GM 6-Speed 6T40 MH8/MHB Hydra-Matic Automatic Transmission
The Hydra-Matic 6T40 (production code MH8/MHB) is a six-speed automatic transmission produced by General Motors for use in front-wheel-drive vehicles with a transverse powertrain orientation. Developed in-house by General Motors, the Hydra-Matic 6T40 is part of the 6TXX family of transmissions that succeeded the 5TXX family. The 6T40 is more robust than GM’s 6T30 gearbox.
Six-speed automatic transmissions are integral to GM’s initiative to offer vehicles with excellent fuel economy. The Hydra-Matic 6T40 transmission delivers on that promise. It was designed for compact and midsize FWD applications and is part of GM’s family of technologically advanced, fuel-saving six-speed automatic transmissions. Shared traits between the 6TXX family – and other Hydra-Matic six-speeds is to reduce complexity, size and mass, including clutch-to-clutch operation that enables the six-speed to be packaged into approximately the same space as a four-speed automatic.
Because of the wide ratio spread, first gear is a very high ratio, which provides brisk acceleration from a stop. Sixth gear, however, is an overdrive ratio, which keeps the engine revolutions lower for highway cruising, reducing engine friction losses and optimizing fuel economy.
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.
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.
|Type:||Six speed front-wheel-drive, electronically controlled, automatic overdrive transaxle with an electronically controlled torque converter clutch|
|Maximum engine power:||180 hp (134 kW)|
|Maximum engine torque||177 lb-ft (240 Nm)|
|Maximum gearbox torque:||277 lb-ft (375 Nm)|
|Final Drive Ratio:||3.87|
|Maximum Input Speed|
|5-6:||6000 rpm (corresponds to 4476 rpm in 6th after the shift is complete)|
|Maximum input in 6th gear:||4476 rpm|
|Maximum validated gross vehicle weight:||1996 kg (4400 lbs)|
|Shifter Positions:||P, R, N, D, M|
|Case material:||Die cast aluminum|
|Shift pattern:||Variable Flow Solenoids|
|Shift quality:||Variable Flow Solenoids|
|Torque converter clutch:||Variable Bleed Solenoid|
|Converter size:||236 mm (diameter of torque converter turbine)|
|Fluid type:||DEXRON VI|
|Transmission weight (wet):||82 kg (180.4 lb)|
|Fluid capacity (approximate):||7.77L (6.56 kg)|
|Bottom pan removal:||NA|
|Pressure taps available:||Line Pressure|
|Transfer design:||Two-axis design, Output Chain|
|Assembly sites:||GMK (Korea), SGM (China), SLP (Mexico), TTO (Toledo)|
|Buick Verano CX, Buick Verano CXL||2.4L I4 LEA NA|
|Buick Encore||1.4L I4 LUV Turbo|
|Buick Regal (Fleet Only)||2.4L I4 LEA NA|
|Chevrolet Cruze||1.4L I4 LUV Turbo|
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|Chevrolet Trax||1.4L I4 LUV Turbo|