The present invention relates to an arrangement and a method for controlling the activation of an auxiliary brake of a vehicle. The arrangement comprises the auxiliary brake (5), a cooling system (1) with a circulating cooling medium adapted to cooling the auxiliary brake (5), a temperature sensor (8) adapted to detecting the temperature (Tout) of the cooling medium after the latter has cooled the auxiliary brake (5), and a control unit (7) adapted to comparing temperature values (Tout) received from the temperature sensor (8) with a reference temperature (Tref). The control unit (7) is adapted to controlling the activation of the auxiliary brake (5) with a reduced braking effect when temperature values (Tout) received exceed a reference temperature (Tref) which varies with time during at least one period of time (t1-t5) of the activation time of the auxiliary brake (5).