What is the function of a vehicle’s retarder?

1. Serve as an alternative to the service brakes (foot brake), in order to stop the vehicle when it is under imminent danger during an emergency.
2. Assist the parking brake (hand brake) while parking on a steep descent.
3. Balance the engine’s function and regulate the motor rotations,
4. Correct answer: To reduce the vehicle speed, without using the vehicle’s braking system, e.g. during a long descent.

WhyA retarder reduces speed without using the main brakes, for example on a long descent. It does not replace the foot brake in sudden danger.