Actuation of retarder, function

W43.30-1163-79

0 Switch position (retarder off)

1 Switch position (retarder stage 1)

2 Switch position (retarder stage 2)

3 Switch position (retarder stage 3)

4 Switch position (retarder stage 4)

5 Switch position (retarder stage 5)

6 Retarder

A3 Drive control (FR) control unit

A6 Engine control (MR) control unit

A11 Electronic brake system (EBS) control unit

A16 Gear control (GS II) control unit

CAN 1 Vehicle CAN

CAN 4 Engine CAN

CAN 5 Transmission CAN

P2 Instrument (INS)

P2h8 Permanent brake indicator lamp

P3 Modular tachograph (MTCO)

S2 Engine control/permanent brake operating lever

Y114 Retarder valve block

A20 Retarder control (RS) control unit

B17 Vehicle speed sensor

B57 Rpm sensor for transmission output speed (only on vehicles with code (GS7) gear control II and 2-gear transfer case, code (GS7) gear control II and code (GE2) Electronic drive control II or with code (GE3) Mercedes PowerShift)

Electrical signals

a Position of engine control/permanent brake operating lever

b Coolant temperature

c Retarder oil temperature

d Actuation of retarder valve block

Versions

A Vehicles without rpm sensor for transmission output speed

B Vehicles with rpm sensor for transmission output speed

CAN message

e Actuation of retarder indicator lamp

General information

The retarder (6) is actuated:

 If the service brake is operated and the brake control control unit (BS) (A11) requests the use of the retarder to assist the braking power (6).

 If the cruise control is activated and an actuation of the retarder (6) is necessary to adapt the vehicle speed (for example when driving downhill).

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

 When the autonomous intelligent cruise control (ART) is activated and an actuation of the retarder (6) is necessary to regulate the distance.

 Only vehicles with code (ET5) Adaptive cruise control.

Requirements

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

 Voltage supply of all control unit affected and networking intact.

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Function sequence

1 By means of the engine speed the engine control (MR) control unit (A6) checks which maximum engine braking torque is available or can be transmitted and sends this information via the engine CAN (CAN 4) to the drive control control unit (FR) (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 rpm sensor for transmission output speed (A):
The retarder speed is determined from the values from the speed sensor (B17), the transmission-driven high drive ratio and the gear ratio of the drivetrain.

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The gear ratio of the drivetrain is vehicle-specific and is determined when the ignition is switched on, while the values from the speed sensor reach the drive control control unit (FR) via the (B17) modular tachograph (MTCO) (P3) (FR) (A3), and from there via the vehicle CAN (CAN 1) to the retarder control control unit (RS) (A20).

 On vehicles with rpm sensor for transmission output speed (B):
The retarder control control unit (RS) (A20) uses the transmission output speed to calculate the retarder speed in addition to the transmission-driven high drive ratio.
This is determined by the gear control control unit (GS) (A16) from the signal from the rpm sensor for transmission output speed (B57) and made available to the drive control control unit (FR) via the transmission CAN (CAN 5) (A3).
The drive control control unit (FR) (A3) finally passes it on to the retarder control control unit (RS) via the vehicle CAN (CAN 1)A20

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3 With the aid of the information available regarding the maximum permanent brake torques the drive control control unit (FR) dividesA3 the specified brake torques into various stages. These are engaged in stages with the engine control/permanent brake operating lever (S2).
Depending whether the request is made by the engine control/permanent brake operating lever (S2), the brake control control unit (BS) (A11), the cruise control or the autonomous intelligent cruise control control unit (ART), the drive control control unit (FR)transmits (A3) a corresponding message with the specified brake torque to the retarder control control unit (RS) (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 (6) by the proportioning valve which is integrated in the retarder valve block (Y114).

 The retarder control control unit (RS) (A20) constantly checks the coolant temperature (b) and the retarder oil temperature (c).

If the retarder oil or coolant reach unacceptably high temperatures, the braking torque of the retarder (6) is limited.