An auxiliary, electric-driven transmission oil pump is added to the 6T40, which keeps the transmission primed and the fluid flowing when the engine shuts down at a stop. That keeps the transmission ready to perform when the driver accelerates, for a seamless, uncompromised driving experience.

On-Axis Design

Instead of “folding” the transmission around the end of a transversely mounted engine, which has been one of the dominant GM transaxle design conventions, the 6T40 contains all of the gearing on the engine crankshaft centerline. The advantages of this layout enables a shorter overall vehicle length, more interior room and lower powertrain height.

Clutch-To-Clutch Shift Operation

Three planetary gearsets are used with three stationary clutches and two rotating clutches, which save space compared to freewheeling designs. Freewheeling mechanisms allow accurate timing between shifts, but also take up more space and add more components to the transmission. Through electronic controls, the clutch-to-clutch concept of the 6T40 delivers optimal shift timing.

Gear changes from second to sixth gear ratios are accomplished with a precise clutch-to-clutch action, where the clutch is engaged in one gear at exactly the same time it is released in another. The first-to-second upshift, however, is a freewheeling action, where the second gear clutch engages while the first gear one-way clutch spins freely. This allows a greater degree of smoothness at lower vehicle speeds.

Adaptive Shift Controls

Adaptive shift controls include automatic grade braking, which commands the transmission to remain in a lower gear if the vehicle is decelerating or coasting on a downgrade. This takes advantage of engine braking to prevent unwanted acceleration. This reduces the number of brake applies for the driver to brake during a hill descent. The control module receives input that monitors brake pedal usage, vehicle acceleration rate, throttle position, and even whether a trailer is connected to the vehicle.

Space-Saving Hyper-Elliptical Torque Converter

The torque converter in the 6T40 uses a single-plate lock-up clutch and features a “hyper-elliptical” oval cross-section shape. This design reduces the thickness of the torque converter, reducing the space it needs and keeping the overall width of the engine and transmission as narrow as possible for packaging advantages. The single-plate converter clutch uses GM’s electronically controlled capacity clutch (ECCC) technology to help reduce engine vibrations and ensure smooth operation.

IX Gear Pump

A compact on-axis, fixed-displacement IX gear-type fluid pump provides hydraulic pressure for shifting and lubrication. The pump features an IX gear (Drive/Driven) with a machined crescent in the pocket to reduce internal leakage and optimizes hydraulic efficiency. The inlet jet nozzle feature in the IX gear-type pump provides increased pump suction velocity, which helps optimize noise and vibration characteristics.

Ground And Honed Gears

To minimize gear noise, as well as vibration, the transmissions’ helical gears are ground and honed to ensure precise dimensions and tolerances. With closer tolerances, the gears are less prone to characteristic whining or humming, allowing the transmission to operate with exceptional quietness.

Maintenance

The 6T40/6T45 is filled for life with DEXRON-VI premium fluid, which does not require changing under normal use. It was developed to have a more consistent viscosity profile, a more consistent shift performance in extreme conditions, and less degradation over time. The fluid was validated to improve durability and shift stability over the life of the transmission.