Actuation of retarder, function

W43.30-1229-79

1 Retarder

A3 Drive control (FR) control unit

A6 Engine control (MR) control unit

A11 Brake control (BS brake control unit)
- Vehicles without code (ZX2)
EPB II

A16 Gear control (GS) control unit

A20 Retarder control (RS) control unit

A110 Central Brake Unit (CBU)
- Vehicles with code (ZX2)
EPB II

A110a1 Brake control (BS brake control unit)
- Vehicles with code (ZX2)
EPB II

Control unit input information

a Position of engine control/permanent brake operating lever (S2)

b Coolant temperature

c Retarder oil temperature

Information on actuation

d Actuation of retarder valve block (Y114)

e Actuation of retarder indicator lamp (P2h8)

A Vehicles without code (GS2) Gear control II

B Vehicles with code (GS2) Gear control

B17 Speed sensor

B57 Transmission output rpm sensor

CAN 1 Vehicle CAN

CAN 4 Engine CAN

P1 Tachograph

P2 Instrument (INS)

P2h8 Permanent brake indicator lamp

S2 Engine control/permanent brake operating lever

Y114 Retarder valve block

General information

The retarder (1) is actuated,

 when the service brake is operated and the brake control (BS) control unit (A11 or A110a1) requests the use of the retarder to assist the braking power (1)

 Only on vehicles with code (BB8) Brake system (BS) with ASR.

 when the cruise control has been activated and an actuation of the retarder (1) is necessary to adapt the vehicle speed (for example when driving down a mountain)

 by manual operation of the engine control/permanent brake operating lever (S2)

Requirements

 Antilock brake system (ABS) and Acceleration slip regulation (ASR) are not in closed-loop operation

 Power supply present for all affected control units and networking intact

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Function sequence

1 By means of engine speed the engine control (MR) control unit (A6) checks which maximum engine braking torque is available or can be converted and transmits this information via the engine CAN (CAN 4) to the drive control (FR) control unit (A3).

2 By means of retarder rpm the retarder control (RS) control unit (A20) simultaneously determines which maximum retarder braking torque can be converted and likewise transmits this to the drive control (FR) control unit (A3).
The retarder rpm required for the calculation can be determined differently in the process:

 On vehicles without code (GS2) gear control (A) the retarder rpm is determined from the values of the speed sensor (B17), the transmission-driven high tractive ratio and the drive train ratio.

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 The drive train ratio is vehicle-specific and is determined when switching on the ignition, while the values from the speed sensor (B17) reach the drive control (FG) control unit via the tachograph (P1) (A3) and from there reach the retarder control (RS) control unit via the vehicle CAN (CAN 1)A20

 On vehicles with code (GS2) Gear control (B) (A20) apart from the transmission-driven high tractive ratio the retarder control (RS) control unit uses the transmission output speed to calculate the retarder rpm.
This is determined by the gear control (GS) control unit (A16) from the signal from the transmission output speed rpm sensor (B57) and made available to the drive control (FR control unit via the vehicle CAN (CAN 1) (A3).
The drive control (FR) control unit (A3) then passes on the CAN message "transmission output speed" to the retarder control (RS) control unit via the vehicle CAN (CAN 1)A20.

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3 With the help of the information on the maximum permanent brake torque available the drive control (FR) control unit (A3) divides up the specified braking torque to the various positions of the engine control/permanent brake operating lever (S2) or in various stages.
Depending on whether the request is made by the engine control/permanent brake operating lever (S2), the brake control (BS) control unit (A11 or A110a1) or the cruise control, the drive control (FR) control unit (A3) transmits a corresponding message with the specified braking torque to the retarder control (RS) control unit (A20).
In the process the positions of the engine control/permanent brake operating lever (S2) are read in by the retarder control (RS) control unit (A20) and transmitted to the drive control (FR) control unit via the vehicle CAN (CAN 1) (A3).

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4 Once the retarder control (RS) control unit (A20) has received the message with the specified braking torque, it calculates from this in connection with the retarder rpm the necessary control current, with which it actuates the retarder valve block (Y114). While the retarder control (RS) control unit (A20) actuates the retarder valve block (Y114), it transmits via the vehicle CAN (CAN 1) the message "Indicator lamp ON" to the instrument (INS) (P2).

5 The instrument (INS) (P2) receives the message "Indicator lamp ON" and as a result of this actuates the permanent brake indicator lamp (P2h8).

6 Depending on the size of the control current a pneumatic actuating pressure is modulated to the retarder diaphragm chamber (1) by the proportioning valve which is integrated in the retarder valve block (Y114).

 The retarder control (RS) control unit (A20) constantly checks the coolant temperature and retarder oil temperature. If the retarder oil or coolant reach unacceptably high temperatures, the braking torque of the retarder (1) is limited.