Transmission control, function - GF27.60-P-2002AHL

TRANSMISSION 722.9 (except 722.930/931) in MODEL 212 

Function requirements for transmission control, general 

• Circuit 87 ON
• Function test of fully integrated transmission control unit (Y3/8n4) carried out

Transmission control, general 

The transmission control system electronically actuates the actuators (control solenoid valves). The valves are located in the hydraulic valve body assembly. This implements the hydraulic functions.

The function of the fully integrated transmission control controller unit (Y3/8) basically consists of evaluating various input signals relevant to the function. Based on this, eight electromechanical hydraulic valves are actuated which determine the gear ratio of the transmission (gear selection), the working pressure and the torque converter lockup.

Transmission control function sequence 

The function sequence is described in the following steps:

• Function sequence for basic shift program 
• Function sequence for electronic and hydraulic transmission control 
• Function sequence for kickdown 
• Function sequence for limp-home mode 

• Model 212.0/2 with engine 271.8 as of 1.3.2011
• Model 212.1 with engine 272.9 as of 1.7.2010
• Model 212.0/2 with engine 274 as of 03/2013
• Model 212.0/2 with engine 276.9 and 278.9 as of 1.6.2011
• Model 212.1 with engine 276 as of 06/2013
• Model 212.0/2 with engine 651.9 as of 1.6.2011
• Model 212.0/2 with engine 642.8 as of 1.12.2011

Depending on the model, the production breakpoint for the ECO start/stop function may be at a later time.

Function sequence for basic shift program 

The basic shift program includes upshift and downshift characteristics for all 7 gears, designed for:

• Empty vehicle
• Warm operating condition
• Smooth road
• Sea level

The basic shift program can be influenced by the driver using the automatic transmission mode button (S16/12) on the lower control panel control unit (N72).

The shift programs are adapted by offsetting the characteristics, depending on:

• Vehicle load
• Change in driving resistance
• Pedal movement

The fully integrated transmission control unit monitors and shifts depending on:

• Vehicle speed
• Selector lever position
• accelerator pedal position
• Transmission mode selection
• Selected gear, for Manual ("M") transmission mode with code 950 (Sport package) except code 494 (USA version)
• Signals on the drive train CAN (CAN C)

Slow accelerator release:

• Undesired downshifts in the upper vehicle speed range are prevented

Rapid accelerator release:

• Upshifting to the next higher gear is prevented and only permitted again when the vehicle accelerates slowly

Transmission mode "M" is only available for code 950 (Sport package). In vehicles without the sport package, transmission mode "M" is not available. In these vehicles, transmission mode "M" is replaced by the "short term mode M".

The gear limitation is replaced by the function "Short-term mode M". The short-term mode M is a temporary program that is activated when the steering wheel gearshift buttons or the DIRECT SELECT lever (S16/13) are operated. Pressing in the positive (+) direction triggers an upshift and pressing in the negative (-) direction triggers a downshift, insofar as this is permissible. If not, the current gear is maintained. After a specified time window, this function deactivates again automatically. The activated time can be extended by a minimum speed being dropped below (e.g. stopping), where lateral acceleration is detected by the yaw rate sensor for lateral and longitudinal acceleration (B24/15) or if the vehicle is in a deceleration phase. If none of the stated criteria are active anymore, the M mode remains active for a prescribed remaining time. After this remaining time expires, the initial shift mode is returned to.

The driver can prematurely cancel the short-term mode M through specific actions, e.g. if he/she operates and holds the upshift steering wheel gearshift button (S111/1) or the DIRECT SELECT lever in the "positive" (+) direction, changes the gear range or changes the transmission mode. If one of these preconditions is met, a switch is made back to the initial shift program.

As of model refinement 13, a gearshift recommendation is also shown in the instrument cluster (A1). This is part of the short-term mode M that is also integrated.

Function sequence for electronic and hydraulic transmission control 

While the electronic transmission control system is responsible for gear selection and adaptation of pressures to the torque to be transmitted, power supply control for the transmission is performed by the hydraulic elements of the fully integrated transmission controller unit. The oil supply for the hydraulic elements such as hydrodynamic torque converter, actuators and hydraulic transmission control is provided through an oil pump (primary pump) connected to the torque converter or through the electric transmission oil pump (M42) (with code (B03) ECO start/stop function). The transmission control system makes possible a precise adaptation of pressures to the respective operating states and to the engine output during the gearshift phase, resulting in a significant improvement in shift quality. It is possible to reach the engine rpm limit in the individual gears using full throttle and kickdown.

The transmission control system prevents impermissible high engine speeds with a downshift lockout device. Furthermore, the transmission control system offers the advantage of a flexible adaptation to various vehicle and engine versions.

The electric transmission oil pump ensures that oil supply continues to the hydraulic system when engine is switched off. It also supports the primary pump when the combustion engine is operating in the low speed range. This is actuated through the gear oil auxiliary pump control unit (N89).

Function sequence for kickdown 

The driver's wish for maximum acceleration is met by the kickdown function.

The kickdown function is activated as of a pedal travel > 90 %.

The "kickdown" signal is routed from the CDI control unit (N3/9) (with diesel engines) or ME-SFI [ME] control unit (N3/10) (with gasoline engines) to the fully integrated transmission control unit over the drive train CAN.

Based on this information the fully integrated transmission control unit actuates the control solenoid valves so that upshifts and downshifts take place at an increased engine speed in order to achieve maximum performance.

The forced downshift into the next lower gear is performed when transmission temperatures are too high and at lower vehicle speed.

Function sequence for limp-home mode 

If the signals are implausible, the transmission control system is only partially operational and switches to the programmed emergency performance map.

The driver can engage the desired gear ranges "P, R, N, D" using the DIRECT SELECT lever.

The driver can choose been forward and reverse travel, engage the park pawl or, when in neutral, interrupt the power flow between torque converter and transmission.