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    An introduction to GM's 2ML70 two mode hybrid transmission

    Jan. 1, 2020
      The GM 2ML70 2 Mode Hybrid system first appeared as a 2008 model in the Chevy Tahoe and the GMC Yukon and was followed by the Cadillac Escalade in 2009. 
    Figure 210

    The GM 2ML70 2 Mode Hybrid system first appeared as a 2008 model in the Chevy Tahoe and the GMC Yukon and was followed by the Cadillac Escalade in 2009. The 2ML70 moved into the Silverado and Sierra 1500 series pickups, with all models using the 6.0 Liter gas engine.

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    The 2ML70 transmission is a four speed rear drive which can be configured in a two or four wheel drive application (Fig. 1). The 2ML70 does not use a torque converter; it uses a dampener assembly to smoothly transmit power from the engine flywheel to the transmission.

    The 2ML70 uses a vane type pump with the dampener housing as the pump body. This system utilizes stop/start technology and therefore has a 12 volt auxiliary pump to keep the 1-2 clutch primed any time the engine shuts down.

    The 2ML70 also utilizes three planetary gear sets, two rotating clutch packs and two fixed clutch packs. In addition it has two 60 kW electric motor/generators which can drive the vehicle on electricity only and can also recharge the 330 volt hybrid battery pack.

    The transmission operation is controlled by a Transmission Control Module (TCM)  which is located inside the transmission and the hybrid systems operation is controlled by the Hybrid Control Processor (HCP) located under the hood. 

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    SHIFT QUADRANT POSITIONS
    P - The rear wheels are locked when in the Park position preventing the vehicle from rolling forward or backward. In this position the “Shift Lock” control is in use causing the driver to have to step on the brake pedal to release the shift lever.

    R - Reverse position allows the vehicle to move in a rearward direction in combination with the internal combustion engine (ICE) and generator Drive Motor 1. If either one becomes inoperable a no reverse condition will be exhibited.

    N – The neutral position allows the transmission to be disengaged without the rear wheels being locked. The circuitry of this position with the Manual Shaft Position Switch enables the engine to start and operate without driving the vehicle. This position should be selected to restart the engine while the vehicle is moving.

    D - In the drive position, the transmission automatically up-shifts throughout all four of its forward gear ranges according to the normal shift pattern programmed into the TCM.

    M – The manual position mode “M” gives a certain degree of control to the driver to manually select forward gear ranges. This is done by tapping the selector lever towards the + for an up-shift and the – for a downshift.

    The “M” position can be selected at anytime while driving forward in the D range. But when selected, the transmission’s current gear will be the highest attainable range with all the lower gears available.

    This “M” position allows the driver to hold the transmission’s lower gears longer under various driving conditions such as, towing, hill climbing or engine braking during downhill driving. This can also be used for higher gear starts during slippery road surface conditions,

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    DRIVING MODES
    ELECTRIC LAUNCH
    When the vehicle has an electric launch, the internal combustion engine (ICE) remains off; the auxiliary fluid pump is then used to provide transmission fluid pressure allowing the hybrid low, 1-2 holding clutch to be applied. Drive Motor 2 then completes the powerflow by driving the third position sun gear shaft.  Once additional power is required to accelerate the vehicle, the ICE is started initiating the EVT Mode 1 (Low).

    EVT MODE 1 (LOW)
    When it is necessary to initiate this mode, Drive Motor 1 is used to start the internal combustion engine (ICE). Once stared, it then becomes a generator for the Hybrid batteries. Drive Motor 1 is also used to provide a driveline load during coast conditions to help slow the vehicle. Depending on the driving conditions of the vehicle, the ICE will operate in either 4 or 8 cylinder mode to optimize fuel consumption while maintaining output power requirements.

    EVT MODE 2 (HIGH)
    As vehicle speed increases, the system will shift to EVT Mode 2. This is similar to EVT Mode 1. The difference is that the Hybrid 2-3-4 clutch is applied rather than the Hybrid Low 1-2 holding clutch.

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    FIXED GEAR OPERATION
    Fixed gear ratio operation is when clutches alone are used to provide power to the drive wheel without the use of electric motors. This configuration allows the advantage of being able to increase engine size without having to increase electric motor size. Depending on vehicle package, this advantage improves towing, climbing, and maximum acceleration performance. Throughout the fixed gear ratio modes the drive/generator motors can be used entirely for power assist or for driveline load during cruising conditions.

    REGENERATIVE BRAKING
    Regenerative braking occurs in both EVT Mode 1 and EVT Mode 2 when the driver is decelerating with his foot on the brake. The electric drive/generator motors assist in this deceleration by applying negative torque to the output shaft while simultaneously generating electricity to recharge the HV battery. The 3 phase AC power generated by the motor is converted to high voltage DC power in the Hybrid Control Module (HCM) and stored in the battery.

    REVERSE
    Just like the Electric Launch, when the vehicle is placed in reverse the hybrid low 1-2 holding clutch is applied. Drive motor 2 completes the powerflow for reverse by driving the third position sun gear opposite of that for a forward launch. Should reverse be selected from the Auto Stop mode (ICE off at a stop in a forward range), the ICE will be commanded to start.

    The following figures provide electrical information, operating information and a look at the inside of this well designed hybrid transmission.

    Figure 3 — Swing the passenger side seat up and remove the end trim panel. Locate the orange hybrid battery switch and pull the handle down and out. Be sure to have your rubber Class 0 gloves on.

    Figure 5 — After the hybrid battery disconnect switch has been removed, wait approximately 5 minutes for the voltage to dissipate. Then with your Class 0 rubber gloves on and your 1000 Volt Cat III voltmeter, remove the 3 Phase cable cover on the side of the transmission and verify that the high voltage has in fact dissipated.

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    AUXILIARY FLUID PUMP
    The Auxiliary Fluid Pump is a 3 Phase 12 Volt AC motor type that is turned on by the Auxiliary Fluid Pump Control Module (AFPCM) when the vehicle comes to a stop and the engine is off. It provides lubrication and keeps the Hybrid Low-1-2 Clutch applied in preparation for initial take off.

    CLUTCH AND MOTOR APPLICATION
    * Drive Motor Position 1 is also used to provide a driveline load to assist in slowing the vehicle and to act as a generator charging the hybrid batteries. Drive Motor Position 1 is also used as a dampener to reduce torque disturbances from engine cylinder firing pulses while Drive Motor Position 2 is used to dampen driveline disturbances. 

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    About the Author

    Wayne Colonna | POWERTRAIN PRO Publisher

    Wayne Colonna is president of the Automatic Transmission Service Group (ATSG) in Cutler Bay, Fla., and a frequent speaker/instructor for transmission training around the globe.

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    Figure 1: The EPS is housed entirely in the steering column assembly on this example displayed. Although this is a typical configuration, it doesn’t mean every vehicle is configured this way.

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