Actuation of retarder, function

W43.30-1142-06

1 Retarder

A3 Drive control (FR) control unit

A20 Retarder control unit

CAN 1 Vehicle CAN bus

S2 Engine control/permanent brake operating lever

Y44 Retarder proportioning valve

Control unit input information:

a Operating lever position

b Retarder oil temperature

d Coolant temperature

Information on actuation:

c Proportioning valve actuation

General

The retarder is actuated,

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

 when the service brake is operated and the BS/EPB control unit requests the use of the retarder for assisting the braking power,

 when the cruise control has been activated and a retarder actuation is necessary to adapt the vehicle speed (when driving down a mountain),

 when the autonomous intelligent cruise control (ART) has been activated and a retarder actuation is necessary to control the distance.

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

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Requirements

 ABS/ASR not in closed-loop operation.

 Voltage supply present at all the control units in question and networking intact.

Function sequence

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

2 Simultaneously by means of retarder speed the retarder control unit (A20) determines which max. retarder braking torque can be converted and transmits this via the vehicle CAN bus (CAN 1) to the drive control (FR) control unit (A3).

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The retarder speed required for the calculation can be determined differently in the process:

 On vehicles with hydraulic or mechanical shift the retarder speed is calculated from the values of the speed sensor, the transmission-driven high-drive ratio and the ratio of the drive train.

The gear ratio of the drive train is vehicle-specific and is determined when the ignition is switched on, whilst the values of the speed sensor reach the drive control (FR) control unit (A3) via the tachograph and from there reach the retarder control unit (A20) via the vehicle CAN bus (CAN 1).

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 On vehicles with gear control and electronic pneumatic shift, the retarder control unit (A20) uses the transmission output speed to calculate the retarder speed, in addition to transmission-driven high-drive gear ratio. This is determined by the transmission output rpm sensor and made available to the drive control (FR) control unit (A3). The drive control (FR) control unit (A3) finally passes it on via the vehicle CAN bus (CAN 1) to the retarder control unit (A20).

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3 Based on the information on the max. permanent braking torque available, the drive control (FR) control unit(A3) divides the specified torque between the various lever positions of the engine control/permanent brake operating lever (S2) or in various stages.

Based on the type of request, in other words depending on whether the request ensues through the engine control/permanent brake operating lever (S2), through the BS/EPB control unit or through the use of cruise control or autonomous intelligent cruise control (ART), the drive control (FR) control unit (A3) transmits a corresponding message with the specified braking torque to the retarder control unit (A20).

The positions of the engine control/permanent brake operating lever (S2) are read in in the process by the retarder control unit (A20) and transmitted to the drive control (FR) control unit via the vehicle CAN bus (CAN 1)(A3)

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4 Once the retarder control unit (A20) has received the message with the specified braking torque, it calculates from this, in conjunction with the retarder speed, the necessary valve current with which the retarder proportioning valve (Y44) has to be actuated in order to convert the specified braking torque.

While the retarder control unit (A20) actuates the retarder proportioning valve (Y44) with the calculated valve current, it transmits the message "indicator lamp ON" to the instrument via the vehicle CAN bus (CAN 1).

5 The instrument receives the message "indicator lamp ON" and as a result of this actuates the indicator lamp.

6 Depending on the size of the control current, a pneumatic actuating pressure to the retarder oil sump is modulated by the retarder proportioning valve (Y44).

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 The level of the valve current and thus the retarder braking torque can be limited if the retarder oil or coolant reach unacceptably high temperatures. For this reason the retarder control unit (A20) permanently checks the coolant temperature and the oil temperature of the retarder.