Street Smart - July - 2019

Generation Gap: 6T40 Gen v3

The 6T40 generation 3 upgrade is mainly aimed at improving fuel economy. The gearbox features changes to the line pressure control system, oil level control system, pump, and lubrication system (figure 1).

Designed for FWD and AWD applications, it can be found in 2013- 2015 Chevrolet Malibu 2.5L and the 2016 Buick Encore Sport Touring 1.4L Turbo. There are some common traits shared between the earlier versions, as well as other GM six-speeds.

This unit uses a clutch-to-clutch operation that allows the six-speed to be packaged into approximately the same space of a four-speed. They all share an overall gear ratio of 6.11:1, which contributes to their signature balance of performance and fuel economy. Three planetary gear sets are used with three stationary brake clutches and two clutch drums. The clutch-to-clutch concept delivers smooth shift timing through specific electronic commands called adaptive shift control. Adaptive shift controls help the transmission shift at its optimum time for usage in heavy or light acceleration. It also includes features that are used during the driving experience, such as “Grade Braking”. Grade Braking 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 and reduces the need for the driver to ride the brake during a hill descent. This feature monitors brake pedal usage, vehicle acceleration rate, throttle position, and even whether a trailer is connected to the vehicle in order to recognize that you’re going downhill and how fast.

Fuel economy is major concern with car manufacturers. GM made improvements to the line pressure control system, oil level control system, pump, and lubrication system with fuel economy in mind.

The pump has been moved off-axis (figure 2). This allows for a more-compact pump that uses less energy to drive.

The Pressure Regulator valve has been changed so it has a lower offset pressure, moving the operating pressures into the more accurate range for Variable Force Solenoid control. The diode one way clutch has been updated to have an aluminum pocket plate with spin loss reducing scallops.

In the lubrication system, a lube bypass valve has been added to the pump assembly. This valve remains closed until line pressure reaches approximately 85 psi. Lube flow passes through one orifice when pressure is below 85 psi and through two orifices in parallel when above. This, too, is a fuel-savings design.

The TECHM (Transmission Electronic Hydraulic Control Module) is mounted inside the transmission, where temperatures remain more constant compared to an externally mounted module (figure 3). Its small size helps minimize the overall size of the transmission. 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, more consistent shift performance in extreme conditions, and less degradation over time. This fluid improves durability and shift stability over the life of the transmission.

Some of you are already seeing this transmission in your shop. I’ll cover these and other features of the 6T40 Gen 3 at this year’s Powertrain Expo at Caesar’s Palace in Las Vegas.

For more information about the show go to https://www.powertrainexpo.com. Hope to see you there! And that’s not just smart. THAT’S STREET SMART!