Enabling back-up mode, function

W26.20-1125-79

1 Without malfunction

2 In the event of failure of the main computer

3 In the event of failure of the speedometer CAN

A3 Drive control (FR) control unit

A7 Base module (GM)

A11 Electronic brake system (EBS) control unit

A16 Gear control (GS II) control unit

B17 Vehicle speed sensor

CAN 1 Vehicle CAN

a CAN message "menu for gear control back-up mode released"

a1 Electrical signal "D3"

b CAN message "moderate speed, front axle"

c CAN message "Mean rear axle speed"

d CAN message "vehicle speed"

e CAN message "request for enable of gear control back-up mode"

f CAN message "request for release of gear control back-up mode (from driver)"

CAN 2 Interior CAN

CAN 5 Transmission CAN

CAN 10 Speedometer CAN

P1 Tachograph

P2 Instrument (INS)

P3 Modular tachograph (MTCO)

S144 Left multifunction steering wheel button group

S145 Right multifunction steering wheel button group

KErs Back-up mode K-line

1 General

The left multifunction steering wheel button group (S144) and right multifunction steering wheel button group (S145) are used to navigate in the menu tree of the driver information system (FIS).

The request from the driver to enable
the back-up mode is initiated by selecting the menu items "Adjustments  Setup 
Release gear control back-up mode". The base module (BM) (A7) reads in the operating commands from the driver, evaluates them and conveys corresponding CAN messages to the instrument cluster (INS) (P2).

The Instrument (INS) (P2) requests the enable for back-up mode from
gear control (GS) control unit (A16). On condition that no current fault is present which would lead to the disabling of back-up mode, the gear control (GS) control unit (A16) checks whether the necessary enable condition "vehicle stationary" is met.

After the enable has been performed, the "Gear control (GC) back up mode" menu is shown in the driver information system (FIS) display.

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2 Determining the enable condition "vehicle stationary"

The modular tachograph (MTCO) (P3) or digital tachograph (P1) receives the electrical signal "D3" (a1) from the road speed pick-up (B17) for the transmission output speed, uses it to calculate the vehicle speed and then forwards it over the
speedometer CAN (CAN 10) to the Instrument (INS) (P2).

 Only if speedometer CAN (CAN 10) fails:
The modular tachograph (MTCO) (P3) or the digital tachograph (P1) transmits the vehicle speed signal to the drive control (FR) control unit (A3) via a separate line.
The signal is converted into the CAN message "vehicle speed" (d) in the drive control (FR) control unit (A3), this message is conveyed to the base module (GM) (A7) and from there it is passed on to the instrument cluster (INS) (P2).
The instrument (INS) (P2) passes on the CAN message "Vehicle speed" (d) to the gear control (GS) control unit (A16) via the back-up mode K-line (KErs).

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Redundancy signal for vehicle speed:
The brake system (BS) control unit (A11) transmits the CAN message "Average speed front axle" (b) to the instrument (INS) (P2) via the vehicle CAN (CAN 1).The CAN message "Average speed rear axle" (c) is available as a redundancy signal.
The Instrument (INS) (P2) routes the CAN messages for "average speed at front axle" (b) or "average speed at rear axle" (c) over the
back-up mode K-line (KErs) to the gear control (GS) control unit (A16).

With functional main processor in gear control (GS) control unit (A16):
The main processor of the gear control (GS) control unit (A16) evaluates the CAN message "vehicle speed" (d) and in this way determines when the vehicle is stationary.

With defective main processor in gear control (GS) control unit (A16):
The auxiliary processor in the gear control (GS) control unit (A16) evaluates the CAN messages "Average speed front axle" (b) or "Average speed rear axle" (c) and in this way determines when the vehicle is stationary.

3 Enabling the "GS back-up mode" menu

After the gear control (GS) control unit (A16) has determined that the vehicle is at a standstill, it uses the
back-up mode K-line (KErs) to send the CAN message "Released gear control back-up mode menu" (a) to the Instrument (INS) (P2).

This enables the back-up mode of the gear control II (GS II) and the "Gear control (GC) back up mode" menu can be opened.