Limiting braking torque by means of coolant temperature, function

W43.30-1177-76

Illustrated on model 930

A20 Retarder control (RS) control unit

B65 Coolant temperature sensor

CAN 1 Vehicle CAN

P2 Instrument (INS)

P2h8 Permanent brake indicator lamp

Y114 Retarder valve block

Function sequence

1 The retarder control unit (A20) is supplied with information regarding the coolant temperature from the coolant temperature sensor (B65).

2 If the coolant temperature rises into the temperature limiting range during a retarder brake application, the control current to the retarder proportioning valve is reduced linearly for a short time. The actuating pressure is limited, thus limiting the retarder braking torque.

3 After this a constant coolant temperature is regulated:
If the measured coolant temperature is smaller than the maximum permissible coolant temperature the actuating pressure is increased continually. In the opposite case the actuating pressure is reduced continuously.

4 The retarder brake torque and thus the heat accumulating reduce until an equilibrium arises between the accumulating retarder brake energy and the thermal energy which can be dissipated via the vehicle cooling system.

While the limitation of retarder brake energy is effective as a result of too high coolant temperature, the permanent brake indicator lamp flashes (P2h8).