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

TRANSMISSION 724.2 in MODEL 212 

Function requirements for transmission control, general 

• Circuit 87 (connected circuit 15) 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, switchover valves). The actuators are located in the hydraulic valve body assembly. This implements the hydraulic functions.

The function of the fully integrated transmission control unit basically consists of the evaluation of various input signals relevant to the transmission function. Depending on these, ten electromechanical hydraulic valves are actuated which in turn determine the gear ratio of the transmission (gear selection), the gear ranges ("P", "R", "N" and "D"), the working pressure and control of the wet clutch.

Transmission control function sequence 

The function sequence is subdivided into the following steps:

• Function sequence for basic shift program 
• Electronic and hydraulic transmission control function sequence 
• Function sequence for kickdown 
• Function sequence for limp-home mode 
• Function sequence for transmission cooling (for engine 651) 

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's transmission mode button (S16/12).

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

• Vehicle load
• Change in driving resistance
• Accelerator pedal movement

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

• Vehicle speed
• Selector lever position
• Accelerator pedal position
• Transmission mode selection
• Gear limitation function
• 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 gear is prevented and only permitted again when the vehicle accelerates slowly

Electronic and hydraulic transmission control function sequence 

While the electronic transmission control is responsible for gear selection and the adaptation of pressures to the torque to be transferred, the power supply control of the transmission occurs via the hydraulic elements of the fully integrated transmission controller unit (Y3/8). The oil is supplied for the hydraulic elements (wet clutch, actuators, hydraulic transmission control) via an oil pump connected to the transmission input shaft or via the electric transmission oil pump (M42).

The transmission control allows for precise adaptation of pressures to the respective operating conditions and to the engine power or the output of the electric machine (A79/1) during the shift phase, which results in a significant increase in shift quality.

The engine speed limit can be reached in the individual gears at full throttle and kickdown.

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

The arrangement of two switchover valves (D/R switchover valve (Y3/8y10) and park pawl switchover valve (Y3/8y11)) on the fully integrated transmission control controller unit allows gear selection to be carried out electrohydraulically, as a result of which the use of an intelligent servo module can be dispensed with.

Function sequence for kickdown 

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

The kickdown function is activated as of a pedal travel > 90 %. The kickdown signal from the accelerator pedal sensor (B37) is read in by the CDI control unit (N3/9) or the ME-SFI [ME] control unit (N3/10), and forwarded over the drive train CAN to the fully integrated transmission control unit.

Based on this information the fully integrated transmission control unit actuates the control solenoid valves so that the upshifts and downshifts take place at higher combustion engine/electric machine speeds 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 in vehicles up to facelift 13 

If the signals are implausible, the transmission control system is only partially operational and switches to the programmed emergency performance map. The fault is stored with the assigned fault code in the fault memory. The CDI control unit or the ME-SFI [ME] control unit also sends an indicator lamp request over the chassis CAN to the instrument cluster (A1). The instrument cluster actuates the engine diagnosis indicator lamp (A1e58).

Due to integration of the DIRECT SELECT shift components (D/R switchover valve and park pawl switchover valve) in the fully integrated transmission control controller unit, there is no longer a hydraulic limp-home function in the event of failure of the fully integrated transmission control unit. If the hydraulic control system fails, e.g. due to interruption of the power supply of the fully integrated transmission control unit, the park pawl is engaged through actuation of the park pawl lift solenoid (Y3/8l1) by the electronic ignition lock control unit (N73) over the AUX power supply line. It is only possible to release the park pawl hydraulically via the park pawl switchover valve.

As of facelift 13 the existing chassis CAN is replaced with chassis 1 (CAN E1). An additional chassis CAN 2 (CAN E2) is used between the instrument cluster and the front SAM control unit with fuse and relay module (N10/1). The ME-SFI [ME] control unit or the CDI control unit forms the interface between the drive train CAN and the chassis CAN 1. The interface between chassis CAN 1 and chassis CAN 2 is formed by the front SAM control unit with fuse and relay module.

Function sequence for transmission cooling (for engine 651) 

To maintain the temperature of the automatic transmission oil at a level suitable for the driving condition, the transmission cooling coolant circulation pump (M13/7) requests a constant flow of coolant through the transmission oil heat exchanger.

The temperature of the automatic transmission oil is detected by the transmission oil temperature sensor (Y3/8s2). The fully integrated transmission control unit sends the temperature value over the drive train CAN to the CDI control unit. This compares the input signals with a threshold value stored in the characteristics map, and actuates the transmission cooling coolant circulation pump over the transmission cooling coolant circulation pump relay (K40/10kS).